A3981KLPTR-T Allegro Microsystems Inc, A3981KLPTR-T Datasheet - Page 34

• Manufacturer Part Number: A3981KLPTR-T
• Description: AUTOMOTIVE PROGRAMMABLE STEPPER DRVR
• Manufacturer: Allegro Microsystems Inc

Specifications of A3981KLPTR-T

• Applications: Stepper Motor Driver, 2 Phase
• Number Of Outputs: 1
• Voltage - Load: 7 V ~ 28 V
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP (0.173", 4.40mm Width) Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

A3981
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
Appendix A. Driving a Stepper Motor
Automotive, Programmable Stepper Driver
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.
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
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