DocID028175 Rev 3

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L6599AF

Application information

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Figure 4. Typical system block diagram

6.1

Oscillator

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

pin 3 (CF) to ground, that is alternately charged and discharged by the current defined with

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

oscillator frequency is. The block diagram of Figure 5 shows a simplified internal circuit that

explains the operation.

1.

operating frequency.

2.

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

primary side; while in operation, the phototransistor modulates the current through this

branch - therefore modulating the oscillator frequency - to perform output voltage

operated at when the phototransistor is fully saturated.

3.

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

a frequency shift to be set up at startup (see Section 6.3: Soft-start on page 18). Note

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