115 Northeast Cutoff, Box 15036

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

2916

DUAL FULL-BRIDGE

MOTOR DRIVER

4

APPLICATIONS INFORMATION

PWM CURRENT CONTROL

The UDN2916B/EB/LB 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

parator, and monostable multivibrator.

When the bridge is turned ON, current increases in

the motor winding and it is sensed by the external sense

resistor until the sense voltage (V

set at 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) because of the

internal logic and switching delays. This delay (t

typically 2 µs. After 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 pF to 1000 pF.

The fixed-off time should be short enough to keep

the current chopping above the audible range (< 46 µs)

and long enough to properly regulate the current. Be-

cause only slow-decay current control is available, short

off times (< 10 µs) require additional efforts to ensure

proper current regulation. Factors that can negatively

affect the ability to properly regulate the current when

using short off times include: higher motor-supply volt-

age, light load, and longer than necessary blank time.

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, main-

taining the average motor winding current at the desired

level.

Loads with high distributed capaci-tances may

result in high turn-ON current peaks. This peak (appear-

ing across R

ing in erroneous current control or high-frequency

oscillations. An external R

to further 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. W M-003-1A

I

OUT

Dwg. E P -006-1

R S

BB

V

BR IDG E ON

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