2559

PROTECTED

QUAD POWER DRIVER

115 Northeast Cutoff, Box 15036

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

TYPICAL OUTPUT

CHARACTERISTIC

CIRCUIT DESCRIPTION AND APPLICATION

INCANDESCENT LAMP DRIVER

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

poor lamp reliability and destroy semiconductor lamp drivers. Warm-

ing or current-limiting 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 700

mA can be driven by these devices without the need for warming

(parallel) or current-limiting (series) resistors.

When an incandescent lamp is initially turned ON, the cold fila-

ment 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 then diverted by the shunting transistor, and the

load current is momentarily limited to approximately 1.0 A. During

this short transition period, the output current is reduced to a value

dependent on supply voltage and filament resistance. During lamp

warmup, the filament resistance increases to its maximum value, the

output stage goes into saturation and applies maximum rated voltage to

the lamp.

INDUCTIVE LOAD DRIVER

Bifilar (unipolar) stepper motors, relays, or solenoids can be driven

directly. The internal flyback 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 inprove performance. However, the peak

FAULT CONDITIONS

In the event of a shorted load, the load current will attempt to

increase. As described above, the drive current to the affected output

stage is reduced, causing the output stage to go linear, limiting the peak

output current to approximately 1 A. As the power dissipation of that

output stage increases, a thermal gradient sensing circuit will become

operational, further decreasing the drive current to the affected output

stage and reducing the output current to a value dependent on supply

voltage and load resistance.

Continuous or multiple overload conditions causing the chip

temperature to reach approximately 165

°C will result in an additional

reduction in output current to maintain a safe level.

If the fault condition is corrected, the output stage will return to its

normal saturated condition.

TYPICAL OUTPUT BEHAVIOR

V

OUT(SAT)

Dwg. GP-013

OUTPUT CURRENT, I

OUT

T < 150

°C

T

= 165

°C

J

J

JUNCTION

TEMP. LIMIT

NOT TO SCALE

TRIP

I

THERMAL

GRADIENT

SENSING

NORMAL LAMP IN-RUSH CURRENT

TIME

THERMAL GRADIENT SENSING

CURRENT LIMIT

Dwg. WP-008

NOT TO SCALE

0