AN3172 datasheet(9/31 Pages) STMICROELECTRONICS

9 / 31 page

AN3172

Main characteristics and circuit description

Doc ID 17230 Rev 1

9/31

1.8

Standby power saving

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.

AN3172 Datasheet(PDF) 9 Page - STMicroelectronics