3966

DUAL FULL-BRIDGE

PWM MOTOR DRIVER

www.allegromicro.com

5

Internal PWM Current Control. The A3966SA and

A3966SLB dual H-bridges are designed to drive both

windings of a bipolar stepper motor. Load current can be

controlled in each motor winding by an internal fixed-

frequency PWM control circuit. The current-control

circuitry works as follows: when the outputs of the H-

bridge are turned on, current increases in the motor wind-

ing. The load current is sensed by the current-control

current continues to increase until it reaches the predeter-

mined value, set by the selection of external current-

according to the equation:

to the base-drive current of the sink driver transistor.

At the trip point, the comparator resets the source-

enable latch, turning off the source driver of that H-bridge.

The source turn off of one H-bridge is independent of the

other H-bridge. Load inductance causes the current to

recirculate through the sink driver and ground-clamp

diode. The current decreases until the internal clock

oscillator sets the source-enable latches of both H-bridges,

turning on the source drivers of both bridges. Load current

increases again, and the cycle is repeated.

FUNCTIONAL DESCRIPTION

The frequency of the internal clock oscillator is set by

be approximately calculated as:

20 k

Ω to 100 kΩ and 470 pF to 1000 pF respectively.

Nominal values of 56 k

Ω and 680 pF result in a clock

frequency of 25 kHz.

Current-Sense Comparator Blanking. When the

source driver is turned on, a current spike occurs due to

the reverse-recovery currents of the clamp diodes and

switching transients related to distributed capacitance in

the load. To prevent this current spike from erroneously

resetting the source enable latch, the current-control

comparator output is blanked for a short period of time

when the source driver is turned on. The blanking time is

tion:

time of 1.3

µs.

The current-control comparator is also blanked when

the H-bridge outputs are switched by the PHASE or

ENABLE inputs. This internally generated blank time is

approximately 1

µs.

+

–

0

Dwg. WM-003-2

I

OUT

t

d

t

blank

INTERNAL

OSCILLATOR

BRIDGE

ON

SOURCE

OFF

BRIDGE

ON

R C

T

T

ALL

OFF

Dwg. EP-006-16

R S

BB

V

BRIDGE ON

SOURCE OFF

ALL OFF