L4970 datasheet(5/21 Pages) STMICROELECTRONICS

5 / 21 page

stability of the loop can be adjusted by an exter-

nal RC network connected to the output of the er-

ror amplifier. A voltage feedforward control has

been added to the oscillator, this maintains supe-

rior line regulation over a wide input voltage

range. Closing the loop directly gives an output

voltage of 5.1V, higher voltages are obtained by

inserting a voltage divider.

At turn on output overcurrents are prevented by

the soft start function (fig. 2). The error amplifier is

initially clamped by an external capacitor Css and

allowed to rise linearly under the charge of an in-

ternal constant current source.

Output overload protection is provided by a cur-

rent limit circuit (fig. 3). The load current is sensed

by an internal metal resistor connected to a com-

parator. When the load current exceeds a preset

threshold the output of the comparator sets a flip

flop which turns off the power DMOS. The next

clock pulse, from an internal 40kHz oscillator will

reset the flip flop and the power DMOS will again

conduct. This current protection method, ensures

a constant current output when the system is

overloaded or short circuited and limits the

switching frequency, in this condition, to 40kHz.

The Reset and Power fail circuitry (fig 4) gener-

ates an output signal when the supply voltage ex-

ceeds a threshold programmed by an external

voltage divider. The reset signal, is generated

with a delay time programmed by an external ca-

pacitor on the delay pin. When the supply voltage

falls below the threshold or the output voltage

goes below 5V the reset output goes low immedi-

ately. The reset output is an open collector-drain.

Fig 4A shows the case when the supply voltage is

higher than the threshold, but the output voltage

is not yet 5V.

Fig 4B shows the case when the output is 5.1V

but the supply voltage is not yet higher than the

fixed threshold.

The thermal protection disables circuit operation

when the junction temperature reaches about

150

°C and has an hysterysis to prevent unstable

conditions.

Figure 4: Reset and Power Fail Functions.

L4970A

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