birdcage/docs/A3981-datasheet.md

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---

## Page 1

The A3981 is a flexible microstepping motor driver with built-in
translator for easy operation. It is a single-chip solution, designed
to operate bipolar stepper motors in full-, half-, quarter- and
sixteenth-step modes, at up to 28 V and ±1.4 A. The A3981
can be controlled by simple Step and Direction inputs, or
through the SPI-compatible serial interface that also can be
used to program many of the integrated features and to read
diagnostic information.
The current regulator can be programmed to operate in fixed
off-time or fixed frequency PWM, with several decay modes
to reduce audible motor noise and increase step accuracy.
In addition the phase current tables can be programmed via
the serial interface to create unique microstep current profiles
to further improve motor performance for specific applications.
The current in each phase of the motor is controlled through a
DMOS full bridge, using synchronous rectification to improve
power dissipation. Internal circuits and timers prevent cross-
conduction and shoot-through, when switching between high-
side and low-side drives.
The outputs are protected from short circuits, and features
for low load current and stalled rotor detection are included.
Chip-level protection includes hot and cold thermal warnings,
overtemperature shutdown, and overvoltage and undervoltage
lockout.
The A3981 is supplied in a 28-pin TSSOP power package with
an exposed thermal pad (package type LP). This package is
lead (Pb) free with 100% matte-tin leadframe plating.
A3981-DS, Rev. 9
MCO-0000738
•	 Peak motor current up to ±1.4 A, 28 V
•	 Low RDS(on) outputs, 0.5 Ω source and sink, typical
•	 Automatic current decay mode detection/selection
•	 Mixed, Fast, and Slow current decay modes
•	 Synchronous rectification for low power dissipation
•	 Internal OVLO, UVLO, and Thermal Shutdown circuitry
•	 Crossover-current protection
•	 Short-circuit and open-load diagnostics
•	 Hot and cold thermal warning
•	 Stall detect features
•	 SPI-compatible or simple Step and Direction motion
control
•	 Highly configurable via SPI-compatible serial interface
APPLICATIONS
•	 Automotive stepper motors
•	 Engine management
•	 Headlamp positioning
Automotive Programmable Stepper Driver
PACKAGE:
28-Pin TSSOP with Exposed Thermal Pad
(suffix LP)
Typical Applications
Not to scale
A3981
Automotive
12V Power Net
Logic
Supply
Serial Interface Control
Micro-
controller
or
ECU
OAP
PGND
VBB
OSC
OAM
OBP
OBM
Stepper
Motor
AGND
VCP
STEP
DIR
MS0
MS1
ENABLE
RESETn
SDI
SDO
SCK
STRn
VDD
DIAG
SENSA
SENSB
REF
VREG
CP1 CP2
Parallel Control
Automotive
12V Power Net
Logic
Supply
or
ECU
OAP
PGND
VBB
OSC
OAM
OBP
OBM
Stepper
Motor
AGND
VCP
STEP
DIR
MS0
MS1
ENABLE
RESETn
SDI
SDO
SCK
STRn
VDD
DIAG
SENSA
SENSB
REF
VREG
CP1 CP2
Micro-
controller
FEATURES AND BENEFITS
DESCRIPTION
July 11, 2024

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## Page 2

Automotive Programmable Stepper Driver
A3981
2
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
SELECTION GUIDE
Part Number
Packing\*
A3981KLPTR-T
4000 pieces per reel
4.4 mm × 9.7 mm, 1.2 mm nominal height TSSOP with
exposed thermal pad
\*Contact Allegro™ for additional packing information.
ABSOLUTE MAXIMUM RATINGS with respect to GND
Characteristic
Symbol
Notes
Rating
Unit
Load Supply Voltage
VBBx
Applies to VBBA and VBBB
–0.3 to 50
V
Logic Supply Voltage
VDD
–0.3 to 6
V
Pin CP1
–0.3 to VBB
V
Pins CP2, VCP
–0.3 to VBB+8
V
Pins STEP, DIR, ENABLE, DIAG
–0.3 to 6
V
Pin VREG
–0.3 to 8.5
V
Pin RESETn
Can be pulled to VBB with 38 kΩ
–0.3 to 6
V
Pin OSC
–0.3 to 6
V
Pins MS0, MS1
–0.3 to 6
V
Pins SDI, SDO, SCK, STRn
–0.3 to 6
V
Pin REF
–0.3 to 6
V
Pins OAP, OAM, OBP, OBM
–0.3 to VBB
V
Pins SENSA, SENSB
–0.3 to 1
V
Ambient Operating Temperature
Range
TA
Range K; limited by power dissipation
–40 to 150
°C
Maximum Continuous Junction
Temperature
TJ(max)
150
°C
Transient Junction Temperature
TtJ
Overtemperature event not exceeding 10 s, lifetime
duration not exceeding 10 hours, guaranteed by design and
characterization
175
°C
Storage Temperature Range
Tstg
–55 to 150
°C
THERMAL CHARACTERISTICS: May require derating at maximum conditions
Characteristic
Symbol
Test Conditions\*
Value
Unit
Package Thermal Resistance
(Junction to Ambient)
RθJA
4-layer PCB based on JEDEC standard
28
°C/W
2-layer PCB with 24.52 cm2 of copper area each side
32
°C/W
Package Thermal Resistance
(Junction to Pad)
RθJP
2
°C/W
\*Additional thermal information available on the Allegro website
SPECIFICATIONS

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## Page 3

Automotive Programmable Stepper Driver
A3981
3
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
DMOS Full Bridge
DMOS Full Bridge
Gate
Drive
Charge
Pump
STEP
REF
6-bit
DAC
6-bit
DAC
Oscillator
+
-
+
-
SENSB
SENSA
VCP
VBBA
OAP
OAM
SENSA
VBBB
OBP
OBM
SENSB
DIR
RESETn
ENABLE
VDD
DIAG
REF
3.3V
VBAT
VBAT
SDI
SDO
SCK
STRn
DAC
REF
PWM
Control
Bridge
Control
Logic
PWM
Control
Translator
Serial Interface
System
Control
and
Registers
Undervoltage, Overvoltage
Cold Warning, Hot Warning, Overtemperature
Short Detect, Open Load Detect
Stall Detect
D
N
G
P
D
N
G
A
Regulator
OSC
VREG
PAD
CP2
CP1
MS1
MS0
Functional Block Diagram

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## Page 4

Automotive Programmable Stepper Driver
A3981
4
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Pinout Diagram
Terminal List Table
Name
Number
Description
AGND
7
Analog reference ground
CP1
23
Charge pump capacitor terminal
CP2
24
Charge pump capacitor terminal
DIAG
16
Diagnostic output
DIR
3
Direction select input
ENABLE
26
Bridge enable input
MS0
13
Microstep select input
MS1
12
Microstep select input
OAM
25
Bridge A negative output
OAP
4
Bridge A positive output
OBM
18
Bridge B negative output
OBP
11
Bridge B positive output
OSC
5
Oscillator input
PAD
–
Connect exposed tab to ground
Name
Number
Description
PGND
21
Power Ground
REF
8
Reference input voltage
RESETn
27
Chip reset
SCK
9
Serial data clock
SDI
6
Serial data input
SDO
17
Serial data output
SENSA
1
Current sense node – bridge A
SENSB
14
Current sense node – bridge B
STEP
19
Step input
STRn
2
Serial data strobe
VBBA
28
Motor supply – bridge A
VBBB
15
Motor supply – bridge B
VCP
22
Above supply voltage
VDD
10
Logic Supply
VREG
20
Regulated voltage
28
27
26
25
24
23
22
21
20
19
18
17
16
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
I/O & Control
Timer
Reg
Ref
VDD
SENSA
STRn
DIR
OAP
AGND
REF
VDD
OBP
SENSB
VBBA
RESETn
ENABLE
OAM
VCP
PGND
VREG
STEP
OBM
DIAG
VBBB
2
P
C
C
S
O
SDI
SCK
CP1
MS1
MS0
SDO
Charge
Pump
Pinout Diagram and Terminal List Table

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## Page 5

Automotive Programmable Stepper Driver
A3981
5
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
ELECTRICAL CHARACTERISTICS [1][2]: Valid at TJ = –40°C to 150°C,  VBB = 7 to 28 V, VDD = 3 to 5.5 V, unless otherwise noted
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
SUPPLIES
Load Supply Voltage Range [3]
VBB
Functional
0
–
50
V
Outputs Driving
7
–
VBBOV
V
Load Supply Quiescent Current
IBBQ
ENABLE = 0
–
–
4
mA
Sleep mode
–
1
10
µA
Logic Supply Voltage Range
VDD
3
–
5.5
V
Logic Supply Quiescent Current
IDDQ
ENABLE = 0
–
–
5
mA
ENABLE=0, VDD > 5 V
–
–
5.5
mA
Sleep mode, VDD = 3.3 V
–
4
15
µA
Sleep mode, VDD = 5 V
–
–
25
µA
Charge Pump Voltage
VCP
With repect to VBB, VBB >7.5 V, ENABLE = 0,
RESETn = 1
–
6.7
–
V
Internal Regulator Voltage
VREG
ENABLE = 0, RESETn = 1, VBB > 7.5 V
–
7.2
–
V
Internal Regulator Dropout Voltage
VREGDO
ENABLE = 0, RESETn = 1, VBB > 5.6 V
–
100
200
mV
MOTOR BRIDGE OUTPUT
High-Side On-Resistance
RONH
VBB = 13.5 V, IOUT = –1 A, TJ = 25°C
–
500
600
mΩ
VBB = 13.5 V, IOUT = –1 A, TJ = 150°C
–
900
1100
mΩ
VBB = 7 V, IOUT = –1 A, TJ = 25°C
–
625
750
mΩ
High-Side Body Diode Forward
Voltage
VFH
IF = 1 A
–
–
1.4
V
Low-Side On-Resistance
RONL
VBB = 13.5 V, IOUT = 1 A, TJ = 25°C
–
500
600
mΩ
VBB = 13.5 V, IOUT = 1 A, TJ = 150°C
–
900
1100
mΩ
VBB = 7 V, IOUT = 1 A, TJ = 25°C
–
625
750
mΩ
Low-Side Body Diode Forward
Voltage
VFL
IF = –1 A
–
–
1.4
V
Output Leakage Current
ILO
ENABLE = 0, RESETn = 1, VO = VBB
–120
–65
–
µA
ENABLE = 0, RESETn = 1, VO = 0 V
–200
–120
–
µA
ENABLE = 0, RESETn = 0, VO = VBB
–
<1.0
20
µA
ENABLE = 0, RESETn = 0, VO = 0 V
–20
<1.0
–
µA
CURRENT CONTROL
Internal Oscillator Frequency
fOSC
OSC = AGND
3.2
4
4.8
MHz
51 kΩ from OSC to VDD
3.6
–
4.4
MHz
External Oscillator Frequency Range
fEXT
3
–
5
MHz
Blank Time [4]
tBLANK
Default Blank-Time
–
1500
–
ns
Off-Time (In Fixed Off-Time Mode) [4]
tOFF
Default Off-Time
–
44
–
µs
PWM Frequency (In Fixed Frequency
Mode) [4]
fPWM
Default PWM Frequency
–
16.7
–
kHz
Continued on the next page…

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## Page 6

Automotive Programmable Stepper Driver
A3981
6
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Continued on the next page…
ELECTRICAL CHARACTERISTICS [1][2] (continued): Valid at TJ = –40°C to 150°C,  VBB = 7 to 28 V, VDD = 3 to 5.5 V,
unless otherwise noted
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
CURRENT CONTROL (continued)
Fast Decay Time [4]
tFAST
Default Fast Decay Time
–
8
–
µs
Reference Input Voltage
VREF
0.8
–
2
V
Internal Reference Voltage
VREFint
REF tied to VDD
1.1
1.2
1.3
V
Reference Input Current
IREF
–3
0
3
µA
Maximum Sense Voltage
VSMAX
–
125
–
mV
Current Trip Point Error [5]
EITrip
VREF = 2 V, MXI0 = MXI1 = 1
–
–
±5
%
LOGIC INPUT AND OUTPUT – DC PARAMETERS
Input Low Voltage
VIL
–
–
0.3 × VDD
V
VDD > 4.5 V
–
–
0.28 × VDD
V
Input High Voltage
VIH
0.7 × VDD
–
–
V
Input Hysteresis
VIhys
250
500
–
mV
Input Current (Except RESETn)
IIN
0 V < VIN < VDD
–1
–
1
µA
Input Pull-Down Resistor (RESETn)
RPD
–
50
–
kΩ
Output Low Voltage
VOL
IOL = 2 mA
–
0.2
0.4
V
Output High Voltage
VOH
IOL = –2 mA
VDD–0.4
VDD–0.2
–
V
Output Leakage (SDO)
IO
0 V < VO < VDD, STRn = 1
–1
–
1
µA
LOGIC INPUT AND OUTPUT – DYNAMIC PARAMETERS
Reset Pulse Width
tRST
0.2
–
4.5
µs
Reset Shutdown Width
tRSD
10
–
–
µs
Input Pulse Filter Time (STEP, DIR)
tPIN
–
35
–
ns
Clock High Time
tSCKH
A in figure 1
50
–
–
ns
Clock Low Time
tSCKL
B in figure 1
50
–
–
ns
Strobe Lead Time
tSTLD
C in figure 1
30
–
–
ns
Strobe Lag Time
tSTLG
D in figure 1
30
–
–
ns
Strobe High Time
tSTRH
E in figure 1
300
–
–
ns
Data Out Enable Time
tSDOE
F in figure 1
–
–
40
ns
Data Out Disable Time
tSDOD
G in figure 1
–
–
30
ns
Data Out Valid Time from Clock
Falling
tSDOV
H in figure 1
–
–
40
ns
Data Out Hold Time from Clock
Falling
tSDOH
I in figure 1
5
–
–
ns
Data In Set-Up Time to Clock Rising
tSDIS
J in figure 1
15
–
–
ns
Data In Hold Time From Clock Rising
tSDIH
K in figure 1
10
–
–
ns
STEP Rising to STRn Rising
Setup Time
tSPS
L in figure 1, only when D15 = 1 and D14 = 0
100
–
–
ns
STEP Rising from STRn Rising
Hold Time
tSPH
M in figure 1, only when D15 = 1 and D14 = 0
300
–
–
ns

