3953

FULL-BRIDGE

PWM MOTOR DRIVER

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

6

FUNCTIONAL DESCRIPTION

Internal PWM Current Control During Forward and

Reverse Operation. The A3953S— contains a fixed off-

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

that can be used to limit the load current to a desired

by the selection of an external current sensing resistor

circuitry compares the voltage across the external sense

resistor to the voltage on the reference input terminal

(REF) resulting in a transconductance function approxi-

mated by:

In forward or reverse mode the current-control cir-

cuitry limits the load current as follows: when the load

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, respec-

tively.

In slow current-decay mode, the selected source

driver is disabled; the load inductance causes the current

to recirculate through the sink driver and ground clamp

diode. In fast current-decay mode, the selected sink and

source driver pair are disabled; the load inductance

causes the current to flow from ground to the load supply

via the ground clamp and flyback diodes.

during which the drivers remain disabled (see “RC Fixed

Off-Time” 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 (see figure 2).

Figure 2

Fast and Slow Current-Decay Waveforms

INTERNAL PWM CURRENT CONTROL

DURING BRAKE-MODE OPERATION

Brake Operation - MODE Input High. The brake circuit

turns off both source drivers and turns on both sink

drivers. For dc motor applications, this has the effect of

shorting the motor’s back-EMF voltage resulting in current

flow that dynamically brakes the motor. If the back-EMF

voltage is large, and there is no PWM current limiting, the

load current can increase to a value that approaches that

of a locked rotor condition. To limit the current, when the

conducting sink drivers. The energy stored in the motor’s

inductance is discharged into the load supply causing the

motor current to decay.

As in the case of forward/reverse operation, the

(see “RC Fixed Off-Time” below). Depending on the

back-EMF voltage (proportional to the motor’s decreasing

the PWM cycle will repeat, limiting the peak load current

to the desired value.

ENABLE

MODE

LOAD

CURRENT

RC

I

TRIP

Dwg. WP-015-1

RC

≈

Dwg. EP-006-13A

R S

BB

V

DRIVE CURRENT

RECIRCULATION (SLOW-DECAY MODE)

RECIRCULATION (FAST-DECAY MODE)

Figure 1 — Load-Current Paths