Microstepping Driver with Translator
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
Device Operation. The A3967 is a complete microstep-
ping motor driver with built in translator for easy operation
with minimal control lines. It is designed to operate bipolar
stepper motors in full-, half-, quarter- and eighth-step
modes. The current in each of the two output full bridges
is regulated with fi xed off time pulse-width modulated
(PWM) control circuitry. The full-bridge current at each
step is set by the value of an external current sense resis-
tor (RS), a reference voltage (VREF), and the DACs output
voltage controlled by the output of the translator.
At power up, or reset, the translator sets the DACs and
phase current polarity to initial home state (see fi gures for
home-state conditions), and sets the current regulator for
both phases to mixed-decay mode. When a step command
signal occurs on the STEP input the translator automati-
cally sequences the DACs to the next level (see table 2 for
the current level sequence and current polarity). The mic-
rostep resolution is set by inputs MS1 and MS2 as shown in
table 1. If the new DAC output level is lower than the pre-
vious level the decay mode for that full bridge will be set
by the PFD input (fast, slow or mixed decay). If the new
DAC level is higher or equal to the previous level then the
decay mode for that Full bridge will be slow decay. This
automatic current-decay selection will improve microstep-
ping performance by reducing the distortion of the current
waveform due to the motor BEMF.
Reset Input (RESET). The RESET input (active low)
sets the translator to a predefi ned home state (see fi gures
for home state conditions) and turns off all of the outputs.
STEP inputs are ignored until the RESET input goes high.
Step Input (STEP). A low-to-high transition on the
STEP input sequences the translator and advances the
motor one increment. The translator controls the input to
the DACs and the direction of current fl ow in each wind-
ing. The size of the increment is determined by the state of
inputs MS1 and MS2 (see table 1).
Microstep Select (MS1 and MS2). Input terminals
MS1 and MS2 select the microstepping format per
table 1. Changes to these inputs do not take effect until the
STEP command (see fi gure).
Direction Input (DIR). The state of the DIRECTION
input will determine the direction of rotation of the motor.
Internal PWM Current Control. Each full bridge is
controlled by a fi xed off-time PWM current-control cir-
cuit that limits the load current to a desired value (ITRIP).
Initially, a diagonal pair of source and sink outputs are
enabled and current fl ows through the motor winding and
RS. When the voltage across the current-sense resistor
equals the DAC output voltage, the current-sense compara-
tor resets the PWM latch, which turns off the source driver
(slow-decay mode) or the sink and source drivers (fast- or
The maximum value of current limiting is set by the
selection of RS and the voltage at the VREF input with a
transconductance function approximated by:
ITRIPmax = VREF/8RS
The DAC output reduces the VREF output to the cur-
rent-sense comparator in precise steps (see table 2 for %
ITRIPmax at each step).
TRIP = (% ITRIPmax/100) x ITRIPmax
Fixed Off-Time. The internal PWM current-control
circuitry uses a one shot to control the time the driver(s)
remain(s) off. The one shot off-time, toff, is determined by
the selection of an external resistor (RT) and capacitor (CT)
connected from the RC timing terminal to ground. The off
time, over a range of values of CT = 470 pF to 1500 pF and
RT = 12 kΩ to 100 kΩ is approximated by:
toff = RTCT