L6566A datasheet(31/51 Pages) STMICROELECTRONICS

31 / 51 page

L6566A

Application information

31/51

Figure 18.

Hiccup-mode OCP: timing diagram

5.8

PFC interface

The device is specifically designed to minimize converter’s losses under light or no-load

conditions, and a special function has been provided to help the designer meet energy

saving requirements even in power-factor-corrected systems where a PFC pre-regulator

precedes the isolated DC-DC converter.

Actually EMC regulations require compliance with low-frequency harmonic emission limits

at nominal load; no limit is envisaged when the converter operates with a light load. Then

the PFC pre-regulator can be turned off, thus saving the no-load consumption of this stage

(0.5

÷1 W).

To do so, the L6566A provides the Vcc_PFC pin (6): this pin is internally connected to the

Vcc pin (5) via a PNP transistor, normally closed, that opens when the voltage VCOMP falls

below VCOMPO, a threshold internally set at a value depending on whether QR operation or

FF operation is selected. This pin is intended for supplying the PFC controller of the pre-

regulator as shown in Figure 16 on page 28. The switch is thermally protected, so that the

IC will stop if an external failure causes the pin to be overloaded for too long time or shorted

to ground.

Vcc

(pin 5)

(pin 4)

OCP latch

VccON

VccOFF

Vccrestart

Secondary diode is shorted here

VCS

(pin 7)

Vcc_OK

1.5 V

Vcc_PFC

(pin 6)

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