2547

PROTECTED

QUAD POWER DRIVERS

115 Northeast Cutoff, Box 15036

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

CIRCUIT DESCRIPTION AND APPLICATION

These devices monitor their outputs for open or shorted conditions.

Both conditions are sensed by comparing the input and output states.

Note that the FAULT output is operational only if the ENABLE input is

high. When a fault condition is sensed, the FAULT output will go to a

low state. An external FAULT output filter capacitor (0.1

µF) is recom-

mended to eliminate erroneous switching.

INCANDESCENT LAMP DRIVER

High incandescent lamp turn-ON/in-rush currents can contribute to

poor lamp reliability and destroy semiconductor lamp drivers.

Warming (parallel) or current-limiting (series) resistors protect both

driver and lamp but use significant power either when the lamp is OFF

or when the lamp is ON, respectively. Lamps with steady-state current

ratings up to 600 mA can be driven without the need for warming or

current-limiting resistors, if lamp turn-ON time is not a concern.

When an incandescent lamp is initially turned ON, the cold filament

is at minimum resistance and would normally allow a 10x to 12x in-rush

current. With these drivers, during turn-ON, the high in-rush current is

sensed by the internal low-value sense resistor, drive current to the

output stage is diverted by the shunting transistor, the output stage

operates in a linear mode, and the load current is limited to approxi-

mately 1.3 A. During lamp warmup, the filament resistance increases

to its maximum value, the output driver goes into saturation and applies

maximum rated voltage to the lamp.

INDUCTIVE LOAD DRIVER

With the addition of external clamp diodes, bifilar (unipolar)

stepper motors and other inductive loads can be driven directly.

The external diodes prevent damage to the output transistors by

suppressing the high-voltage spikes that occur when turning

OFF an inductive load. For rapid current decay (fast turn-OFF speeds),

the use of Zener diodes will raise the flyback voltage and improve

performance. However, the peak voltage must not exceed the speci-

OVER-CURRENT CONDITIONS

In the event of a shorted load, or stalled motor, the load current

will attempt to increase. As described above, the drive current to the

affected output stage is linearly reduced (limiting the load current to

about 1.3 A), causing the output stage to go linear. As the junction

temperature of the output stage increases, the thermal shutdown circuit

will shut OFF the affected output. If the fault condition is corrected, the

output driver will return to its normal saturated condition.

NORMAL LAMP IN-RUSH CURRENT

TIME

Dwg. WP-005A

NOT TO SCALE

CURRENT LIMIT

THERMAL SHUTDOWN

THERMAL HYSTERESIS

SHORT CIRCUIT