L6599AD datasheet(16/35 Pages) STMICROELECTRONICS

16 / 35 page

Application information

L6599A

16/35

Doc ID 15308 Rev 7

Figure 21.

Typical system block diagram

7.1

Oscillator

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

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

network connected to pin 4 (RFmin). The pin provides an accurate 2 V reference 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 22 shows a simplified internal circuit that

explains the operation.

The network that loads the RFmin pin is generally made up of 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 modulates the current

through this branch - therefore modulating the oscillator frequency - to perform output

voltage regulation; the value of RFmax determines the maximum frequency the half

bridge is operated at when the phototransistor is fully saturated.

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 7.3: Soft-start). Note that the

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

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