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

20/38

Doc ID 022835 Rev 2

the operating frequency is essentially determined by the current sunk by the optocoupler's 
phototransistor. 

During this frequency sweep phase the operating frequency decays following the 
exponential charge of C

SS

: then, it initially changes relatively quickly but the rate of change 

gets slower and slower. This counteracts the non-linear frequency dependence of the tank 
circuit that makes the converter's input impedance change little as frequency is away from 
resonance and change very quickly as frequency approaches resonance frequency (see 

As a result, the DC input current smoothly increases, without the peaking that occurs with 
linear frequency sweep, and the output voltage reaches the regulated value with almost no 
overshoot. 

Typically, R

SS

 and C

SS

 is selected based on the following relationships:

Equation 5

where f

start

 is recommended to not be over 300 kHz. Please refer to the timing diagram of 

 to see some significant signals during the soft-start phase.

         

         

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SS

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