L4963 datasheet(4/17 Pages) STMICROELECTRONICS

4 / 17 page

CIRCUIT DESCRIPTION (Refer to Block Dia-

gram)

The L4963 is a monolithic stepdown regulator pro-

viding 1.5A at 5.1V working in discontinuous vari-

able frequency mode. In normal operation the

device resonates at a frequency dependingprimar-

ily on the inductance value, the input and output

voltage and the load current. The maximum switch-

ing however can be limited by an internal oscillator,

which can be programmed by only one external

resistor.

The fondamental regulation loop consists of two

comparators, a precision 5.1V on-chip reference

and a drive latch. Briefly the operation is as follows:

when the choke ends its discharge the catch free-

wheeling recirculation filter diode begins to come

out of forward conduction so the output voltage of

the device approaches ground. When the output

voltage reaches –0.1V the internal comparator sets

the latch and the power stage is turned on. Then

the inductor current rises linearly until the voltage

sensed at the feedback input reaches the 5.1V

reference.

The second comparator then resets the latch and

the output stage is turned off. The current in the

choke falls linearly until it is fully discharged, then

the cycle repeats. Closing the loop directly gives an

output voltage of 5.1V. Higher output voltages are

obtained by inserting a voltage divider and this

method of control requires no frequency compen-

sation network. At output voltages greater than

5.1V the available output current must be derated

due to the increased power dissipation of the de-

vice.

Output overload protection is provided by an inter-

nal current limiter. The load current is sensed by a

on-chip metal resistor connected to a comparator

which resets the latch and turns off the power stage

in overload condition. The reset circuits (see fig. 1)

generates an output high signal when the output

voltage value is correct. It has an open collector

output and the output signal delay time can be

programmed with an external capacitor. A power-

fail circuit is also available and is used to monitor

the supply voltage. Its output goes high when the

supply voltage reaches a pre-programmedtreshold

set by a voltage divider to its input from the supply

to ground. With the input left open the threshold is

approximately equal to 5.1V. The output of the

power fail is an open collector.

A TTL level inhibit is provided for applications such

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

a low logic level and disables circuits operation.

The thermal overload circuit disables the device

when the junction temperature is about 150

°C and

has hysteresis to prevent unstable conditions.

Figure 1: Reset and Power Fail Function

L4963 - L4963D

4/17

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