L296 데이터 시트보기 (PDF) - STMicroelectronics

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L296

HIGH CURRENT SWITCHING REGULATORS

STMicroelectronics 

L296 - L296P

CIRCUIT OPERATION

(refer to the block diagram)

The L296 and L296P are monolithic stepdown

switching regulators providing output voltages from

5.1V to 40V and delivering 4A.

The regulationloop consists of a sawtoothoscillator,

error amplifier, comparatorand the outputstage. An

error signal is produced by comparing the output

voltage with a precise 5.1V on-chipreference(zener

zap trimmed to ± 2 %). This error signalis then com-

pared with the sawtooth signal to generate the fixed

frequencypulse width modulatedpulseswhich drive

the output stage.The gain and frequencystability of

the loop can be adjustedby an externalRC network

connectedto pin9. Closing the loop directlygives an

outputvoltageof 5.1V.Higher voltagesare obtained

by inserting a voltage divider.

Output overcurrents at switch on are prevented by

the soft start function. The error amplifier output is

initially clamped by the external capacitor Css and

allowed to rise, linearly, as this capacitor is charged

by a constant current source.

Outputoverloadprotection is provided in the form of

a current limiter. The load current is sensed by an

internal metal resistor connected to a comparator.

When the load current exceeds a preset threshold

this comparator sets a flip flop which disables the

outputstage and discharges the soft start capacitor.

A second comparator resets the flip flop when the

voltage across the soft start capacitor has fallen to

Figure 1 : Reset Output Waveforms

0.4V. The output stage is thus re-enabled and the

output voltage rises under control of the soft start

network. If the overload condition is still present the

limiter will trigger again when the threshold current

is reached. The average short circuit current is lim-

ited to a safe value by the dead time introduced by

the soft start network.

The reset circuit generates an output signal when

the supply voltage exceeds a threshold pro-

grammed by an external divider. The reset signal is

generatedwith a delay time programmed by an ex-

ternal capacitor. When the supply falls below the

threshold the reset output goes low immediately.

The reset output is an open collector.

The scrowbar circuit senses the output voltage and

the crowbar output can provide a current of 100mA

to switch on an externalSCR. This SCR is triggered

when the output voltage exceeds the nominal by

20%. There is no internal connection between the

outputand crowbar sense input thereforethe crow-

bar can monitor either the input or the output.

A TTL - level inhibit input is providedfor applications

such as remote on/offcontrol. This input is activated

by high logic level and disables circuit operation.Af-

ter an inhibit the L296 restarts under control of the

soft start network.

The thermal overload circuit disables circuit opera-

tion when the junction temperature reaches about

150 °C and has hysteresis to prevent unstable con-

ditions.

3/22

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