2919

DUAL FULL-BRIDGE

MOTOR DRIVER

115 Northeast Cutoff, Box 15036

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

APPLICATIONS INFORMATION

PWM CURRENT CONTROL

The A2919SB/SLB dual bridges are

designed to drive both windings of a bipolar

stepper motor. Output current is sensed and

controlled independently in each bridge by

an external sense resistor (R

comparator, and monostable multivibrator.

When the bridge is turned ON, current

increases in the motor winding and flows

through the external sense resistor until the

sense voltage (V

the comparator’s input:

I

The comparator then triggers the

monostable, which turns OFF the source

driver of the bridge. The actual load current

peak will be slightly higher than the trip point

(especially for low-inductance loads) be-

cause of the internal logic and switching

delays. This delay (t

turn-off, the motor current decays, circulating

through the ground-clamp diode and sink

transistor. The source driver’s OFF time

(and therefore the magnitude of the current

decrease) is determined by the monostable’s

external RC timing components, where

t

100 k

Ω and 100 pF to 1000 pF.

When the source driver is re-enabled,

the winding current (the sense voltage) is

again allowed to rise to the comparator’s

threshold. This cycle repeats itself, maintain-

ing the average motor winding current at the

desired level.

Loads with high distributed capacitances

may cause current spikes capable of tripping

the comparator, resulting in erroneous

current control. An external R

should be used to delay the action of the

comparator. Depending on load type, many

applications will not require these external

components (SENSE connected to E).

PWM OUTPUT CURRENT WAVE FORM

LOAD CURRENT PATHS

+

–

0

Dwg. WM-003-1A

I

OUT

t

d

t

off

I

TRIP

Dwg. EP-006-1

R S

BB

V

BRIDGE ON

SOURCE OFF

ALL OFF