STMicroelectronics 19V - 90W Adapter with PFC for Laptop computers using the L6563H and L6699 EVL6699-90WADP EVL6699-90WADP Ficha De Dados

Doc ID 022835 Rev 2

13/38

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 (

RF

min

). 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 

 shows a simplified internal circuit that explains the 

operation.

The network that loads the 

RF

min

 pin generally comprises three branches:

1.

a resistor 

RF

min

 connected between the pin and ground that determines the minimum 

operating frequency.

2. 

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.

3. 

an R-C series circuit (C

SS

+ R

SS

) connected between the pin and ground that enables 

the setting up of a frequency shift at startup (see 

). 

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

The following approximate relationships hold for the minimum and the maximum oscillator 
frequency respectively:

Equation 1

!-V

,







2&MIN



3

1

2









#&

)

2

q+

-

q)

2

+

-

q)

2

2

&MIN

2

&MAX

2

33

#

33

+

-

q)

2

(

)

max

min

max

min

min

RF

//

RF

 

·

 

CF

 

·

 

3

1

=

f

;

RF

 

·

 

CF

 

·

 

3

1

=

f