AN3142 datasheet(26/44 Pages) STMICROELECTRONICS

26 / 44 page

Designing a fixed-off-time PFC

AN3142

26/44

Doc ID 17005 Rev 3

voltage feedforward with a technique that makes use of just two external parts and that limits

the feedforward time constant trade-off issue to only one direction.

Figure 16.

Mains detector and discharge resistor allow fast response to sudden line

drops not depending on the external RC

A capacitor CFF and a resistor RFF, both connected from the VFF (pin 5) pin to ground,

complete an internal peak-holding circuit that provides a DC voltage equal to the peak of the

rectified sine wave applied on the MULT pin (pin 3). In a case where the VFF pin is

connected directly to the RUN pin as in

Figure 17, the following value can be selected:

RFF provides a means to discharge CFF when the line voltage decreases (see Figure 16). In

this way, in the case of a sudden line voltage rise, CFF is rapidly charged through the low

impedance of the internal diode and no appreciable overshoot is visible at the preregulator's

output; in the case of line voltage drop, CFF is discharged with the time constant RFF·CFF,

which can be in the hundred ms to achieve an acceptably low steady-state ripple and have

low current distortion; consequently the output voltage can experience a considerable

undershoot, like in systems with no feedforward compensation.

Pin 10 (RUN): Remote ON/OFF control. A voltage below 0.8 V shuts down (not latched) the

IC and brings its consumption to a considerably lower level. PWM_STOP is asserted low.

The IC restarts as the voltage at the pin goes above 0.88 V.

The Brownout function can be easily implemented by connecting to the RUN pin through a

divider to the VFF pin as shown in Figure 17.

(16)

VFF

L6563S

MULT

MU LTIPL IER

1/V

AC rectified

DETECTOR

MAINS DROP

CFF

Ω

= M

RFF

AN3142 Datasheet(PDF) 26 Page - STMicroelectronics