Full-Bridge PWM Motor Driver

A4973

4

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

FUNCTIONAL DESCRIPTION

Internal PWM Current Control During Forward

off-time pulse width modulated (PWM) current-control circuit

that can be used to limit the load current to a desired value. The

across the external sense resistor to the voltage on the reference

input terminal (REF) resulting in a transconductance function

approximated by:

In forward or reverse mode the current-control circuitry limits

the comparator resets a latch that turns off the selected source

driver or selected sink and source driver pair depending on

whether the device is operating in slow or fast current-decay

mode, respectively.

In slow current-decay mode, the selected source driver is

disabled and both sinks are turned on. The load inductance

causes the current to recirculate through the sink drivers. In fast

current-decay mode, the selected sink and source driver pair are

disabled, then the opposite pair is turned on. The load inductance

causes the current to flow from ground to the load supply via the

motor winding and the opposite pair of transistors (see figure 1).

drivers remain disabled (see RC Fixed Off-Time section, below).

At the end of the RC interval, the drivers are enabled allowing

the load current to increase again. The PWM cycle repeats,

maintaining the peak load current at the desired value (figure 2).

Figure 2

Fast and Slow Current-Decay Waveforms

to ensure that the motor’s current does not exceed the ratings

of the device. The braking current can be measured by using

an oscilloscope with a current probe connected to one of the

motor’s leads, or if the back-EMF voltage of the motor is known,

approximated by:

circuitry uses a one shot to control the time the driver(s)

Figure 1 — Load-Current Paths

Drive Current (Normal)

Recirculation (Fast Decay)

Recirculation (Slow Decay)

ENABLE

MODE

LOAD

CURRENT

RC

I

TRIP

Dwg. WP-015-1

RC