CIRCUIT OPERATION (refer to the block diagram)

The L4960 is a monolithic stepdownswitching regu-

lator providing output voltagesfrom 5.1V to 40V and

delivering 2.5A.

The regulation loop consists of a sawtooth oscilla-

tor, error amplifier, comparator and the output

stage. An error signal is produced by comparing the

output voltage with a precise 5.1V on-chip refer-

ence (zener zap trimmed to

± 2%).

This error signal is thencompared with the sawtooth

signal to generate the fixed frequency pulse width

modulated pulses which drive the output stage.

The gain and frequency stability of the loop can be

adjusted by an external RC network connected to

pin 3. Closing the loop directly gives an output

voltage of 5.1V. 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

allowed to rise, linearly, as this capacitor is charged

by a constant current source. Output overload pro-

tection 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 com-

parator resets the flip flop when the voltage across

the soft start capacitor has fallen to 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 limited

to a safe value by the dead time introduced by the

soft start network. The thermal overload circuit dis-

ables circuit operation when the junction tempera-

ture reaches about 150

°C and has hysteresis to

prevent unstable conditions.

Figure 1. Soft start waveforms

Figure 2. Current limiter waveforms

5/16

L4960