a4980klp-t Allegro MicroSystems, Inc., a4980klp-t Datasheet - Page 26

• Manufacturer Part Number: a4980klp-t
• Description: Automotive, Programmable Stepper Driver
• Manufacturer: Allegro MicroSystems, Inc.

A4980
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.
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 A4980
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
Automotive, Programmable Stepper Driver
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 A4980, 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 the Phase Current Table
The full phase current table in the A4980 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 12 shows the default phase table values
plotted by Step Angle Number. Similar information is provided in
table 7.
The diagram in figure 12 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.
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
Value 0
Value
Step
Step
0
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
1
5 11 18 23 29 35 40 44 48 52 55 58 60 62 63
5 11 18 23 29 35 40 44 48 52 55 58 60 62 63 63
2
3
4
5
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
6
7
8
9 10 11 12 13 14 15
Q1
Q2
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