2918

DUAL FULL-BRIDGE

PWM MOTOR DRIVER

115 Northeast Cutoff, Box 15036

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

PWM OUTPUT CURRENT WAVEFORM

APPLICATIONS INFORMATION

PWM Current Control

The A2918SWH/V 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

monostable multivibrator.

When the bridge is turned ON, current increases

comparator’s input:

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

2

µs typically. After turn-off, the motor current decays,

circulating through the ground clamp diode and sink

therefore the magnitude of the current decrease, is

determined by the monostable’s external RC timing

k

Ω to 100 kΩ and 200 pF to 500 pF.

When the source driver is re-enabled, the winding

current (the sense voltage) again is 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

This circuitry prevents the source driver from being

re-enabled until the load current has decayed to

Loads with high distributed capacitances may result

in high turn-ON current peaks. This peak, appearing

in possible erroneous current control or high-frequency

used to delay the action of the comparator, and thus

ignore turn-on spikes.

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Dwg. WM-003-1A

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