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## Page 7

Automotive Programmable Stepper Driver
A3981
7
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
LOGIC INPUT AND OUTPUT – DYNAMIC PARAMETERS (continued)
Step Low Time
tSTPH
1
–
–
µs
Setup Time Control Input Change
to STEP
tSU
MS1, MS2, DIR
200
–
–
ns
Hold Time Control Input Change
from STEP
tH
MS1, MS2, DIR
200
–
–
ns
Wake-Up from RESET
tEN
–
–
1
ms
DIAGNOSTICS AND PROTECTION
VBB Overvoltage Threshold
VBBOV
VBB rising
32
34
36
V
VBB Overvoltage Hysteresis
VBBOVHys
2
–
4
V
VREG Undervoltage Threshold
VREGUV
VREG falling
5.1
–
5.4
V
VREG Undervoltage Hysteresis
VRGUVHys
–
1
–
V
VDD Undervoltage Threshold
VDDUV
VDD falling
2.6
–
2.9
V
VDD Undervoltage Hysteresis
VDDUVHys
50
100
–
mV
VDD Power-On Reset Threshold6
VDDPOR
VDD falling
0.8
–
1.5
V
OSC Timeout
tWD
Bit 13 = 1
0.5
1
1.5
µs
High-Side Overcurrent Threshold
IOCH
Sampled after tSCT
1.4
2.05
2.65
A
High-Side Current Limit
ILIMH
Active during tSCT
3
5.5
8
A
Low-Side Overcurrent Sense Voltage
VOCL
Sampled after tSCT
210
250
290
mV
Overcurrent Fault Delay
tSCT
Default Fault Delay
1500
2000
2700
ns
Open Load Current Threshold Error
EIOC
VREF = 2 V, MXI0 = MXI1 = 1
–
–
±10
%
Temperature Voltage Output Offset
VTO
Temperature output selected on DIAG pin
–
1440
–
mV
Temperature Voltage Output Slope
AT
–
–3.92
–
mV/°C
Cold Temperature Warning Threshold
TJWC
Temperature decreasing
–20
–10
0
°C
Cold Temperature Warning Hysteresis
TJWChys
–
15
–
°C
Hot Temperature Warning Threshold
TJWH
Temperature increasing
125
135
145
°C
Hot Temperature Warning Hysteresis
TJWHhys
–
15
–
°C
Overtemperature Shutdown Threshold
TJF
Temperature increasing
155
170
–
°C
Overtemperature Hysteresis
TJhys
Recovery = TJF – TJhys
–
15
–
°C
[1] For input and output current specifications, negative current is defined as coming out of (sourcing) the specified device pin.
[2] All references to “VBB” apply to VBBA and VBBB.
[3] Function is correct but parameters are not guaranteed above or below the general limits (7 to 28 V). Outputs not operational above VBBOV or below VREGUV .
[4] Assumes a 4 MHz clock.
[5] Current Trip Point Error is the difference between actual current trip point and the target current trip point, referred to maximum full scale (100%) current:  EItrip = 100 ×
[ItripActual – ItripTarget ] / IFullScale (%).
[6] Ensured by design and characterization.
ELECTRICAL CHARACTERISTICS [1][2] (continued): Valid at TJ = –40°C to 150°C,  VBB = 7 to 28 V, VDD = 3 to 5.5 V,
unless otherwise noted

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## Page 8

Automotive Programmable Stepper Driver
A3981
8
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Figure 1: Serial Interface Timing Diagram
Figure 2: Control Input Interface Timing Diagram
C
A
B
D
E
J
K
F
I
G
0
D
4
1
D
5
1
D
'0
D
'4
1
D
'5
1
D
STRn
SCK
SDI
SDO
H
Z
STEP
L
M
No rise when
D15=1 and D14=0
Z
X
X
X
X
Key
Characteristic
Key
Characteristic
A
Clock High Time
H
Data Out Valid Time from Clock Falling
B
Clock Low Time
I
Data Out Hold Time from Clock Falling
C
Strobe Lead Time
J
Data In Set-Up Time to Clock Rising
D
Strobe Lag Time
K
Data In Hold Time From Clock Rising
E
Strobe High Time
L
STEP Rising to STRn Rising Setup Time
F
Data Out Enable Time
M
STEP Rising from STRn Rising Hold Time
G
Data Out Disable Time
X
“Don’t care”
Z
High-impedance (tristate)
STEP
DIR, MS0, MS1
tSTPL
tSTPH
tH
tSU
RESETn
tEN
ENABLE\*
* ENABLE(Pin) OR RUN[EN] bit

![Image 1 from page 8](pdf-image://page_8_img_1)

## Page 9

Automotive Programmable Stepper Driver
A3981
9
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
FUNCTIONAL DESCRIPTION
The A3981 is an automotive stepper motor driver suitable for
high-temperature applications such as headlamp bending and
leveling, throttle control, and gas recirculation control. It is also
suitable for other low-current stepper applications such as air
conditioning and venting. It provides a highly flexible microstep­
ping motor driver that can be configured via the SPI-compatible
serial interface. It can be controlled with simple Step and Direc­
tion inputs, for high-speed stepping applications, or directly
through the serial interface by writing a step change value.
The two DMOS full bridges are capable of driving bipolar step­
per motors in full-, half-, quarter-, eighth- and sixteenth-step
modes, at up to 28 V and ±1.4 A. The current in each phase of the
stepper motor is regulated by a peak detect PWM current control
scheme that can be programmed to operate in fixed off-time or
fixed frequency. Several decay modes can be selected to reduce
audible motor noise and increase step accuracy. In addition the
phase current tables, which default to a sinusoidal current profile,
can be programmed via the serial interface to create unique mic­
rostep current profiles to further improve motor performance for
specific applications.
The outputs are protected from short circuits, and features for
open load and stalled rotor detection are included. Chip level pro­
tection includes hot and cold thermal warning, overtemperature
shutdown, and overvoltage and undervoltage lockout.
Pin Functions
VBBA, VBBB. Main motor supply and chip supply for internal
regulators and charge pump. VBBA and VBBB should be con­
nected together and each decoupled to ground with a low ESR
electrolytic capacitor and a good ceramic capacitor.
Note: Any reference to “VBB” in this specification is defined as
applying to both VBBA and VBBB.
CP1, CP2. Pump capacitor connection for charge pump. Con­
nect a 100 nF (50 V) ceramic capacitor between CP1 and CP2.
VCP. Above-supply voltage for high-side drive. A 100 nF (16 V)
ceramic capacitor should be connected between VCP and VBB to
provide the pump storage reservoir.
VDD. Logic supply. Compatible with 3.3 V and 5 V logic. Should
be decoupled to ground with a 100 nF (10 V) ceramic capacitor.
VREG. Regulated supply for bridge gate drive. Should be
decoupled to ground with a 220 nF (10 V) ceramic capacitor.
AGND. Analog reference ground. Quiet return for measurement
and input references. Connect to PGND (see Layout section).
PGND. Digital and power ground. Connect to supply ground and
AGND (see Layout section).
OAP, OAM. Motor connection for phase A. Positive motor phase
current direction is defined as flowing from OAM to OAP.
OBP, OBM. Motor connection for phase B. Positive motor phase
current direction is defined as flowing from OBM to OBP.
SENSA. Phase A current sense. Connect sense resistor between
SENSA and PGND.
SENSB. Phase B current sense. Connect sense resistor between
SENSB and PGND.
REF. Reference input to set absolute maximum current level for
both phases. Defaults to internal reference when tied to VDD.
STEP. Step logic input. Motor advances on rising edge. Filtered
input with hysteresis.
DIR. Direction logic input. Direction changes on the next STEP
rising edge. When high, the Phase Angle Number is increased
on the rising edge of STEP. Has no effect when using the serial
interface. Filtered input with hysteresis.
MS0. Microstep resolution select input.
MS1. Microstep resolution select input.
RESETn. Resets faults when pulsed low. Forces low-power
shutdown (sleep) when held low for more than the Reset Shut­
down Width, tRSD . Can be pulled to VBB with 30 kΩ resistor.
ENABLE. Controls activity of bridge outputs. When held low,
deactivates the outputs, that is, turns off all output bridge FETs.
Internal logic continues to follow input commands.
SDI. Serial data input. 16-bit serial word input MSB first.
SDO. Serial data output. High impedance when STRn is high.
Outputs bit 15 of the diagnostic registers (Fault Register 0 and Fault
Register 1), the Fault Register flag, as soon as STRn goes low.
SCK. Serial interface clock. Data is latched in from SDI on the
rising edge of the SCK clock signal. There must be 16 rising
edges per write and SCK must be held high when STRn changes.
STRn. Serial data strobe and serial access enable. When STRn
is high any activity on SCK or SDI is ignored, and SDO is high
impedance allowing multiple SDI slaves to have common SDI,
SCK, and SDO connections.

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## Page 10

Automotive Programmable Stepper Driver
A3981
10
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
DIAG. Diagnostic output. Function selected via the serial inter­
face, setting Configuration Register 1. Default is Fault output.
OSC. With bit 13 in Configuration Register 1 set to 0, either con­
nect this pin to AGND to use the internal oscillator running at the
default frequency of 4 MHz, or connect a resistor to VDD to set
the internal oscillator frequency. ( The approximate frequency is
calculated from:
fOSC = 10 000 / (48 ROSC – 20)
where fOSC is the internal oscillator frequency in MHz, and ROSC
is the value, in kΩ of the resistor between OSC and VDD.)
If bit 13 in Configuration Register 1 is set to 1, then OSC is the
input for an external system clock, which must have a frequency
between 3 and 5 MHz. In this mode a watchdog is provided to
detect loss of the system clock. If the OSC pin remains high or
low for more than the watchdog time, tWD , 1 µs typical, then the
Fault Register flag (bit 15 in the diagnostic registers) is set and
the outputs are disabled until the clock restarts.
Driving a Stepper Motor
A two-phase stepper motor is made to rotate by sequencing
the relative currents in each phase. In its simplest form, each
phase is simply fully energized in turn by applying a voltage to
the winding. For more precise control of the motor torque over
temperature and voltage ranges, current control is required. For
efficiency this is usually accomplished using pulse width modula­
tion (PWM) techniques. In addition current control also allows
the relative current in each phase to be controlled, providing more
precise control over the motor movement and hence improve­
ments in torque ripple and mechanical noise. Further details of
stepper motor control are provided in Appendix A.
For bipolar stepper motors the current direction is significant,
so the voltage applied to each phase must be reversible. This
requires the use of a full bridge (also known as an H-bridge)
which can switch each phase connection to supply or to ground.
PHASE CURRENT CONTROL
In the A3981, current to each phase of the two-phase bipolar
stepper motor is controlled through a low impedance N-channel
DMOS full bridge. This allows efficient and precise control of
the phase current using PWM switching. The full-bridge con­
figuration provides full control over the current direction during
the PWM on-time, and over the current decay mode during the
PWM off-time. Due to the flexibility of the A3981 these control
techniques can be completely transparent to the user or can be
partially- or fully-programmed through the serial interface.
Each leg (high-side, low-side pair) of a bridge is protected from
shoot-through by a fixed dead time. This is the time between
switching off one FET and switching on the complementary FET.
Cross-conduction is prevented by lock-out logic in each driver pair.
The phase currents and in particular the relative phase currents
are defined in the Phase Current table (table 7). This table defines
the two phase currents at each microstep position. For each of the
two phases, the currents are measured using a sense resistor, RS,
with voltage feedback to the respective SENSx pin. The target
current level is defined by the voltage from the digital-to-analog
converter (DAC) for that phase. The sense voltage is amplified by
a fixed gain and compared to the output of the DAC.
There are two types of maximum current: the absolute maximum,
ISMAX , the maximum possible current defined by the sense resis­
tor and the reference input; and the phase maximum, IPMAX , the
maximum current delivered to a motor phase.
The absolute maximum current, ISMAX, is defined as:
ISMAX = VREF / (16 × RS )
where VREF is the voltage at the REF pin, and RS is the sense
resistor value.
The phase maximum, IPMAX , is the 100% reference level for the
phase current table and may be a fraction of the absolute maxi­
mum current, ISMAX , depending on the value of the MXI0 and
MXI1 bits in Configuration Register 0.
For example:
•	 if RS = 180 mΩ and VREF = 2 V, then ISMAX = 694 mA
•	 if MXI1= 1 and MXI0 = 0, then IPMAX = 520 mA
The actual current delivered to each phase at each Step Angle
Number is determined by the value of IPMAX and the contents
of the Phase Current table. For each phase, the value in the table
is passed to the DAC, which uses IPMAX as the reference 100%
level (code 63) and reduces the current target depending on the
DAC code. The output from the DAC is used as the input to the
current comparators.
The current comparison is ignored at the start of the PWM
on‑time for a duration referred to as the blank time. The blank
time is necessary to prevent any capacitive switching currents
from causing a peak current detection.

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## Page 11

Automotive Programmable Stepper Driver
A3981
11
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
The PWM on-time starts at the beginning of each PWM period.
The current rises in the phase winding until the sense voltage
reaches the required current level. At this point the PWM off-time
starts and the bridge is switched into one of two decay modes,
slow decay or fast decay:
•	 Slow decay is most effective when the current is rising
from step to step, and it occurs when the phase winding is
effectively shorted by switching-on either both high-side FETs
or both low-side FETs in the full bridge.
•	 Fast decay is most effective when the current is falling from
step to step, and it occurs when the voltage on the phase is
reversed.
One disadvantage of fast decay is the increased current ripple in
the phase winding. However, this can be reduced while main­
taining good current control, by using a short time of fast decay
followed by slow decay for the remainder of the PWM off-time.
This technique is commonly referred to as mixed decay.
The A3981 provides two methods to determine the PWM
frequency: fixed off-time and fixed frequency. At power-up the
default mode is fixed off-time. Fixed frequency can be selected
through the serial interface. Fixed off-time provides a marginal
improvement in current accuracy over a wide range of current
levels. Fixed frequency provides a fixed fundamental frequency
to allow more precise supply filtering for EMC reduction. In both
cases the PWM off-time will not be present if the peak current
limit is not attained during the PWM on-time.
PHASE CURRENT TABLE
The relative phase currents are defined by the Phase Current table
(Table 7). This table contains 64 lines and is addressed by the
Step Angle Number, where Step Angle Number 0 corresponds to
0° or 360°. The Step Angle Number is generated internally by the
step sequencer, which is controlled either by the STEP and DIR
inputs or by the step change value from the serial input. The Step
Angle Number determines the motor position within the 360°
electrical cycle and a sequence of Step Angle Numbers deter­
mines the motor movement. Note that there are four full mechani­
cal steps per 360° electrical cycle.
Each line of the Phase Current table (Table 7) has a 6-bit value
per phase to set the DAC level for that phase, plus an additional
bit per phase to determine the current direction for that phase.
The Step Angle Number sets the electrical angle of the stepper
motor in one-sixteenth microsteps, approximately equivalent to
electrical steps of 5.625°.
On first power-up or after a VDD power-on reset, the Phase Cur­
rent table values are reset to define a sinusoidal current profile
and the Step Angle Number is set to 8, equivalent to the electri­
cal cycle 45° position. This position is defined as the “home”
position. The maximum current in each phase, IPMAX , is defined
by the sense resistor and the Maximum Current setting (bits
MXI[0..1]) in Configuration Register 0. The phase currents for
each entry in the Phase Current table are expressed as a percent­
age of this maximum phase current.
When using the STEP and DIR inputs to control the stepper
motor, the A3981 automatically increases or decreases the Step
Angle Number according to the step sequence associated with
the selected step mode. The default step mode, reset at power-
up or after a power on reset, is full step. Half-, quarter-, and
sixteenth‑step sequences are also available when using the STEP
and DIR inputs, and are selected using the logical OR of the MS0
and MS1 inputs and the MS0 and MS1 bits in Configuration Reg­
ister 0. The eighth-step sequence is shown in the Phase Current
table for reference only.
When using the serial interface to control the stepper motor, a
step change value (6-bit) is input through the serial interface to
increase or decrease the Step Angle Number. The step change
value is a two’s complement (2’sC) number, where a positive
value increases the step angle and a negative value decreases
the step angle. A single step change in the Step Angle Number is
equivalent to a single one-sixteenth microstep. Therefore, for cor­
rect motor movement, the step change value should be restricted
to no greater than 16 steps, positive or negative.
This facility enables full control of the stepper motor at any
microstep resolution up to and including sixteenth‑step, plus
the ability to change microstep resolution “on-the-fly” from one
microstep to the next.
In both control input method cases, the resulting Step Angle
Number is used to determine the phase current value and current
direction for each phase, based on the Phase Current table. The
decay mode is determined by the position in the Phase Current
table and the intended direction of rotation of the motor.
Diagnostics
The A3981 integrates a number of diagnostic features to protect
the driver and load as far as possible from fault conditions and
extreme operating environments. At the system level the supply

