L4964HT datasheet(3/13 Pages) STMICROELECTRONICS

3 / 13 page

CIRCUIT OPERATION (refer to the block diagram)

The L4964 is a monolithic stepdown switching regu-

lator providing output voltages from 5.1 V to 28 V

and delivering 4A.

The regulation loop consists of asawtooth oscillator,

error amplifier, comparator and the output stage. An

error signal is produced by comparing the output

voltage with a precise 5.1 V on-chip reference

(zener zap trimmed to

± 3 %). This error signal is

then compared with the sawtooth signal to generate

the fixed frequency pulse width modulated pulses

which drive the output stage. The gain and fre-

quency stability of the loop can be ajusted by an ex-

ternal RC network connected to pin 9. Closing the

loop directly gives an output voltage of 5.1 V. Higher

voltages are 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 al-

lowed to rise, linearly, as this capacitor is charged

by a constant current source.

Output overload protection 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

output stage and discharges the soft start capacitor.

A second comparator resets the flip flop when the

voltage across the soft start capacitor has fallen to

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

outputvoltage rises undercontro of the soft startnet-

work. If the overload condition is still present the

limiter will trigger again when the thershold 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

generated with 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.

A TTL - level input is provided for applications such

as remote on/off control. This input is activated by

high level and disables circuit operation. After an in-

hibit the L4964 restarts under control of the soft start

network.

The thermal overload circuit disables circuit opera-

tion when the junction temperature reaches about

150 and has hysteresis to prevent unstable condi-

tions.

Figure 1 : Reset Output Waveforms

L4964

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