L6599ATN datasheet(13/31 Pages) STMICROELECTRONICS

13 / 31 page

L6599AT

Application information

Doc ID 15534 Rev 3

13/31

Figure 4.

Typical system block diagram

6.1

Oscillator

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

(CF) to ground, which 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, the higher the

oscillator frequency. The block diagram in Figure 5 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, which 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 - thus modulating the oscillator frequency - to perform output voltage regulation.

The value of RFmax determines the maximum frequency at which the half-bridge is

operated when the phototransistor is fully saturated.

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

the setup of a frequency shift at startup (see Section 6.3: Soft-start). Note that the

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

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