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## Page 12

Automotive Programmable Stepper Driver
A3981
12
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
voltages and the chip temperature are monitored. A number of
these features automatically disable the current drive to protect
the outputs and the load. Others only provide an indication of
the likely fault status, as shown in the Fault table (Table 1). A
single diagnostic output pin (DIAG) can be programmed through
the serial interface to provide several different internal signals.
At power-up, or after a power-on-reset the DIAG pin outputs a
simple Fault Output flag which will be low if a fault is present.
The Fault Output flag remains low while the fault is present or if
one of the latched faults (for example, a bridge short circuit) has
been detected and the outputs disabled.
Alternative to the Fault Output flag, the DIAG output can be pro­
grammed via the serial interface to output: the stall detect signal,
which goes low when a stall is detected; the phase A PWM-on
signal, which is high during the phase A PWM on-time; or an
analog signal indicating the silicon temperature.
If required, specific fault information can be determined by read­
ing the diagnostic registers (see Serial Interface section).
The first bit (bit 15) in both diagnostic registers contains a com­
mon Fault Register flag which will be high if any of the fault bits
in either register has been set. This allows a fault condition to be
detected using the serial interface, by simply taking STRn low.
As soon as STRn goes low the fist bit in the diagnostic registers
can be read to determine if a fault has been detected at any time
since the last diagnostic registers reset. In all cases the fault bits
in the diagnostic registers are latched and only cleared after a
diagnostic registers reset.
Note that the Fault Register flag in the diagnostic registers, does
not provide the same function as the Fault Output flag on the
DIAG pin. The Fault Output flag on the DIAG pin provides an
indication that either a fault is present or the outputs have been
disabled due to a short circuit fault. The Fault Register flag sim­
ply provides an indication that a fault has occurred since the last
diagnostic registers reset and has been latched.
At the system level the supply voltages and chip temperature are
monitored.
SUPPLY VOLTAGE MONITORS
The logic supply, the motor supply, and the regulator output are
monitored: the motor supply for overvoltage, and the regulator
output and logic supply for undervoltage.
•	 If the motor supply voltage, VBBA and VBBB , goes above
the VBB overvoltage threshold, the A3981 will disable the
outputs and indicate the fault. When the motor supply voltage
goes below the VBB overvoltage threshold, the outputs will
be re-enabled and the fault flag removed. The fault bits in the
diagnostic registers remain set until cleared by a diagnostic
registers reset.
•	 If the output of the internal regulator, VREG , goes below the
VREG undervoltage threshold, the A3981 will disable the
outputs and indicate the fault. When the regulator output rises
above the VREG undervoltage threshold, the outputs will be
re-enabled and the fault flag removed. The fault bits in the
diagnostic registers remain set until cleared by a diagnostic
registers reset.
•	 If the logic supply voltage, VDD , goes below the VDD
undervoltage threshold, then the outputs will be immediately
disabled. When the logic supply rises above the VDD
undervoltage threshold, the outputs will be enabled.
•	 If the logic supply voltage, VDD , goes below the VDD
power–on reset threshold, a power-on reset will take place and
all registers will be reset to their default state. The fault bits in
the diagnostic registers remain set until cleared by a diagnostic
registers reset.
TEMPERATURE MONITORS
Three specific temperature thresholds are provided: a hot
warning, a cold warning, and an overtemperature shutdown. In
addition, the analog internal signal used to determine the chip
temperature can be selected in Configuration Register 1 as the
Table 1: Fault Table
Diagnostic
Action
Latched
VBB Overvoltage
Disable outputs, set Fault
Register flag
No
VREG Undervoltage
Disable outputs, set Fault
Register flag
No
VDD Undervoltage
Disable outputs
No
Temperature Warning
Set Fault Register flag
No
Overtemperature
Disable outputs, set Fault
Register flag
No
Bridge Short
Disable outputs, set Fault
Register flag
Yes
Bridge Open
Set Fault Register flag
No
Stall Detect
Set ST flag
No

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## Page 13

Automotive Programmable Stepper Driver
A3981
13
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
output on the DIAG pin through the serial interface. The analog
scale is TJ ≈ (VDIAG – VTO ) / AT  .
Hot Warning If the chip temperature rises above the Hot Tem­
perature Warning Threshold, TJWH , the Fault flag will go low and
the Hot Warning bits will be set in the diagnostic registers. No
action will be taken by the A3981. When the temperature drops
below the Hot Temperature Warning Threshold, the Fault flag
will go high but the Hot Warning bits remain set in the diagnostic
registers until reset.
Cold Warning If the chip temperature falls below the Cold
Temperature Warning Threshold, TJWC , the Fault flag will go low
and the Cold Warning bits will be set in the diagnostic registers.
No action will be taken by the A3981. When the temperature rises
above the Cold Temperature Warning Threshold, the Fault flag
will go high but the Cold Warning bits remain set in the diagnos­
tic registers until reset.
Overtemperature Shutdown If the chip temperature rises
above the Overtemperature Shutdown Threshold, TJF , the Fault
flag will go low and the Thermal Shutdown bits will be set in the
diagnostic registers. The A3981 will disable the outputs to try to
prevent a further increase in the chip temperature. When the tem­
perature drops below the Overtemperature Shutdown Threshold,
the Fault flag will go high but the Thermal Shutdown bits remain
set in the diagnostic registers until reset.
BRIDGE AND OUTPUT DIAGNOSTICS
The A3981 includes monitors that can detect a short to supply or
a short to ground at the motor phase connections. These condi­
tions are detected by monitoring the current from the motor
phase connections through the bridge to the motor supply and to
ground.
Low current comparators and timers are provided to help detect
possible open load conditions.
Short to Supply A short from any of the motor connections to
the motor supply (VBBA or VBBB) is detected by monitoring the
voltage across the low-side current sense resistor in each bridge.
This gives a direct measurement of the current through the low
side of the bridge.
When a low-side FET is in the On state, the voltage across the
sense resistor, under normal operating conditions, should never
be more than the Maximum Sense Voltage, VSMAX. In this state,
an overcurrent is determined to exist when the voltage across the
sense resistor exceeds the Low-Side Overcurrent Sense Voltage,
VOCL , typically 2 × VSMAX . This overcurrent must be continu­
ously present for at least the Overcurrent Fault Delay, tSCT ,
before the short fault is confirmed by setting the relevant bit in
FAULT0 and driving the DIAG output low if the Fault Output
flag is selected. The output is switched off and remains off until a
fault reset occurs.
Note that the sense resistor cannot distinguish which low-side
FET is in an overcurrent state. So, if more than one low-side FET
is active when the fault is detected, for example during low-side
recirculation with synchronous rectification, then the shorted con­
nection is determined from the internal PWM state.
The actual overcurrent that VOCL represents is determined by the
value of the sense resistor and is typically 2 × ISMAX .
Short to Ground A short from any of the motor connections
to ground is detected by directly monitoring the current through
each of the high-side FETs in each bridge.
When a high-side FET is in the On state the maximum current
is typically always less than 1 A. In this state, an overcurrent is
determined to exist when the current through the active high-side
FET exceeds the High-Side Overcurrent Threshold, IOCH .
This overcurrent must be present for at least the Overcurrent
Fault Delay, tSCT , before the short fault is confirmed by setting
the relevant bit in FAULT0 and driving the DIAG output low if
the Fault Output flag is selected. The output is switched off and
remains off until a fault reset occurs.
Note that when a short to ground is present the current through
the high-side FET is limited to the High-Side Current Limit,
ILIMH , during the Overcurrent Fault Delay, tSCT  . This prevents
large negative transients at the phase output pins when the out­
puts are switched off.
Shorted Load A short across the load is indicated by concurrent
short faults on both high side and low side.
Short Fault Blanking All overcurrent conditions are ignored
for the duration of the Overcurrent Fault Delay, tSCT . The short
detection delay timer is started when an overcurrent first occurs.
If the overcurrent is still present at the end of the short detection
delay time then a short fault will be generated and latched. If the
overcurrent goes away before the short detection delay time is
complete, then the timer is reset and no fault is generated.
This prevents false short detection caused by supply and load

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## Page 14

Automotive Programmable Stepper Driver
A3981
14
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Figure 3. Effect of stall condition on current rise
transients. It also prevents false short detections resulting from
current transients generated by the motor or wiring capacitance
when a FET is first switched on.
Short Fault Reset and Retry When a short circuit has been
detected all outputs for the faulty phase are disabled until the
next occurrence of: the next rising edge on the STEP input, the
RESETn input is pulsed low, or until the diagnostic registers
are reset by writing to one of the registers through the serial
interface. At the next STEP command or after a fault reset, the
Fault Register flag is cleared, the outputs are re-enabled, and the
voltage across the FET is resampled. Note that the diagnostic
registers are not cleared by the rising edge of the STEP input.
While the fault persists the A3981 will continue this cycle,
enabling the outputs for a short period then disabling the out­
puts. This allows the A3981 to handle a continuous short circuit
without damage. If, while stepping rapidly, a short circuit appears
and no action is taken, the repeated short circuit current pulses
will eventually cause the temperature of the A3981 to rise and an
overtemperature fault will occur.
Open Load Detection Open load conditions are detected
by monitoring the phase current when the phase DAC value
is greater than 31. The Open Load Current Threshold, IOL , is
defined by the OL0 and OL1 bits in the Run register as a percent­
age of the maximum (100%) phase current, IPMAX , defined in the
Phase Current table. The 100% level in the Phase Current table is
defined by the sense resistor value and the contents of the MXI0
and MXI1 bits in Configuration Register 0.
For example:
•	 if RS = 180 mΩ and VREF = 2 V,	then ISMAX = 694 mA
•	 if MXI1 = 1 and MXI0 = 0, then IPMAX = 520 mA
•	 if OL1=0 and OL0=1, then IOL = 156 mA
The open load current monitor is only active after a blank
time from the start of a PWM cycle. An open load can only be
detected if the DAC value for the phase is greater than 31 and the
current has not exceeded the Open Load Current Threshold for
more than 15 PWM cycles.
The A3981 continues to drive the bridge outputs under an open
load condition and clears the Fault Register flag as soon as the
phase current exceeds the Open Load Current Threshold or the
DAC value is less than 32. The diagnostic registers retain the
open load fault bits, OLA and OLB, and will not be cleared until
RESETn is pulsed low or one of the diagnostic registers is written
through the serial interface.
Stall Detection For all motors it is possible to determine the
mechanical state of the motor by monitoring the back-EMF
(BEMF) generated in the motor phase windings. A stalled motor
condition is when the phase currents are being sequenced to step
the motor but the motor remains stationary. This can be due to
a mechanical blockage such as an end stop or the step sequence
exceeding the motor capability for the attached load.
A PWM monitor feature is included in the A3981 to assist in
detecting the stall condition of the stepper motor. This feature
uses the effect of the BEMF on the current rise time by compar­
ing the PWM count during the current rise quadrant to determine
the point at which a stall occurs. Reliable stall detection in a
simple stepper driver is only possible by combining the PWM
monitor with a continuous step sequence at a sufficiently high
step rate.
When a motor is running normally, at speed, the BEMF, gen­
erated by the magnetic poles in the motor passing the phase
windings, acts against the supply voltage and reduces the rise
rate of the phase current, as shown in Figure 3. The PWM current
control does not activate until the current reaches the set trip level
for the microstep position. When a motor is stopped, as in a stall
Effect of stall
condition
Normal running
condition
Increased number of
PWM cycles at each
microstep

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## Page 15

Automotive Programmable Stepper Driver
A3981
15
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
condition, the BEMF is reduced. This allows the current to rise to
the limit faster and the PWM current control to activate sooner.
Assuming a constant step rate and motor load this results in an
increase in the number of PWM cycles for each step of the motor.
The A3981 uses this difference to detect a motor changing from
continuous stepping to a stalled condition.
The PWM monitor feature assumes the following factors:
•	 The motor must be stepping fast enough for the BEMF to
reduce the phase current slew rate. Stall detection reliability
improves as the current slew rate reduces.
•	 The motor is not being stepped in full step mode.
Although stall detection cannot be guaranteed when using the
integrated features of the A3981, good stall detection reliability
can be achieved by careful selection of motor winding resistance
and inductance, motor speed, count difference, stall detection
scheme, and by conforming to the above requirements.
The A3981 includes circuits to allow the PWM monitor to oper­
ate in two ways: compare opposite phases and compare each
phase.
Stall Detection Scheme: Compare Opposite Phases
The default stall detection scheme in the A3981, selected when
STS[1..0] = 00, is the compare opposite phases scheme.
When this scheme is selected, two PWM counters, one for each
phase, accumulate the number of PWM cycles when the phase
current is stepped from zero to full-scale current. At the end of
each phase current rise, the counter for that phase is compared
to the count result for the previous current rise in the opposite
phase, as shown in Figure 4. If the difference is greater than the
PWM count difference in the CONFIG1 register (CD[7:0]), then
the ST bit in the diagnostic registers is set. In addition, if the ST
signal is selected as the output on the DIAG pin, then the pin will
go low.
Figure 4: Stall Detect by PWM Count Comparing Opposite Phases, STS[1..0] = 00

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## Page 16

Automotive Programmable Stepper Driver
A3981
16
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Stall Detection Scheme: Compare Each Phase
In some motors the winding differences can cause false stall
detection. This can be overcome by changing the comparison cir­
cuits to operate on each phase independently. The compare each
phase scheme is selected when STS[1..0] = 01.
When this scheme is selected, two PWM counters, one for each
phase, accumulate the number of PWM cycles when the phase
current is stepped from zero to full-scale current. At the end of
each phase current rise, the counter for that phase is compared to
the count result for the previous current rise in the same phase,
as shown in Figure 5. If the difference is greater than the PWM
count difference in the CONFIG1 register (CD[7:0]), then the ST
bit in the diagnostic registers is set. In addition, if the ST signal is
selected as the output on the DIAG pin, then the pin will go low.
In addition to using the integrated features of the A3981, it is
also possible to perform stall detection by examining the PWM
on-time for a single phase using an external microcontroller. In
the A3981 the PWM-on signal for phase A can be selected as the
output on the DIAG pin by using the serial interface.
Figure 5: Stall Detect by PWM Count Comparing Each Phase Independently, STS[1..0] = 01

