UDN2917EB datasheet(4/8 Pages) ALLEGRO

4 / 8 page

2917

DUAL FULL-BRIDGE

PWM MOTOR DRIVER

115 Northeast Cutoff, Box 15036

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

–

Dwg. WM-003-1A

VPHASE

OUT

toff

ITRIP

APPLICATIONS INFORMATION

PWM CURRENT CONTROL

The UDN2917EB dual bridge is designed

to drive both windings of a bipolar stepper

motor. Output current is sensed and con-

trolled independently in each bridge by an

external sense resistor (RS), internal com-

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 (VSENSE) reaches the level set

at the comparator’s input:

ITRIP = VREF/10 RS

The comparator then triggers the mon-

ostable which turns off the source driver of

the bridge. The actual load current peak will

be slightly higher than the trip point (espe-

cially for low-inductance loads) because of

the internal logic and switching delays. This

delay (td) is typically 2

µs. After turn-off, the

motor current will normally decay, circulating

through the ground clamp diode and sink

transistor. The source driver’s off time (and

therefore the magnitude of the current de-

crease) is determined by the monostable’s

external RC timing components, where toff =

RTCT within the range of 20 k

Ω to 100 kΩ and

200 pF to 500 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, maintaining the

average motor winding current at the desired

level.

Special circuitry has been included to

prevent runaway current control when the

fixed off time (toff) is set too short. This

circuitry prevents the source driver from being

re-enabled until the load current has decayed

to below the ITRIP level.

PWM OUTPUT CURRENT WAVEFORM

LOAD CURRENT PATHS

Dwg. EP-006-1

R S

BRIDGE ON

SOURCE OFF

ALL OFF

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