3957

FULL-BRIDGE PWM

MICROSTEPPING

MOTOR DRIVER

Functional Description

Two A3957S— full-bridge PWM microstepping motor

drivers are needed to drive the windings of a bipolar stepper

motor. Internal pulse-width modulated (PWM) control circuitry

regulates each motor winding’s current. The peak motor

current is set by the value of an external current-sense resistor

To improve motor performance, especially when using

sinusoidal current profiles necessary for microstepping, the

A3957S— has three distinct current-decay modes: slow decay,

fast decay, and mixed decay.

PHASE Input. The PHASE input controls the direction of

current flow in the load (table 1). An internally generated dead

time of approximately 1.5

µs prevents crossover currents that

could occur when switching the PHASE input.

to digitally control the output current. The output of the DAC is

used to set the trip point of the current-sense comparator. Table

3 shows DAC output voltages for each input condition. When

transistors are turned off.

Internal PWM Current Control. Each motor driver IC

contains an internal fixed off-time PWM current-control circuit

diagonal pair of source and sink transistors are enabled and

When the voltage across the sense resistor equals the DAC

output voltage, the current-sense comparator resets the PWM

latch, which turns off the source drivers (slow-decay mode) or

the sink and source drivers (fast- or mixed-decay mode).

with a transconductance function approximated by:

current of the sink transistor (typically 30 mA). The actual peak

internal logic and switching delays. The driver(s) remain off

for a time period determined by a user-selected external

off time, the driver(s) are re-enabled, allowing the load current

The current-sense comparator has a fixed offset of approxi-

The DAC data input lines are used to provide up to eight

levels of output current. The internal 4-bit digital-to-analog

converter reduces the reference input to the current-sense

comparator in precise steps (the step reference current ratio or

SRCR) to provide half-step, quarter-step, eighth-step, or

“microstepping” load-current levels.

in slow current-decay mode (the source drivers are disabled

the load inductance causes the current to recirculate through the

motor winding, sink driver, ground clamp diode, and sense

resistor (see figure 1). Slow-decay mode produces low ripple

current for a given fixed off time (see figure 2). Low ripple

current is desirable because the average current in the motor

winding is more nearly equal to the desired reference value,

resulting in increased motor performance in microstepping

applications.

For a given level of ripple current, slow decay affords the

lowest PWM frequency, which reduces heating in the motor and

driver IC due to a corresponding decrease in hysteretic core

losses and switching losses respectively. Slow decay also has

the advantage that the PWM load current regulation can follow

a more rapidly increasing reference before the PWM frequency

drops into the audible range. For these reasons slow-decay

mode is typically used as long as good current regulation can be

maintained.

Figure 1 — Load-Current Paths

Dwg. EP-006-15

R S

BB

V

DRIVE CURRENT

RECIRCULATION

(SLOW-DECAY MODE)

RECIRCULATION

(FAST-DECAY MODE)