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## Page 17

Automotive Programmable Stepper Driver
A3981
17
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
SERIAL INTERFACE DESCRIPTION
A three wire synchronous serial interface, compatible with
SPI, can be used to configure and control all the features of the
A3981. A fourth wire can be used to provide diagnostic feedback.
The registers that are accessible through the serial interface are
defined in Table 2.
The A3981 can be operated without using the serial interface,
by using the default configuration and control register settings
and the STEP and DIR logic inputs for motor control. However,
application-specific configurations are only possible by setting
the appropriate register bits through the serial interface. In addi­
tion to setting the configuration bits, the serial interface can also
be used to control the motor directly.
The serial interface timing requirements are specified in the Elec­
trical Characteristics table, and illustrated in Figure 1.
Writing to Configuration and Control Regis­
ters
When writing to the serial register, data is received on the SDI
pin and clocked through a shift register on the rising edge of the
clock signal input on the SCK pin. STRn is normally held high,
and is only brought low to initiate a serial transfer. No data is
clocked through the shift register when STRn is high, thus allow­
ing multiple SDI slave units to use common SDI, SCK, and SDO
connections. Each independent slave requires a dedicated STRn
connection.
The serial data word has 16 bits, MSB input first. After 16 data
bits have been clocked into the shift register, STRn must be taken
high to latch the data into the selected register. When this occurs,
the internal control circuits act on the new configuration and
control data, and the diagnostic registers are reset.
Table 2. Serial Register Definition\*
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Configuration and Control Registers (Write)
Configuration
Register 0
(CONFIG0)
0
0
SYR
MS1
MS0
MXI1
MXI0
PFD2
PFD1
PFD0
TBK1
TBK0
TOF2
TOF1
TOF0
PWM
FRQ2
FRQ1
FRQ0
1
0
0
1
1
1
0
0
0
1
1
1
0
0
Configuration
Register 1
(CONFIG1)
0
1
OSC
TSC1
TSC0
CD7
CD6
CD5
CD4
CD3
CD2
CD1
CD0
DIAG1
DIAG0
0
1
0
0
0
0
0
0
1
0
0
0
0
0
Run Register
(RUN)
1
0
EN
OL1
OL0
HLR
SLEW
BRK
DCY1
DCY0
SC5
SC4
SC3
SC2
SC1
SC0
0
0
1
0
1
0
0
1
0
0
0
0
0
0
Table Load
Register
(TBLLD)
1
1
STS1
STS1
PTP
PT5
PT4
PT3
PT2
PT1
PT0
0
0
0
0
0
0
0
1
0
0
0
1
0
1
Diagnostic Registers (Read)
Fault Register
0 (FAULT0)
FF
TW1
TW0
OV
UV
ST
OLB
OLA
BML
BMH
BPL
BPH
AML
AMH
APL
APH
Fault
Register 1
(FAULT1)
FF
TW1
TW0
OV
UV
ST
OLB
OLA
0
0
SA5
SA4
SA3
SA2
SA1
SA0
\*Power-on reset value shown below each input register bit.

![Image 1 from page 17](pdf-image://page_17_img_1)

## Page 18

Automotive Programmable Stepper Driver
A3981
18
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
If there are more than 16 rising edges on SCK, or if STRn goes
high and there are fewer than 16 rising edges on SCK, the write
will be cancelled without writing data to the configuration and
control registers. In addition the diagnostic registers will not be
reset. Instead the FF bit will be set to 1 in the diagnostic registers,
to indicate a data transfer error.
The first two bits of the serial word are used to select the register
to be written. This provides access to four writable registers:
•	 The Configuration registers are used for system configuration:
CONFIG0 for system parameters, and CONFIG1 for system
and diagnostic parameters.
•	 The RUN register contains motor drive settings used to control
the motor movement and phase current.
•	 The fourth writable register, TBLLD, is used for diagnostic
configuration and to program the phase current table.
Reading from Diagnostic Registers
In addition to the writable registers there are two diagnostic
registers. The first eight (most significant) bits of both diagnostic
registers contain the same flags, only the last eight (least signifi­
cant) bits differ, as follows:
•	 FAULT0 contains the short-circuit fault flags
•	 FAULT1 contains the present Step Angle Number
Each time a configuration and control register is written, one
of the diagnostic registers can be read, MSB first, on the serial
output pin, SDO (see timing in Figure 1). FAULT1 is made the
active register for serial transfer and output on SDO only while
CONFIG1 is being written, that is, only when the first bit of the
input word is 0 and the second bit is 1. FAULT0 is the active
register for serial transfer and output on SDO during writes to any
other configuration or control register.
When STRn goes low to start a serial write, SDO comes out of its
high impedance state and outputs the serial register Fault Register
flag. This allows the main controller to poll the A3981 through
the serial interface to determine if a fault has been detected. If no
faults have been detected then the serial transfer may be termi­
nated without generating a serial read fault by ensuring that SCK
remains high while STRn is low. When STRn goes high the trans­
fer will be terminated and SDO will go into its high impedance
state. Configuration and Run Registers
These registers are used for system configuration and motor con­
trol. Access is described in the section Writing to Configuration
and Control Registers, above.
CONFIG0 sets certain system parameters, and CONFIG1 sets
system and diagnostic output selection parameters. The RUN
register contains motor drive settings used to control the motor
movement and phase current.
Phase Table Load Register
This is one of the configuration and control registers, accessed
when both address bits are 1, and can be used to write a sequence
of values to the phase current table in the A3981. This allows the
current at each Step Angle Number to be tailored to suit the mic­
rostep current profile requirements of a specific motor. In most
cases this feature will not be required and the default sinusoidal
profile will suffice. However for some motor / load combinations,
altering the current profile can improve torque ripple, resulting in
lower mechanical vibration and noise.
Although the phase current table contains 64 entries for each of
two phases, only 16 distinct values are required. These 16 values
correspond to one quadrant of the table for a single phase, and
they are repeated for the other three quadrants and again for the
four quadrants of the other phase. So each of the 16 values writ­
ten to the Phase Table Load register are written to 8 locations in
the phase current table.
The 16 values must be entered by sequential writes to the Phase
Table Load register. The first write to the register after writing to
any other register, or after a reset (RESETn pulse low or power-
on), puts that value, PT[5..0], into the first phase table address,
a 6-bit field defined as PT(0). Subsequent writes put values into
successive addresses: PT(1), PT(2), and so forth up to PT(15).
After the sixteenth value has been written, no more values are
accepted and any writes to the Phase Table Load register are
ignored. As each value is received, it is effectively distributed to
all eight required locations in the phase current table.
An optional simple odd parity scheme is included to provide
some measure of error checking, if required. Each 6-bit value
can be supplemented with an additional parity bit, PTP, to ensure
an odd number of 1s in the transmission. This is checked by the
A3981 and if a the number of 1s in the value plus parity bit is not
odd, the FF bit will be set and the SDO pin will go high the next
time STRn is taken low, indicating a parity error. That data will

![Image 1 from page 18](pdf-image://page_18_img_1)

## Page 19

Automotive Programmable Stepper Driver
A3981
19
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
still be written to the next phase table value address; it is incum­
bent upon the external controller to take action, if required.
If the write sequence is broken (by a reset, by writing to another
register, or by a data transfer error) before the sequence has been
completed, then the phase table value address will be reset to
PT(0). If it is required to load the table, then the entire 16-value
sequence must be sent.
After loading, although the phase current table is volatile, a reset
using a low pulse on the RESETn pin does not corrupt the table.
The table is only reset to default values on a power-on reset.
The Phase Table Load register also contains the diagnostic
parameter used to select the stall detection scheme, STS[1..0].
When writing to the Phase Table Load register to set the
STS[1..0] bits, the remaining bits in the serial transfer, PT[5..0],
must match the phase table value for the first phase table
address, PT(0). Before re-writing the STS[1..0] bits, a write to
another register is required to ensure that the phase table value
address is reset to PT(0).
Diagnostic Registers
The diagnostic registers comprise two read-only fault data regis­
ters. Access is described in the section Reading from Diagnostic
Registers, above.
The diagnostic registers contain fault flags for each fault condi­
tion and are reset to all 0s on the completion of each serial access.
They are also reset to all 0s each time the RESETn input is low
for longer than the Reset Pulse Width, tRST . FAULT0 is set to
all 1s at power-up or after a power-on reset. This indicates to the
external controller that a power-on reset has taken place and all
registers have been reset. Note that a power-on reset occurs when
power is first applied or the logic supply, VDD , drops below the
VDD Power-On Reset Threshold, VDDPOR .
Power-on reset function is not affected by the state of the motor
supply or VREG .
The first bit in both registers is the Fault Register flag, FF. This
is high if any bits in FAULT0 are set, or if a serial write error or
parity error has occurred.

![Image 1 from page 19](pdf-image://page_19_img_1)

## Page 20

Automotive Programmable Stepper Driver
A3981
20
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
CONFIG 0
0
0
SYR
MS1
MS0
MXI1
MXI0
PFD2
PFD1
PFD0
TBK1
TBK0
TOF2
FRQ2
TOF1
FRQ1
TOF0
FRQ0
PWM
1
0
0
1
1
1
0
0
0
1
1
1
0
0
Configuration Register 0
PWM
PWM configuration
PWM
MODE
Default
0
Fixed off-time
D
1
Fixed frequency
TOF[2..0]
Off time (only valid when PWM bit = 0) Re­
places FRQ bits
Assumes 4-MHz clock
TOF2
TOF1
TOF0
Off Time
Default
0
0
0
20 µs
0
0
1
24 µs
0
1
0
28 µs
0
1
1
32 µs
1
0
0
36 µs
1
0
1
40 µs
1
1
0
44 µs
D
1
1
1
48 µs
FRQ[2..0]
Frequency (only valid when PWM bit = 1)
Replace TOF bits
Assumes 4-MHz clock
FRQ2
FRQ1
FRQ0
Period / Frequency
Default
0
0
0
24 µs / 41.7 kHz
0
0
1
32 µs / 31.3 kHz
0
1
0
40 µs / 25.0 kHz
0
1
1
46 µs / 21.7 kHz
1
0
0
52 µs / 19.2 kHz
1
0
1
56 µs / 17.9 kHz
1
1
0
60 µs / 16.7 kHz
D
1
1
1
64 µs / 15.6 kHz
PFD[2..0]
Fast decay time for mixed decay
Assumes 4-MHz clock
PFD2
PFD1
PFD0
Fast Decay Time
Default
0
0
0
2 µs
0
0
1
3 µs
0
1
0
4 µs
0
1
1
6 µs
1
0
0
8 µs
D
1
0
1
10 µs
1
1
0
14 µs
1
1
1
20 µs
MXI[1..0]
Max phase current as a percentage of ISMAX
MXI1
MXI0
Maximum Current
Default
0
0
25%
0
1
50%
1
0
75%
1
1
100%
D
MS[1..0]
Microstep mode for external STEP input control
MS1
MS0
Microstep Mode
Default
0
0
Full Step
D
0
1
Half Step
1
0
Quarter Step
1
1
Sixteenth Step
TBK[1..0]
Blank Time
Assumes 4-MHz clock
TBK1
TBK0
Blank Time
Default
0
0
1 µs
0
1
1.5 µs
D
1
0
2.5 µs
1
1
3.5 µs
SYR
Synchronous rectification
SYR
Synchronous Rectification
Default
0
Diode recirculation
1
Synchronous
D

![Image 1 from page 20](pdf-image://page_20_img_1)

## Page 21

Automotive Programmable Stepper Driver
A3981
21
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
CONFIG 1
0
1
OSC
TSC1
TSC0
CD7
CD6
CD5
CD4
CD3
CD2
CD1
CD0
DIAG1 DIAG0
0
1
0
0
0
0
0
0
1
0
0
0
0
0
RUN
1
0
EN
OL1
OL0
HLR
SLEW
BRK
DCY1
DCY0
SC5
SC4
SC3
SC2
SC1
SC0
0
0
1
0
1
0
0
1
0
0
0
0
0
0
Configuration Register 1
Run Register
SC[5..0]
Step change number
2’s complement format
Positive value increases Step Angle Number
Negative value decreases Step Angle Number
DCY[1..0] Decay mode selection
DCY1
DCY0
Decay Mode
Default
0
0
Slow
0
1
Mixed—PFD fixed
D
1
0
Mixed—PFD auto
1
1
Fast
DIAG[1..0] Selects signal routed to DIAG output
DIAG1 DIAG0
Signal on DIAG Pin
Default
0
0
Fault–low true
D
0
1
ST–low true
1
0
PWM-on, Phase A
1
1
Temperature
TSC[1..0]
Overcurrent fault delay
Assumes 4-MHz clock
TSC1
TSC0
Detect Delay Time
Default
0
0
0.5 µs
0
1
1 µs
1
0
2 µs
D
1
1
3 µs
OL[1..0]
Open  load current threshold as a percentage of
maximum current defined by ISMAX and MXI[1..0]
OL1
OL0
Open Load Current
Default
0
0
20%
0
1
30%
D
1
0
40%
1
1
50%
CD[7..0]
PWM count difference for ST detection
Default to 8
OSC
Selects clock source
OSC
Clock Source
Default
0
Internal
D
1
External
BRK
Brake enable
BRK
Brake
Default
0
Normal operation
D
1
Brake active
SLEW
Slew rate control
SLEW
Slew Rate Control
Default
0
Disable
1
Enable
D
HLR
Selects slow decay and brake recirculation path
HLR
Recirculation Path
Default
0
High side
D
1
Low side
EN
Phase current enable
OR with ENABLE pin
EN
Phase Current Enable
Default
0
Output bridges disabled if ENABLE
pin = 0
D
1
Output bridges enabled

![Image 1 from page 21](pdf-image://page_21_img_1)

