L6699D datasheet(13/38 Pages) STMICROELECTRONICS

13 / 38 page

L6699

Application information

Doc ID 022835 Rev 2

13/38

6.1

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; the higher the current sourced by the pin, the higher the oscillator

frequency. The block diagram of

Figure 5 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 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

the setting up of a frequency shift at startup (see

Section 6.3: Safe-start procedure).

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

Figure 5.

Oscillator's internal block diagram

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

frequency respectively:

Equation 1

()

max

min

max

min

;

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