Application information

L6599

16/36

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

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:

1.

operating frequency;

2.

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

be operated at when the phototransistor is fully saturated;

3.

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

+

-

4

RFmin

3

CF

S

Q

R

+

-

1 V

4 V

+

-

CF

0.9V

3.9V

f

min

1

3CFRF

min

⋅⋅

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

=

f

max

1

3CF

RF

min

RF

max

||

()

⋅⋅

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

=