L6599 datasheet(16/36 Pages) STMICROELECTRONICS

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Application information

L6599

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7.1

Oscillator

The oscillator is programmed externally by means of a capacitor (CF), connected from pin 3

(CF) to ground, that will be alternately charged and discharged by the current defined with

the network connected to pin 4 (RFmin). The pin provides an accurate 2V reference with

about 2mA source capability and the higher the current sourced by the pin is, the higher the

oscillator frequency will be. The block diagram of Figure 21 shows a simplified internal

circuit that explains the operation.

The network that loads the RFmin pin generally comprises three branches:

A resistor RFmin connected between the pin and ground that determines the minimum

operating frequency;

A resistor RFmax connected between the pin and the collector of the (emitter-grounded)

phototransistor that transfers the feedback signal from the secondary side back to the

primary side; while in operation, the phototransistor will modulate the current through

this branch - hence modulating the oscillator frequency - to perform output voltage

regulation; the value of RFmax determines the maximum frequency the half-bridge will

be operated at when the phototransistor is fully saturated;

An R-C series circuit (CSS + RSS) connected between the pin and ground that enables

to set up a frequency shift at start-up (see Chapter 7.3: Soft-start). Note that the

contribution of this branch is zero during steady-state operation.

Figure 21. Oscillator's internal block diagram.

The following approximate relationships hold for the minimum and the maximum oscillator

frequency respectively:

L6599

2 V

RFmin

1 V

4 V

2·KM·IR

KM·IR

RFmin

RFmax

RSS

CSS

KM·IR

0.9V

3.9V

min

3CFRF

min

⋅⋅

-------------------------------------------

max

3CF

min

max

()

⋅⋅

--------------------------------------------------------------------------

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