A3981 Allegro Micro Systems, Inc., A3981 Datasheet - Page 40

• Manufacturer Part Number: A3981
• Manufacturer: Allegro Micro Systems, Inc.

A3981
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
I
I
A
B
Figure A10. Phase current sequence for eighth step
100%
100%
100%
100%
98%
92%
83%
70%
56%
38%
19%
19%
38%
56%
70%
83%
92%
98%
98%
92%
83%
70%
56%
38%
19%
19%
38%
56%
70%
83%
92%
98%
24 25 26 27 28 29 30 31
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
Automotive, Programmable Stepper Driver
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.
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
40