A3981 Allegro Micro Systems, Inc., A3981 Datasheet - Page 27

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

A3981
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,
I
rent control. Other numbers represent a percentage of I
example, the number 23 sets the phase current trip point to 23/63
= 36.51% of I
There are two restrictions when using the phase table load capa-
bility:
• The required current profile must conform to the symmetry
shown in figure 10. 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 sym-
metrical 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.
Figure 11. Example current profile
PMAX
, defined in the section of the specification on phase cur-
PMAX
.
Automotive, Programmable Stepper Driver
PMAX
. For
Figure 12. Digitizing the example current profile
Phase Current Table Programming Example
As an example of programming the phase current table, consider
the current profile shown in figure 11. 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 specific motor,
it is only shown as an example.)
Figure 13 shows the required current for each phase at each
Step Angle Number as a percentage of the maximum phase
current, I
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 12.
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, I
0 to 63. The value 63 represents 100% of I
PMAX
, defined above. The waveform conforms to the
PMAX
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, is digitized to a 6-bit value from
PMAX
, 32 represents
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