STMicroelectronics 19 V - 90 W adapter with PFC for laptop computers using the L6563H and L6599A EVL6599A-90WADP EVL6599A-90WADP Fiche De Données

Doc ID 17230 Rev 1

9/31

The board implements a burst-mode function allowing a significant power saving during 
light-load operation. 

The L6599A’s STBY pin (#5) senses the optocoupler’s collector voltage which is related to 
the feedback control and is proportional to the output load. This signal is compared to an 
internal reference (1.24 V); if the load decreases and the voltage on the STBY pin goes 
lower than the reference, the IC enters an idle state and its quiescent current is reduced. 
Once the voltage exceeds the reference by 50 mV, the controller restarts switching. Burst-
mode operation load threshold can be programmed by properly choosing the resistor 
connecting the optocoupler to the RFMIN pin (R34). On this board the controller operates in 
burst-mode if the load falls below ~15 W.

The L6563H implements its own burst-mode function. If the COMP voltage falls below 2.5 V, 
the IC stops switching, causing an output voltage decrease, as a consequence the COMP 
voltage rises again and the IC restarts switching. 

In order to achieve a better load transient response, the PFC burst-mode operation is 
partially forced by the resonant converter; once the L6599A stops switching due to load 
drops, its PFC_STOP pin pulls down the L6563H’s PFC_OK pin, disabling PFC switching. 
Thanks to this solution, the PFC is forced into idle state when the resonant stage is not 
switching and rapidly wakes-up when the downstream converter restarts switching. This 
solution prevents a significant drop of the bulk voltage in the case of abrupt load rising.