## Page 22

Automotive Programmable Stepper Driver
A3981
22
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
FF
Fault register flag
TW1
Temperature diagnostic
TW0
Temperature diagnostic
OV
Overvoltage on VBB detected
UV
Undervoltage on VREG
ST
Stall detected
OLB
Open load detected on phase B
OLA
Open load detected on phase A
BML
Overcurrent detected on BM output low side
BMH
Overcurrent detected on BM output high side
BPL
Overcurrent detected on BP output low side
BPH
Overcurrent detected on BP output high side
AML
Overcurrent detected on AM output low side
AMH
Overcurrent detected on AM output high side
APL
Overcurrent detected on AP output low side
APH
Overcurrent detected on AP output high side
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
TBLLD
1
1
STS1
STS0
PTP
PT5
PT4
PT3
PT2
PT1
PT0
0
0
0
0
0
0
0
1
0
0
0
1
0
1
Fault 0
FF
TW1
TW0
OV
UV
ST
OLB
OLA
BML
BMH
BPL
BPH
AML
AMH
APL
APH
Fault 1
FF
TW1
TW0
OV
UV
ST
OLB
OLA
0
0
SA5
SA4
SA3
SA2
SA1
SA0
Table Load Register
Fault Register 0
Fault Register 1
Table Load Register Mapping
Step Angle Number
Phase A
Phase B
0%
0
32
16
48
PT(0)
1
31
33
63
15
17
47
49
PT(1)
2
30
34
62
14
18
46
50
PT(2)
3
29
35
61
13
19
45
51
PT(3)
4
28
36
60
12
20
44
52
PT(4)
5
27
37
59
11
21
43
53
PT(5)
6
26
38
58
10
22
42
54
PT(6)
7
25
39
57
9
23
41
55
PT(7)
8
24
40
56
8
24
40
56
PT(8)
9
23
41
55
7
25
39
57
PT(9)
10
22
42
54
6
26
38
58
PT(10)
11
21
43
53
5
27
37
59
PT(11)
12
20
44
52
4
28
36
60
PT(12)
13
19
45
51
3
29
35
61
PT(13)
14
18
46
50
2
30
34
62
PT(14)
15
17
47
49
1
31
33
63
PT(15)
16
48
0
32
FF
Fault register flag
TW1
Temperature diagnostic
TW0
Temperature diagnostic
OV
Overvoltage on VBB detected
UV
Undervoltage on VREG
ST
Stall detected
OLB
Open load detected on phase B
OLA
Open load detected on phase A
SA[5..0]
Step Angle Number read back
TW[1..0]
Temperature diagnostic
TW1
TW0
Thermal Indicator
0
0
No Fault
0
1
Cold Warning
1
0
Hot Warning
1
1
Overtemperature Shutdown
PTP
Parity bit (odd parity)
PT(0..15)[5..0] Phase Table Value
STS[1..0]
Selects stall detection scheme
STS1
STS0
Stall Detection Scheme
Default
0
0
Compare opposite phases
D
0
1
Compare each phase
1
0
Reserved
1
1
Disable stall detection

![Image 1 from page 22](pdf-image://page_22_img_1)

## Page 23

Automotive Programmable Stepper Driver
A3981
23
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Motor Movement Control
The A3981 provides two independent methods to control the
movement of a stepper motor. The simpler is the Step and Direc­
tion method, which only requires two control signals to control
the stepper motor in either direction. The other method is through
the serial interface, which provides more flexible control capa­
bility. Both methods can be used together (although it is not
common), provided the timing restrictions of the STEP input in
relation to the STRn input are preserved.
PHASE TABLE AND PHASE DIAGRAM
The key to understanding both of the available control methods
lies in understanding the Phase Current table (Table 7). This table
contains the relative phase current magnitude and direction for
each of the two motor phases at each microstep position. The
maximum resolution of the A3981 is one-sixteenth microstep.
That is 16 microsteps per full step. There are 4 full steps per elec­
trical cycle, so the phase current table has 64 microstep entries.
The entries are numbered from 0 to 63. This number represents
the phase angle within the full 360° electrical cycle and is called
the Step Angle Number. This is illustrated in Figure 6.
Figure 6 shows the contents of the phase current table as a phase
diagram. The phase B current, IB, from the phase current table, is
plotted on horizontal axis and the phase A current, IA, is plotted
on the vertical axis. The resultant motor current at each microstep
is shown as numbered radial arrows. The number shown corre­
sponds to the one-sixteenth microstep Step Angle Number in the
phase current table.
Figure 7 shows an example of calculating the resultant motor
current magnitude and angle for step number 28. The target is to
have the magnitude of the resultant motor current be 100% at all
microstep positions. The relative phase currents from the phase
current table are:
IA = 37.50%
IB = –92.19%
Assuming a full scale (100%) current of 1A means that the two
phase currents are:
IA = 0.3750 A
IB = -0.9219 A
The magnitude of the resultant will be the square root of the sum
of the squares of these two currents:
9953 (A)
.0
8499
.0
1406
.0
|
|
2
2
28
=
+
=
+
=
B
A
I
I
I
APPLICATION INFORMATION
IA
IB
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
23
22
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45 46 47
48 49
50 51 52
53
54
55
56
57
58
59
60
61
62
63
Figure 6: A3981 Phase Current Table as a Phase
Diagram
Values shown are referred to as the Step Angle Number.
Figure 7: Calculation of Resultant Motor Current
IA
24
28
32
IB
IA28
IB28= –92.19%
α28=
157.9°
=37.5%
31
30
29
27
26
25

![Image 1 from page 23](pdf-image://page_23_img_1)

## Page 24

Automotive Programmable Stepper Driver
A3981
24
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
So the resultant current magnitude is 99.53% of full scale. This
is within 0.5% of the target (100%) and is well within the ±5%
accuracy of the A3981.
The reference angle, zero degrees (0°), within the full electrical
cycle (360°), is defined as the angle where IB is at +100% and IA
is zero. Each full step is represented by 90° in the electrical cycle
so each one-sixteenth microstep is: 90°/16 steps = 5.625°. The
target angle of each microstep position with the electrical cycle
is determined by the product of the Step Angle Number and the
angle for a single microstep. So for the example of figure 7:
°
=
°
×
=
5.
157
625
.5
28
)
(
28 TARGET
α
The actual angle is calculated using basic trigonometry as:






+
=
−
28
28
1
)
(
28
tan
180
B
A
ACTUAL
I
I
α
(
)
°
=
−
+
=
9.
157
1.
22
180
So the angle error is only 0.4°. Equivalent to about 0.1% error in
360° and well within the current accuracy of the A3981.
Note that each phase current in the A3981 is defined by a 6-bit
DAC. This means that the smallest resolution of the DAC is
100 / 64 = 1.56% of the full scale, so the A3981 cannot produce
a resultant motor current of exactly 100% at each microstep. Nor
can it produce an exact microstep angle. However, as can be seen
from the calculations above, the results for both are well within
the specified accuracy of the A3981 current control. The resultant
motor current angle and magnitude are also more than precise
enough for all but the highest precision stepper motors.
With the phase current table, control of a stepper motor is simply
a matter of increasing or decreasing the Step Angle Number
to move around the phase diagram of Figure 7. This can be in
predefined multiples using the STEP input, or it can be variable
using the serial interface.
USING STEP AND DIRECTION CONTROL
The STEP input moves the motor at the microstep resolution
defined by the two microstep select variables, MS0 and MS1,
logic levels. The DIR input defines the motor direction. These
inputs define the output of a translator which determines the
required Step Angle Number in the phase current table. The MS0
and MS1 can be set to select full step, half step, quarter step, or
sixteenth step microstepping as follows:
MS1
MS0
Microstep Mode
0
0
Full step
0
1
Half step
1
0
Quarter step
1
1
Sixteenth step
MS0 and MS1 can be accessed through the serial interface or
directly on pins 13 and 12 respectively. The values of MS0 and
MS1 are defined as the logical OR of the logic level on the input
pins and the value in Configuration Register 0. The bits in the
register default to 0 so if the serial interface is not used then MS0
and MS1 are defined by the input pins alone. If only the serial
interface is used to set the microstep resolution, then the MS0 and
MS1 logic input pins should be tied low to ensure that the register
retains full control over all resolutions. Note that the microstep
select variables, MS0 and MS1, are only used with the STEP
input; they can be ignored if the motor is fully controlled through
the serial interface.
In sixteenth step mode the translator simply increases or
decreases the Step Angle Number on each rising edge of the
STEP input, depending on the logic state of the DIR input. In the
other three microstep resolution modes the translator outputs spe­
cific Step Angle Numbers as defined in the phase current table.
Full step uses four of the entries in the phase current table. These
are 8, 24, 40, and 56 as shown in Figure 8. Note that the four
positions selected for full step are not the points at which only
one current is active, as would be the case in a simple on-off full
step driver. There are two advantages in using these positions
rather than the single full current positions. With both phases
active, the power dissipation is shared between two drivers. This
slightly improves the ability to dissipate the heat generated and
reduces the stress on each driver.
The second reason is that the holding torque is slightly improved
because the forces holding the motor are mainly rotational rather
than mainly radial.
Half step uses eight of the entries in the phase current table.
These are 0, 8, 16, 24, 32, 40, 48, and 56 as shown in Figure 9.
Quarter step uses sixteen of the entries in the phase current table.
These are 0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56,
and 60 as shown in Figure 10.
In half step and in quarter step, the single phase active positions
are used to preserve symmetry. However, if the motor is required

![Image 1 from page 24](pdf-image://page_24_img_1)

## Page 25

Automotive Programmable Stepper Driver
A3981
25
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
to stop with a significant holding torque for any length of time
it is recommended that the 45° positions be used; those are Step
Angle Numbers 8, 24, 40, and 56, as used with full-step resolu­
tion.
The following table summarizes the Step Angle Numbers used
for the four resolutions available when using the STEP input to
control the output of the A3981:
Mode
Step Angle Numbers used
Full
8, 24, 40, 56
Half
0, 8, 16, 24, 32, 40, 48, 56
Quarter
0, 4, 8, 12, 16, 20, 24, 28, 32,
36, 40, 44, 48, 52, 56, 60
Sixteenth
All
The microstep select inputs can be changed between each rising
edge of the STEP input. The only restriction is that the MSO and
MS1 logic inputs must comply with the set-up and hold timing
constraints. When the microstep resolution changes, the A3981
moves to the next available Step Angle Number on the next rising
edge of the STEP input. For example, if the microstep mode is
sixteenth and the present Step Angle Number is 59, then with the
direction forwards (increasing Step Angle Number), changing
to quarter step mode will cause the phase number to go to 60 on
the next rising edge of the STEP input. If instead the microstep
mode is changed to half step then the phase number will go to 0
on the next rising edge of the STEP input. If the microstep mode
is changed to full step then the phase number will go to 8 on the
next rising edge of the STEP input.
CONTROL THROUGH THE SERIAL INTERFACE
The A3981 provides the ability to directly control the motor
movement using only the serial interface. In fact, all features
of the A3981, except sleep mode, can be controlled through the
serial interface thus removing the requirement for individual
control inputs. This can reduce the interface requirement from
multiple I/O signals to a single four wire interface.
Motor movement is controlled using the serial interface by
increasing or decreasing the Step Angle Number. Note that the
maximum value of the Step Angle Number is 63 and the mini­
mum number is 0.Therefore, any increase or decrease in the
microstep number is performed using modulo 64 arithmetic. This
means that increasing a Step Angle Number of 63 by 1 will pro­
duce a Step Angle Number of 0. Increasing by two from 63 will
produce 1 and so on. Similarly in the reverse direction, decreasing
a Step Angle Number of 0 by 1 will produce a Step Angle Num­
ber of 63. Decreasing by two from 0 will produce 62 and so on.
The least significant six bits of the Run register, bits 0 to 5, are
the step change number, SC[5..0]. This number is a two’s comple­
ment number that is added to the Step Angle Number causing it
to increase or decrease. Two’s complement is the natural integer
number system for most microcontrollers. This allows standard
arithmetic operators to be used, within the microcontroller, to
determine the size of the next step increment. Table 6 shows the
IA
0
4
8
12
16
20
24
28
IB
32
36
40
44
48
52
56
60
0
8
16
24
32
40
48
56
IB
IA
8
24
40
56
IB
IA
Figure 8: Full-step Phase Diagram Using STEP Input
Figure 9: Half-step Phase Diagram Using STEP Input
Figure 10: Quarter-step Phase Diagram Using STEP
Input

![Image 1 from page 25](pdf-image://page_25_img_1)

## Page 26

Automotive Programmable Stepper Driver
A3981
26
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
binary equivalent of each decimal number between –16 and  +16.
Each increase in the Step Angle Number represents a forwards
movement of one-sixteenth microstep. Each decrease in the Step
Angle Number represents a reverse movement of one-sixteenth
microstep.
To move the motor one full step, the Step Angle Number must be
increased or decreased by 16. To move the motor one half step,
the Step Angle Number must be increased or decreased by 8. For
quarter step the increase or decrease is 4 and for eighth step, 2.
So, for example, to continuously move the motor forwards in
quarter-step increments, the number 4 (000100) is repeatedly
written to SC[5..0] through the serial interface Run register (see
Figure 11). To move the motor backwards in quarter step incre­
ments, the number -4 (111100) is repeatedly written to SC[5..0]
(see Figure 12). The remaining bits in the Run register should be
set for the required configuration and sent with the step change
number each time.
The step rate is controlled by the timing of the serial interface.
It is the inverse of the step time, tSTEP , shown in Figure 11. The
motor step only takes place when the STRn goes from low to
high when writing to the Run register. The motor step rate is
therefore determined by the timing of the rising edge of the STRn
input. The clock rate of the serial interface, defined by the fre­
quency of the SCK input, has no effect on the step rate.
Table 6. Binary Equivalents
Decimal
2’s Complement
Decimal
2’s Complement
0
000000
1
000001
–1
111111
2
000010
–2
111110
3
000011
–3
111101
4
000100
–4
111100
5
000101
–5
111011
6
000110
–6
111010
7
000111
–7
111001
8
001000
–8
111000
9
001001
–9
110111
10
001010
–10
110110
11
001011
–11
110101
12
001100
–12
110100
13
001101
–13
110011
14
001110
–14
110010
15
001111
–15
110001
16
010000
–16
110000
SDI
SCK
STRn
1 0 1 0
1 0 1 0
1 0 1 1 1 1 0 0
-4
SDI
SCK
STRn
1 0 1 0
1 0 1 0
1 0 0 0 0 1 0 0
+4
tSTEP
Figure 11: Serial Interface Sequence for Quarter Step in Forward Direction
Figure 12: Serial Interface Sequence for Quarter Step in Reverse Direction

![Image 1 from page 26](pdf-image://page_26_img_1)

