a4980klp-t Allegro MicroSystems, Inc., a4980klp-t Datasheet - Page 39

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

A4980
in the first instance, by the resistance of the winding and the
applied voltage.
From figure A2(b) it is also apparent that varying the relative
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 figure 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
Figure A3. Phase current sequence for simple half step
Current
Current
Phase
Phase
B
A
F
6
H
7
F
0
H
1
F
2
H
3
F
4
H
5
F
6
H
7
F
0
H
1
F
2
H
3
Automotive, Programmable Stepper Driver
F
4
H
5
F
6
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:
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.
Figure A4. Phase diagram for simple half step
H
H
F
4
3
5
1
2

115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
1
2
Phase A
Current
2
6

F
F
2

1
Resultant
4 1 .
1
7
H
0
F
H
Phase B
Current
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