## Page 27

Automotive Programmable Stepper Driver
A3981
27
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Using the Phase Table Load Capability
TORQUE RIPPLE REDUCTION
The performance and audible noise of any motor drive system is
defined, to a large extent, by the torque ripple generated by both
the motor and the load. In most cases, when using a stepper motor
as the mechanical drive, the torque ripple of the load is not related
to the mechanical steps of the motor and must be reduced by
means unrelated to the motor and its drive system. However, for
stepper motors in particular, torque ripple produced by the motor
can be reduced by improvements in the mechanical design of the
motor and by improvements in the phase current control system.
Torque ripple will naturally be high when driving a stepper motor
in full step mode, due to the nature of stepping. However the
torque ripple can be reduced by using microstepping. Increas­
ing the number of microsteps per mechanical step will result in
reduced torque ripple. This is one of the major reasons for using
microstepping.
In the majority of cases the standard sinusoidal, microstep current
profile will be sufficient to achieve a good performance with
a good quality motor. In a few cases, further improvements in
torque ripple performance may be achieved by modifying the
microstep current profile to more closely match the motor charac­
teristics. This is usually only necessary for higher quality, higher
power stepper motors.
When using microstepping, the torque ripple is defined by the
variation in torque at each microstep. In a hybrid stepper motor
this is mostly determined by the mechanical construction of the
motor, particularly the shape of the teeth on the poles of the sta­
tor. The shape of these teeth determine the variation in the torque
constant, the ratio between current and torque, as the motor
rotates. The variation in the torque constant can be seen by mea­
suring the back EMF of the motor when being driven as a genera­
tor, that is when the shaft is driven by external means and the
phase voltage is monitored. The back EMF represents the motor
constant, which is essentially proportional to the torque constant.
If such torque ripple reduction measures are required, the A3981
provides the ability to modify the microstep current profile by
programming the internal phase current table through the serial
interface. The modified profile is then used, in place of the default
sinusoidal profile, to compensate for any variation in motor torque
constant. The current at each Step Angle Number can be set to suit
the microstep current profile requirements of a specific motor.
Note: This is an advanced feature of the A3981, which will not be
required for most applications. In general the default sinusoidal
profile will suffice and therefore the phase current table does not
have to be loaded.
LOADING PHASE CURRENT TABLE
The full phase current table in the A3981 contains one 6-bit value
for each phase, at each microstep position. With 16 microsteps
per mechanical step, 4 mechanical steps per electrical cycle, and
2 phases this gives a total of 128 values. However, due to symme­
try, described below,  this reduces to 17 independent values, one
of which is always zero. The remaining 16 values can be loaded
sequentially through the serial interface using the Phase Table
Load register. Figure 13 shows the default phase table values
plotted by Step Angle Number. Similar information is provided in
Table 7.
The diagram in Figure 13 is marked with four quadrants, Q1 to
Q4. The set of phase table values is the same in each quadrant in
each phase. Consider phase A (bottom graph), quadrant 1 (Q1).
This contains Step Angle Numbers 0 to 15. The default values
in these 16 positions are selected to produce one quarter of a
sinusoid.
Now consider the next quadrant (Q2) of phase A. The sequence
of values in this quadrant form a mirror image, by Step Angle
Number, of the values in Q1 so the same values are used but
entered in the reverse sequence.
The following table shows the Step Angle Number in the first
row increasing from 0 to 15, from left to right, and the default
values also increasing from left to right in the second row. These
first two rows are the entries for Q1 of phase A.
Step
0
1
2
3
4
5
6
7
8
9
10
11
12 13 14 15
Q1
Value
0
5
11
18 23 29 35 40 44 48 52 55 58 60 62 63
Step
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Q2
Value
5
11
18 23 29 35 40 44 48 52 55 58 60 62 63 63
The second two rows are the entries for Q2 of phase A. The Step
Angle Number in the third row increases from 16 to 31, this time
from right to left, but the same default values still increase from
left to right. A single value is therefore placed in more than one
location in the table. Shown outlined above, steps 4 and 28 both
contain the value 23.
The same principal can be applied to Q3 and Q4 of phase A. In
this case the mirror image is in the horizontal axis, about the zero
reference value. Although the current in Q3 and Q4 for phase A is
effectively negative, the negation is provided by controlling the

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## Page 28

Automotive Programmable Stepper Driver
A3981
28
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
(DAC value)
0
2
3
4
5
6
7
8
9
14
15
16
17
18
19
20
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
1
21
11
10
13
12
48
Q1
Q2
Q3
Q4
Q1
Q2
Q3
Q4
IB
(forwards)
63
62
58
52
44
35
23
11
11
23
35
44
52
58
62
63
5
18
29
40
48
55
60
5
18
29
40
48
55
60
63
62
58
52
44
35
23
11
11
23
35
44
52
58
62
63
5
18
29
40
48
55
60
5
18
29
40
48
55
60
IB
(reverse)
IA
(forwards)
IA
(reverse)
Figure 13. Default Phase Table Values
Step Angle Number
(DAC value)
(DAC value)
(DAC value)

![Image 1 from page 28](pdf-image://page_28_img_1)

## Page 29

Automotive Programmable Stepper Driver
A3981
29
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Table 7: Phase Current Table (default, power-on content)
Step Angle Number
Phase Cur­
rent
(% of IPMAX)
Step
Angle
Phase
DAC
Step Angle Number
Phase Current
(% of IPMAX)
Step
Angle
Phase
DAC
Full
1/2
1/4
1/8
1/16
A
B
A
B
A
B
Full
1/2
1/4
1/8
1/16
A
B
A
B
A
B
0
0
0
0
0.00 100.00
0.0
0
0
0
63
4
8
16
32
0.00
-100.00 180.0
0
1
0
63
1
9.38 100.00
5.4
0
0
5
63
33
-9.38
-100.00 185.4
1
1
5
63
1
2
18.75 98.44
10.8
0
0
11
62
17
34
-18.75
-98.44 190.8
1
1
11
62
3
29.69 95.31
17.3
0
0
18
60
35
-29.69
-95.31 197.3
1
1
18
60
1
2
4
37.50 92.19
22.1
0
0
23
58
9
18
36
-37.50
-92.19 202.1
1
1
23
58
5
46.88 87.50
28.2
0
0
29
55
37
-46.88
-87.50 208.2
1
1
29
55
3
6
56.25 82.81
34.2
0
0
35
52
19
38
-56.25
-82.81 214.2
1
1
35
52
7
64.06 76.56
39.9
0
0
40
48
39
-64.06
-76.56 219.9
1
1
40
48
0
1
2
4
8
70.31 70.31
45.0
0
0
44
44
2
5
10
20
40
-70.31
-70.31 225.0
1
1
44
44
9
76.56 64.06
50.1
0
0
48
40
41
-76.56
-64.06 230.1
1
1
48
40
5
10
82.81 56.25
55.8
0
0
52
35
21
42
-82.81
-56.25 235.8
1
1
52
35
11
87.50 46.88
61.8
0
0
55
29
43
-87.50
-46.88 241.8
1
1
55
29
3
6
12
92.19 37.50
67.9
0
0
58
23
11
22
44
-92.19
-37.50 247.9
1
1
58
23
13
95.31 29.69
72.7
0
0
60
18
45
-95.31
-29.69 252.7
1
1
60
18
7
14
98.44 18.75
79.2
0
0
62
11
23
46
-98.44
-18.75 259.2
1
1
62
11
15
100.00 9.38
84.6
0
0
63
5
47
-100.00
-9.38
264.6
1
1
63
5
2
4
8
16
100.00 0.00
90.0
0
0
63
0
6
12
24
48
-100.00
0.00
270.0
1
1
63
0
17
100.00 -9.38
95.4
0
1
63
5
49
-100.00
9.38
275.4
1
0
63
5
9
18
98.44 -18.75 100.8
0
1
62
11
25
50
-98.44
18.75
280.8
1
0
62
11
19
95.31 -29.69 107.3
0
1
60
18
51
-95.31
29.69
287.3
1
0
60
18
5
10
20
92.19 -37.50 112.1
0
1
58
23
13
26
52
-92.19
37.50
292.1
1
0
58
23
21
87.50 -46.88 118.2
0
1
55
29
53
-87.50
46.88
298.2
1
0
55
29
11
22
82.81 -56.25 124.2
0
1
52
35
27
54
-82.81
56.25
304.2
1
0
52
35
23
76.56 -64.06 129.9
0
1
48
40
55
-76.56
64.06
309.9
1
0
48
40
1
3
6
12
24
70.31 -70.31 135.0
0
1
44
44
3
7
14
28
56
-70.31
70.31
315.0
1
0
44
44
25
64.06 -76.56 140.1
0
1
40
48
57
-64.06
76.56
320.1
1
0
40
48
13
26
56.25 -82.81 145.8
0
1
35
52
29
58
-56.25
82.81
325.8
1
0
35
52
27
46.88 -87.50 151.8
0
1
29
55
59
-46.88
87.50
331.8
1
0
29
55
7
14
28
37.50 -92.19 157.9
0
1
23
58
15
30
60
-37.50
92.19
337.9
1
0
23
58
29
29.69 -95.31 162.7
0
1
18
60
61
-29.69
95.31
342.7
1
0
18
60
15
30
18.75 -98.44 169.2
0
1
11
62
31
62
-18.75
98.44
349.2
1
0
11
62
31
9.38 -100.00 174.6
0
1
5
63
63
-9.38
100.00 354.6
1
0
5
63
4
8
16
32
0.00 -100.00 180.0
0
1
0
63
0
0
0
0
0.00
100.00
0.0
0
0
0
63
direction of the current. The current control scheme still operates
using positive values.
As shown below, the table of values can be extended to include
Q3 and Q4 with the current direction indicated in the last column.
Note that the same value is now applied to four locations in the
full 360-degree electrical cycle.
Step
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Q1 FWD
Value
0
5
11 18 23 29 35 40 44 48 52 55 58 60 62 63
Step
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Q2 FWD
Value
5
11 18 23 29 35 40 44 48 52 55 58 60 62 63 63
Step
32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
Q3 Rev
Value
0
5
11 18 23 29 35 40 44 48 52 55 58 60 62 63
Step
63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48
Q4 Rev
Value
5
11 18 23 29 35 40 44 48 52 55 58 60 62 63 63
Shown outlined above, steps 4, 28, 36, and 60 all contain the
value 23.
The other phase, phase B, uses the same values as phase A but
shifted back by 16 Step Angle Numbers. The full distribution of
the value entered in step 4 of phase A is highlighted in Figure 13
(and shown in Table 7). This single value is used in a total of
eight locations. The same distribution of values applies to all the
values in steps 1 to 15. These values are defined in the A3981 as
PT(0) to PT(14), respectively.
There are two exceptions to this data distribution principal. These
are the zero value and the maximum value:
•	 The values in phase A steps 0 and 32 and phase B steps 16 and
48 are always set to zero and cannot be programmed.
•	 The maximum value, PT(15), is distributed to only two Step
Angle Numbers in each phase. These are the points in the
cycle where the peak current is required, namely phase A steps
16 and 48 and phase B steps 0 and 32.

![Image 1 from page 29](pdf-image://page_29_img_1)

## Page 30

Automotive Programmable Stepper Driver
A3981
30
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Each of the 16 values written to the phase table is a 6-bit num­
ber that determines the current trip point for the associated step.
The highest value, 63, represents the maximum phase current,
IPMAX , defined in the section of the specification on phase cur­
rent control. Other numbers represent a percentage of IPMAX . For
example, the number 23 sets the phase current trip point to 23/63
= 36.51% of IPMAX .
There are two restrictions when using the phase table load capa­
bility:
•	 The required current profile must conform to the symmetry
shown in Figure 14. The forward (positive) current part must
be symmetrical about Step Angle Number16 for phase A and
about 0 for phase B. The reverse (negative) current part must
be symmetrical about Step Angle Number 48 for phase A and
about 32 for phase B. The forward and reverse profiles for
each phase must be the same.
•	 The phase current must be zero at Step Angle Numbers 0 and
32 for phase A and Step Angle Numbers 16 and 48 for phase
B.
PHASE CURRENT TABLE PROGRAMMING EXAMPLE
As an example of programming the phase current table, consider
the current profile shown in Figure 14. This shows a profile
where the torque from each phase is required to be relatively
higher at the detent points, that is, the points where only one
phase is active. (This current profile does not relate to any spe­
cific motor, it is only shown as an example.)
Figure 14 shows the required current for each phase at each
Step Angle Number as a percentage of the maximum phase
current, IPMAX , defined above. The waveform conforms to the
required symmetry and zero crossing restrictions, so the profile
for phase A for Step Angle Numbers from 0 to 16 (outlined and
shaded) can be used to determine the phase table contents.
The first step is to digitize the profile into microsteps and the
percentage values into 6-bit numbers, as shown in Figure 15.
At each of the one-sixteenth microsteps, identified by Step Angle
Number, the value of the phase current, as a percentage of the
maximum phase current, IPMAX , is digitized to a 6-bit value from
Figure 14: Example Current Profile
Phase Current Table Value
Figure 15: Digitizing the Example Current Profile

![Image 1 from page 30](pdf-image://page_30_img_1)

![Image 2 from page 30](pdf-image://page_30_img_2)

![Image 3 from page 30](pdf-image://page_30_img_3)

## Page 31

Automotive Programmable Stepper Driver
A3981
31
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
0 to 63. The value 63 represents 100% of IPMAX , 32 represents
32/63=50.8% and so on. The value at each Step Angle Number is
then assigned to its corresponding phase table values as follows:
(
)
n
DI
n
PT
=
−1
where DIn represents the digitized value of the current at Step
Angle Number n.
A selection of the values and the corresponding phase current
table entries is shown in Figure15. The full set of phase current
table values is shown in the table below.
Step
1
2
3
4
5
6
7
8
9
10
11
12 13 14 15 16
Value
10 20 25 28 29 30 31 32 35 40 50 58 60 62 63 63
PT
0
1
2
3
4
5
6
7
8
9
10
11
12 13 14 15
These 16 values are then loaded sequentially into the phase
current table through the Phase Table Load register of the serial
interface. Each value is then distributed to the appropriate Step
Angle Numbers as described above and as shown in table 4C in
the Phase Table Load Register section.
A representation of the final result is shown in Figure 16. This
is the digitized version of the required current profile shown in
Figure 14.
Figure 16: Resulting Example Current Profile

![Image 1 from page 31](pdf-image://page_31_img_1)

![Image 2 from page 31](pdf-image://page_31_img_2)

## Page 32

Automotive Programmable Stepper Driver
A3981
32
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Power Dissipation
The A3981 is a power circuit, therefore careful consideration
must be given to power dissipation and the effects of high cur­
rents on interconnect and supply wiring.
A first order approximation of the power dissipation in the A3981
can be determined by examining the power dissipation in each of
the two bridges during each of the operation modes. When syn­
chronous rectification is used current will flow most of the time
through the DMOS transistors that are switched on. When syn­
chronous rectification is not used the current will flow through
the body diode of the DMOS transistors during the decay phase.
The use of fast or slow decay will also affect the dissipation. All
the above combinations can be calculated from five basic DMOS
output states as shown in Figure 17.
Figure 17. Basic Output States
•
Diagonally opposite DMOS
output transistors are on
•
Current flows from ground
through load to positive supply
•
Dissipation is I2R losses in the
DMOS transistors:
PD(SF) = I2 × (RDS(on)H+RDS(on)L )
•
Both low-side DMOS output
transistors are on
•
Current circulates through both
transistors and the load
•
Dissipation is I2R losses in the
DMOS transistors:
PD(SS) = I2 × (2 × RDS(on)L )
•
Diagonally opposite body diodes
conducting
•
Current flows from ground
through load to positive supply
•
Dissipation is IV losses in the
diodes:
PD(NF) = I × ( VFH + VFL )
•
One low-side DMOS output
transistor and one body diode
conducting
•
Current circulates through the
diode, the transistor and the load
•
Dissipation is I2R losses in the
DMOS transistors plus IV loss in
the diode:
PD(NS) = (I2 × RDS(on)L ) ⁄ ( I × VF )
•
(Used in all combinations)
•
Diagonally opposite DMOS
output transistors are on
•
Current flows from positive
supply through load to
ground
•
Dissipation is I2R losses in
the DMOS transistors:
PD = I2 × (RDS(on)H + RDS(on)L )
Synchronous Fast Decay
Non-Synchronous Fast Decay
Synchronous Slow Decay
Non-Synchronous Slow Decay
Drive Current Ramp-up

![Image 1 from page 32](pdf-image://page_32_img_1)

## Page 33

Automotive Programmable Stepper Driver
A3981
33
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
The total power dissipation for each of the four decay modes,
PD(TOT) XX, is the average power for the drive current ramp por­
tion, PD , and the drive current decay portion, PD(XX) of the PWM
cycle. For slow decay the current will be rising for approximately
20% of the cycle and decaying for approximately 80%. For fast
decay the ratio will be approximately 50%. Note that these are
approximate figures and will vary slightly depending on the
motor characteristics and the use of synchronous rectification.
The following formulas may be used to estimate total power dis­
sipation:
•	 Synchronous slow decay mode
PD(TOT)SS = 0.2 × PD + 0.8 × PD(SS)
PD(TOT)SS = 0.2 (I 2 [RDS(on)H + RDS(on)L ]) + 0.8 (I 2 × 2 × RDS(on)L)
•	 Non-synchronous slow decay mode
PD(TOT)NS = 0.2 × PD + 0.8 × PD(NS)
PD(TOT)NS = 0.2 ( I 2 [RDS(on)H + RDS(on)L ]) + 0.8 (I 2 × RDS(on)L + I × VF)
•	 Synchronous fast decay mode
PD(TOT)SF = 0.5 × PD + 0.5 × PD(SF)
PD(TOT)SF = I 2 (RDS(on)H + RDS(on)L )
•	 Non-synchronous fast decay mode
PD(TOT)NF = 0.5 × PD + 0.5 × PD(NF)
PD(TOT)NF = 0.5( I 2 [RDS(on)H + RDS(on)L ] ) + 0.5( I × [VFH + VFL ] )
An approximation of the total dissipation can be calculated by
summing the total power dissipated in both bridges and adding
the control circuit power due to VBB × IBB and VDD × IDD .
The total power at the required ambient temperature can then be
compared to the allowable power dissipation shown in figure 18.
For critical applications, where the first order power estimate is
close to the allowable dissipation, the power calculation should
take several other parameters into account including: motor
parameters, dead time, and switching losses in the controller.
Ambient Temperature (°C)
Allowable Power Dissipation (W)
150
125
100
75
50
25
5
4
3
2
1
0
RθJA = 28 °C/W
(on 4-layer PCB)
RθJA = 32 °C/W
(on 2-layer PCB)
Figure 18: Allowable Power Dissipation, on Typical PCBs

![Image 1 from page 33](pdf-image://page_33_img_1)

## Page 34

Automotive Programmable Stepper Driver
A3981
34
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Layout
TRACES
PCB The printed circuit board (PCB, or printed wiring board)
should use a higher weight copper thickness than a standard small
signal or digital circuit board. This helps to reduce the impedance
of the printed traces when conducting high currents. PCB traces
carrying switching currents should be as wide and short as pos­
sible to reduce the inductance of the trace. This will help reduce
any voltage transients caused by current switching during PWM
current control.
For optimum thermal performance, the exposed thermal pad on
the underside of the A3981 should be soldered directly onto the
board. A solid ground plane should be added to the opposite side
of the board, and multiple vias through the board to the ground
plane should be placed in the area under the thermal pad.
DECOUPLING
All supplies should be decoupled with an electrolytic capacitor in
parallel with a ceramic capacitor. The ceramic capacitor should
have a value of 100 nF and should be placed as close as pos­
sible to the associated supply and ground pins of the A3981. The
electrolytic capacitor connected to VBB should be rated at least
1.5 times the maximum circuit voltage, and selected to support
the maximum ripple current provided to the motor. The value of
the capacitor is unimportant but should be the lowest value with
the necessary ripple current capability.
The pump capacitor between CP1 and CP2, the pump storage
capacitor between VCP and VBB, and the compensation capaci­
tor between VREG and ground should be connected as close as
possible to the respective pins of the A3981.
GROUNDING
A star ground system, with the common star point located close to
the A3981, is recommended. The reference ground, AGND (pin
7), and the power ground, PGND (pin 21), must be connected
together externally. The copper ground plane located under the
exposed thermal pad is typically used as the star ground point.
CURRENT SENSE REGISTER
In sensing the output current level, to minimize inaccuracies
caused by ground-trace IR drops, the current sense resistor (RS)
should have an independent ground return to the star ground
point. This path should be as short as possible. For low-value
sense resistors, the IR drop in the PCB trace to the sense resis­
tor can be significant and should be taken into account. Surface
mount chip resistors are recommended to minimize contact
resistance and parasitic inductance. The value, RS , of the sense
resistor is given by:
SMAX
REF
S
I
V
R
×
= 16
There is no restriction on the value of RS or VREF , other than the
range of VREF over which the output current precision is guaran­
teed. However, it is recommended that the value of VREF be kept
as high as possible to improve the current accuracy. The table
below provides increasing values of ISMAX for suggested values
of VREF and standard E96 values of RS .
Suggested Values
ISMAX
(mA)
RS
(mΩ)
VREF
(V)
100
499
0.8
200
499
1.6
300
417
2.0
405
309
2.0
501
249
2.0
610
205
2.0
702
178
2.0
812
154
2.0
912
137
2.0
1008
124
2.0

![Image 1 from page 34](pdf-image://page_34_img_1)

## Page 35

Automotive Programmable Stepper Driver
A3981
35
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
INPUT/OUTPUT STRUCTURES
DIR
STEP
MS1
MS0
2 kΩ
8 V
6 V
VDD
47 kΩ
1 pF
OSC
8 V
6 V
VDD
6 V
EN
8 V
6 V
VDD
1 pF
STRN
SCK
SDI
8 V
6 V
VDD
SDO
8 V
VDD
OAP
OAM
OBP
OBM
VBB
VCP
VBB
18 V
18 V
14 V
VDD
8 V
CP1
CP2
VREG
8 V
7.5 V
AGND
PGND
REF
5 kΩ
8 V
6 V
6 V
RESETn
8 V
6 V
VDD
1 pF
50 kΩ
6 V
DIAG
8 V
VDD
6 V
SENSA
SENSB
22 V
VREG
2 kΩ
25 Ω
25 Ω
2 kΩ
51 kΩ
51 kΩ
500 Ω
120 Ω
Figure 19a: Supplies and Reference
Figure 19b: Sense Inputs
Figure 19e: STRN, SCK, SDI Inputs
Figure 19h: SDO Output
Figure 19c: DIR, STEP, MS1, MS0 Inputs
Figure 19f: OSC Input
Figure 19i: Phase Outputs
Figure 19d: EN Input
Figure 19g: RESETn Input
Figure 19j: DIAG Output

![Image 1 from page 35](pdf-image://page_35_img_1)

## Page 36

Automotive Programmable Stepper Driver
A3981
36
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Figure 20: Package LP, 28-Pin TSSOP with Exposed Thermal Pad
For Reference Only – Not for Tooling Use
(Reference Allegro DWG-0000379, Rev. 3 and JEDEC MO-153AET)
Dimensions in millimeters – NOT TO SCALE
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
A
1.20 MAX
0.15
0.025
0.30
0.19
0.20
0.09
8º
0º
0.60 ±0.15 1.00 REF
C
SEATING
PLANE
C
0.10
28X
0.65 BSC
0.25 BSC
2
1
28
9.70 ±0.10
4.40±0.10
6.40±0.20
GAUGE PLANE
SEATING PLANE
B
Branded Face
6.10
0.65
0.45
1.65
3.00
5.00
28
2
1
C
5.08 NOM
3 NOM
PCB Layout Reference View
A
B
C
Exposed thermal pad (bottom surface)
Terminal \#1 mark area
Reference land pattern layout (reference IPC7351 SOP65P640X120-29CM);
All pads a minimum of 0.20 mm from all adjacent pads; adjust as necessary
to meet application process requirements and PCB layout tolerances; when
mounting on a multilayer PCB, thermal vias at the exposed thermal pad land
can improve thermal dissipation (reference EIA/JEDEC Standard JESD51-5)
Branding scale and appearance at supplier discretion.
D
Standard Branding Reference View
Lines 1, 2, 3 = 10 characters
Line 1: Part Number
Line 2: Logo A, 4-digit Date Code
Line 3: Characters 5, 6, 7, 8 of
Assembly Lot Number
E
XXXXXXX
Date Code
Lot Number
CUSTOMER PACKAGE DRAWING

![Image 1 from page 36](pdf-image://page_36_img_1)

## Page 37

Automotive Programmable Stepper Driver
A3981
37
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
APPENDIX A: DRIVING A STEPPER MOTOR
A stepper motor is a particular form of brushless DC motor. As
for any electric motor, motion is created by magnetic interaction
between the stationary part of the motor, known as the stator, and
the moving part of the motor, known as the rotor. The information
presented here concentrates on a specific type of motor known as
a hybrid stepper motor. This is the most common type of small
stepper motor. It uses permanent magnets in the rotor to produce
one set of constant magnetic fields and electromagnets in the
stator to produce another set of varying magnetic fields. The term
hybrid relates to the use of both electromagnets and permanent
magnets.
Comparing Bipolar and Unipolar Motors
There are two options in small hybrid stepper motor construction.
In the first, known as a unipolar stepper motor, there are indepen­
dent electromagnets to generate each magnetic polarity, so two
electromagnets are required per phase. Each of these is energized
with current in only one direction, producing a single magnetic
field direction (unipolar). Because the current in each electromag­
net only flows in a single fixed direction, the control circuit can
be very simple. The drawback is that only one electromagnet per
phase can be energized at any time so, at most, only half of the
motor volume is ever used to create torque on the rotor.
A bipolar motor, in contrast, uses each electromagnet to pro­
duce two opposing fields (bipolar) at different times, by allow­
ing the current to flow in both directions. This means that the
motor volume required for a bipolar motor is half of the volume
required for a unipolar motor for the same torque output. The
minor drawback is that a bipolar motor requires a more complex
drive circuit in order to reverse the forcing voltage across the coil
of the electromagnet. However, if the drive circuit is integrated
into a single IC then the drive becomes cost effective. This, along
with the improvement in torque output makes the bipolar motor
a better solution for applications where the volume available is
restricted. For this reason the following information will relate
only to bipolar motors.
In order to create continuous motion in one direction it is neces­
sary to have two or more sets of electromagnets, that is, two or
more phases. The simplest and most cost effective configuration
for a stepper motor is to have two phases. For some applications
that require an extremely low torque ripple, 3 phase, 5 phase, and
even 9 phase stepper motors are sometimes used. However, the
remainder of the information presented here relates specifically to
2-phase bipolar motors.
Moving a 2-Phase Bipolar Stepper Motor
Figure A1 shows the four possible current combinations in two
phase windings, A and B, and the effect on a simplified repre­
sentation of part of a stepper motor. In each case the stator with
the electromagnets is shown at the top of the diagram and the
rotor with the permanent magnets is shown at the bottom of the
diagram.
In Figure A1 the stator consists of alternate phase A and phase
B electromagnets. The winding direction of the electromagnet
changes for each sequential electromagnet in each phase as indi­
cated by the overbar above the phase letter and identified below
as A-bar and B-bar. The result is that the magnetic poles will
alternate for each sequential electromagnet of each phase. That
means, for example, when the A electromagnet produces a north
(N) magnetic pole at the end nearest to the rotor, then the A-bar
electromagnet will produce a south (S) magnetic pole at the end
nearest to the rotor.
The windings for all the A and A-bar electromagnets are con­
nected in series and driven by a single full bridge. Similarly the
windings for all the B and B-bar electromagnets are connected
in series and driven by another single full bridge. So a 2-phase
bipolar stepper motor requires two full bridges for full control.
The rotor is much simpler than the stator, and consists of a solid
base holding permanent magnets with alternating pole directions.
There are no windings on the rotor, so there is no requirement to
conduct current to the moving part of the motor. In addition the
lack of current and windings means that there is no heat generated
in the rotor, making cooling of the moving parts much simpler.
The diagrams in Figure A1 provide a representation of a small
section of the mechanics of the motor. In practice the motor struc­
ture is a little different from this, but the principle of operation is
the same.
Starting at the top, panel (a) in Figure A1, the current is flowing
down through the phase A winding from top to bottom and there
is no current in phase B. The result is an N magnetic pole on the
A electromagnets and an S pole on the A-bar electromagnets. The
rotor position is such that that the poles of the permanent magnets
align with the poles of the electromagnets, N to S.
In the next panel, panel (b), the current is flowing down through
the phase B winding from top to bottom and there is no current
in phase A. The result is an N pole on the B electromagnets and
an S pole on the B-bar electromagnets. These magnetic poles will
attract and repel the permanent magnets on the rotor producing a

![Image 1 from page 37](pdf-image://page_37_img_1)

## Page 38

Automotive Programmable Stepper Driver
A3981
38
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
A
S
\_
A
N
B
\_
B
A
S
\_
A
N
B
\_
B
Stator
S
N
N
S
S
N
N
S
N
S
Rotor
B
A
A
\_
A
B
N
\_
B
S
A
\_
A
B
N
\_
B
S
Stator
S
N
N
S
S
N
N
S
N
S
Rotor
A
B
N
S
A
N
\_
A
S
B
\_
B
A
N
\_
A
S
B
\_
B
Stator
N
S
S
N
N
S
S
N
Rotor
B
A
A
\_
A
B
S
\_
B
N
A
\_
A
B
S
\_
B
N
N
S
S
N
N
S
S
N
S
N
Stator
Rotor
A
B
Figure A1. Basic Principle of Bipolar Stepper Motor Operation
(a)
• Phase A energized in positive direction
• Phase B not energized
Permanent magnet poles on the rotor aligned with
electromagnet poles on the stator
(b)
• Phase A not energized
• Phase B energized in positive direction
Rotor moves to the right to realign permanent
magnet poles on the rotor to the electromag­
net poles on the stator.
(c)
• Phase A energized in negative direction.
• Phase B not energized
Rotor moves to the right to realign permanent
magnet poles on the rotor to the electromag­
net poles on the stator.
(d)
• Phase A not energized.
• Phase B energized in negative direction
Rotor moves to the right to realign permanent
magnet poles on the rotor to the electromagnet
poles on the stator.

![Image 1 from page 38](pdf-image://page_38_img_1)

## Page 39

Automotive Programmable Stepper Driver
A3981
39
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
Figure A2: Half Step Operation
force that moves the rotor from left to right in the diagram until
the poles of the permanent magnets again align with the poles of
the electromagnets.
In panel (c), the current is flowing up through the phase A wind­
ing from bottom to top and there is no current in phase B. This
reverses the pole orientation from the top panel, such that there
is an S pole on the A electromagnets and an N pole on the A-bar
electromagnets. As before, these magnetic poles will attract and
repel the permanent magnets on the rotor producing a force that
moves the rotor from left to right in the diagram, until poles of
the permanent magnets again align with the poles of the electro­
magnets.
The bottom panel, panel (d), shows the final combination with
current flowing up through the phase B winding from bottom to
top and there is no current in phase A. This produces an N pole
on the B electromagnets and a S pole on the B-bar electromag­
nets. As before, these magnetic poles will attract and repel the
permanent magnets on the rotor producing a force that moves the
rotor from left to right until poles of the permanent magnets again
align with the poles of the electromagnets.
Each of the four steps in Figure A1 represents a single full
mechanical step of the stepper motor. The four steps together
represent a single electrical cycle.
The step resolution depends entirely on the mechanical construc­
tion of the motor and typically there will be 200 or more full
steps per mechanical revolution of the motor. A 200-step motor
will provide a resolution of 360 / 200 = 1.8° of rotation per step.
Stepping in the opposite direction to that described above is sim­
ply a case of changing the step sequence or inverting one of the
phase current directions.
Microstepping
In many applications it is necessary to improve the resolution of
the stepper motor, for more precise positioning control, or simply
to increase the number of steps per revolution to reduce the
torque ripple and therefore the vibration and noise of the motor.
Fortunately this can be achieved by driving both phases at the
same time in order to move the rotor to a position between two
electromagnets. This is known generically as microstepping.
Figure A2 shows the basic principle of microstepping. Panels (a)
and (c) of Figure A2 correspond to panels (a) and (b) of figure
A1. Panel (b) shows each phase energized such that there are now
two adjacent N poles and two adjacent S poles. In this example
the currents in both phases is the same, and so the S and N poles
of the rotor now move to half way between the positions in
diagrams (a) and (c). Figure A2 only shows a single mechanical
step in total, which is one quarter of a full electrical cycle. This
sequence is the lowest resolution form of microstepping, known
as half step, and is the simplest method of driving a stepper motor
in half-step mode.
The currents are switched-on in the correct direction in sequence
and no current control is required. The current is simply defined,
in the first instance, by the resistance of the winding and the
applied voltage.
A
\_
A
B
N
\_
B
S
A
Stator
S
N
N
S
N
A
B
Rotor
A
N
\_
A
S
B
N
\_
B
S
A
N
Stator
S
N
N
S
S
N
A
B
Rotor
A
N
\_
A
S
B
\_
B
A
N
Stator
N
S
S
N
S
N
B
A
Rotor
(a) Same as
figure A1(a)
(b) Half-step
position
(c) Same as
figure A1(b)

![Image 1 from page 39](pdf-image://page_39_img_1)

## Page 40

Automotive Programmable Stepper Driver
A3981
40
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
From Figure A2(b) it is also apparent that varying the rela­
tive current in each phase will make it possible to move the
rotor to any intermediate position between the four positions of
Figure A1, which occur when only a single phase is energized.
When there is one intermediate position this is known as half
step. When there are three intermediate positions this is known
as quarter step and so on. Higher resolution microstepping is
described in more detail below.
PHASE CURRENT-SEQUENCE DIAGRAMS
Figure A3 shows the full sequence of the two phase currents illus­
trated in Figure A2. This shows two electrical cycles, equivalent
to 4 full mechanical steps (8 half steps). The full-step positions are
marked F and the half-step positions are marked H. Each half step
in the electrical cycle is numbered, from 0 to 7, for reference later.
This figure shows that, when discussing stepper motor control, it
is necessary to know the relative magnitude and direction of the
current in each phase. So, rather than use physical representations
of the motor, such as in Figures A1 and A2, or simple time-based
current waveforms, such as Figure A3, it is simpler to use a phase
diagram. For a 2-pole bipolar motor this diagram is created by
plotting the current in the two phases as orthogonal vectors, that
is, as vectors at 90° to each other as shown in Figure A4.
PHASE CURRENT-PHASE DIAGRAMS
Figure A4 shows the currents of Figure A3 plotted on a phase
diagram where the phase A current is represented by the vertical
line and the phase B current by the horizontal line. The half-step
numbers correspond to the numbers in Figure A3. For example,
at step 1 in Figure A3, the phase A current and the phase B cur­
rent are both positive and with the same magnitude. These two
currents are shown in figure A4 as the two solid arrows. Adding
these two current vectors together gives the resultant motor cur­
rent vector indicated. The resultant is the hypotenuse of a right-
angled triangle with the two other sides equal. If the other two
sides are assumed to be 1 then the magnitude of the hypotenuse
will be:
41.1
2
1
1
2
2
=
=
+
So the resultant current vector will be 141% of the value of the
current in phase A or B, positioned at 45°.
Torque Ripple
Now, the torque output of any electrical motor is directly propor­
tional to the magnitude of the motor current, and the motor cur­
rent is the resultant phase current. It is clear from Figure A4 that
the resultant phase current at the half-step position is higher than
the current at the full-step position. This means that the motor
torque will be changing as the motor rotates, resulting in what is
known as torque ripple. Torque ripple in any rotating system will
cause mechanical vibration and will result in increased audible
noise and possible wear on other mechanical components. Torque
ripple can be reduced by ensuring that the resultant current at the
half-step point has the same magnitude as the full current in the
single phase at the full-step positions.
F
H
F
H
F
H
F
H
F
H
F
H
F
H
F
F
H
Phase
B
Current
Phase
A
Current
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
6
7
F
0
1
2
3
4
5
6
7
H
F
F
F
H
H
H
Phase B
Current
Phase A
Current
Resultant
Figure A3: Phase Current Sequence for
Uncompensated Half Step
Figure A4: Phase Diagram for Uncompensated Half
Step

![Image 1 from page 40](pdf-image://page_40_img_1)

## Page 41

Automotive Programmable Stepper Driver
A3981
41
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
COMPENSATED HALF STEP
Figure A5 shows a circle superimposed on the phase diagram.
This circle represents the required locus of the resultant phase cur­
rent vectors to maintain 100% current magnitude. At the full-step
positions, 0, 2, 4, and 6, only one phase is active and the magni­
tude of the phase current is at 100%. At the half-step positions, 1,
3, 5, and 7, both phases are active. To ensure that the magnitude
of the resultant current is 100%, the magnitude of each phase cur­
rent must be 70.7%. Calculating the value of the resultant current
as before gives a resultant current of 100%.
=
0.7072 + 0.7072
0.5 + 0.5 =
1 = 1
The current vectors at half-step position 1 are shown specifically
to illustrate that the magnitude of the resultant sits on the 100%
circle.
For a standard stepper motor to operate with minimum torque
ripple, the resultant current must always lie on the constant torque
circle irrespective of the number of microsteps. For higher resolu­
tion microstepping this then defines the relative phase currents at
each microstep position.
QUARTER STEP
For example consider the next resolution in microstepping; quar­
ter step. The locus of the required phase currents are shown in
Figure A6. The required current level in each phase can be calcu­
lated using simple trigonometry. For example, consider microstep
position 7 in Figure A6 as detailed in Figure A7.
There are 4 quarter steps for each full step. A full step on the
phase diagram is represented by 90°. So each quarter step incre­
ments the phase angle by 90° / 4 = 22.5°.
In Figure A7 the resultant motor current at quarter-step posi­
tion 8 is one quarter step from the horizontal, so it is at 22.5°. The
magnitude of the current in phase A at quarter-step position 7,
IA7 , is therefore sin 22.5°, which is equal to 0.383 or 38.3% of
the maximum current.
Similarly, the magnitude of the current in phase B at quarter-step
position 7, IB7 , is therefore cos 22.5°, which is equal to 0.924 or
92.4% of the maximum current.
At the 45° positions, 2, 6, 10 and 14, the magnitude of the current
in phase A and phase B will be cos 45° = 0.707 or 70.7%, which
is the same magnitude as in the half-step case shown in figure A5.
Due to symmetry, the phase A current is the same at quarter-step
F
0
1
2
3
4
5
6
7
H
F
F
F
H
H
H
Phase B
Current
Phase A
Current
Resultant
IA
0
1
2
3
4
5
6
7
IB
8
9
10
11
12
13
14
15
IA
6
7
8
IB
IA7
IB7=-cos22.5°
22.5°
=sin22.5°
Figure A5: Phase Diagram for Compensated Half Step
Figure A6: Phase Diagram for Quarter Step
Figure A7: Calculating Phase Current Magnitudes

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## Page 42

Automotive Programmable Stepper Driver
A3981
42
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
positions 7 and 1. The phase A current at quarter-step positions
9 and 15 also has the same magnitude, but the current is in the
opposite direction. In addition the phase B current at quarter-
step positions 3, 5, 11, and 13 also have the same magnitude as
that of phase A at quarter-step position 7, with a positive current
direction for steps 3 and 13 and a negative direction for steps 5
and 11. Similar symmetry can be applied to the phase B current at
quarter-step position 7, calculated above.
This means that only five discrete current magnitudes are
required, including 0% and 100%, in order to drive the stepper
motor to all 16 quarter-step positions. Using the same nomencla­
ture as Figure A7, that is, IPn , where P is the phase, A or B, and
n is the quarter-step number from Figure A6, Table A1 shows
where each of the five magnitude values are used.
Figure A8 shows these values plotted as a current sequence
diagram. This figure is therefore the time-based equivalent of the
phase diagram in Figure A6.
HIGHER MICROSTEP RESOLUTION
The principles described above can easily be extended to higher
microstep resolutions. As the microstep resolution increases, it
becomes more apparent that the phase current sequences approxi­
mate ever closer to a sin and cosine function. Figure A9 shows
the measured phase current sequence of the A3981 running in
sixteenth-step mode. The phase current sequences for eighth-step
and sixteenth-step resolutions are shown in figures A10 and A11.
Most applications using small motors are limited to sixteenth-step
mode due to the mechanical precision of the motor. Larger, high-
precision stepper motors are sometimes driven at 32, 64, or even
up to 256 microsteps in some extreme cases.
Practical Implementation
A system to drive a stepper motor with microstep capability
requires sequencers, current reference generators, and current
controllers. Developing such a system from discrete components,
or even using a fast microcontroller, is a complex task. The
A3981 is one of several fully integrated stepper drivers that are
available with microstep resolutions, from compensated half step
to sixteenth step and higher, using programmable current tables.
All aspects of the stepper control system are included in these
single chip solutions and many of them can be controlled by a
simple Step and Direction interface.
IB
14 15 0
1
2
3
4
5
6
7
8
9 10 11 12
12 13
IA
38%
70%
92%
100%
0
-38%
-70%
-92%
-100%
38%
70%
92%
100%
0
-38%
-70%
-92%
-100%
Figure A8: Phase Current Sequence for Quarter Step
Figure A9: Measured Sixteenth-step Phase Current
Sequence
Table A1: Quarter-Step Phase Current Magnitudes
Magnitude
(%)
Phase B
Phase A
0.
IA0
–
IA8
–
–
IB4
–
IB12
38.3
IA1
IA7
IA9
IA15
IB3
IB5
IB11
IB13
70.7
IA2
IA6
IA10
IA14
IB2
IB6
IB10
IB14
92.4
IA3
IA5
IA11
IA13
IB1
IB7
IB9
IB15
100.
–
IA4
–
IA12
IB0
–
IB8
–

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## Page 43

Automotive Programmable Stepper Driver
A3981
43
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
PRACTICAL LIMITATIONS
The information presented here assumes ideal stepper motors
being stepped slowly, with accurate, efficient current control
circuits. In practice the stepper motor phase windings are repre­
sented by two non-ideal inductors and the motor may be driven at
a high stepping rate.
A high stepping rate will produce a back EMF, like any other
motor, that will act against any current control circuits. The
current control circuits must also be able to work with inductive
loads. In general the current control circuit will be a PWM cur­
rent control scheme to make the driver as efficient as possible and
reduce the dissipation in the driver.
Like any other motor, the back EMF will also limit the maximum
stepping rate of the motor. As the motor speed increases the back
EMF will increase. When it reaches a value close to the supply
voltage the resulting voltage difference will be insufficient to
drive the phase current required to produce the necessary output
torque. When this occurs the motor will stall and slip out of syn­
chronization with the driving circuit.
The mechanical precision of the motor will also have an effect
on the overall performance of the system. If the effect of the
motor windings on the rotor are non-linear then the relationship
between current and torque may not be linear. The magnitude of
the currents at each microstep may then require a relationship
other than sinusoidal. The A3981 and a few other integrated driv­
ers are able to accommodate this by allowing the phase current
values for each microstep position to be reprogrammed. In most
systems this effect will be very small and can be ignored but in
some cases some improvement in torque ripple and audible noise
can be achieved.
Figure A10: Phase Current Sequence for Eighth Step
100%
98%
92%
83%
70%
56%
38%
19%
19%
38%
56%
70%
83%
92%
98%
100%
100%
98%
92%
83%
70%
56%
38%
19%
19%
38%
56%
70%
83%
92%
98%
100%
IB
IA
0 1 2 3 4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
24 25 26 27 28 29 30 31

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## Page 44

Automotive Programmable Stepper Driver
A3981
44
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
100%
98%
92%
83%
70%
56%
38%
19%
19%
38%
56%
70%
83%
92%
98%
100%
IB
10%
29%
47%
63%
77%
88%
96%
99%
10%
29%
47%
63%
77%
88%
96%
99%
100%
98%
92%
83%
70%
56%
38%
19%
19%
38%
56%
70%
83%
92%
98%
100%
10%
29%
47%
63%
77%
88%
96%
99%
10%
29%
47%
63%
77%
88%
96%
99%
IA
Figure A11: Phase Current Sequence for Sixteenth Step

![Image 1 from page 44](pdf-image://page_44_img_1)

## Page 45

Automotive Programmable Stepper Driver
A3981
45
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
For the latest version of this document, visit our website:
www.allegromicro.com
Revision History
Number
Date
Description
4
June 21, 2012
Update Electrical Characteristics table.
5
January 23, 2015
Update stall detection, VDD power-on reset, pin-out, and miscellaneous changes.
6
January 29, 2016
Corrected Figure 19j.
7
November 22, 2019
Minor editorial updates
8
February 3, 2022
Updated package drawing (page 36)
9
July 11, 2024
Removed tube packing option from selection guide (page 2)
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Allegro MicroSystems reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit
improvements in the performance, reliability, or manufacturability of its products.  Before placing an order, the user is cautioned to verify that the
information being relied upon is current.
Allegro’s products are not to be used in any devices or systems, including but not limited to life support devices or systems, in which a failure of
Allegro’s product can reasonably be expected to cause bodily harm.
The information included herein is believed to be accurate and reliable.  However, Allegro MicroSystems assumes no responsibility for its use; nor
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![Image 1 from page 45](pdf-image://page_45_img_1)