AN4339 datasheet(7/27 Pages) STMICROELECTRONICS

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DocID025039 Rev 1

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AN4339

Description

Figure 4. PFC_OK function in flyback operation

1.3.2

VFF (feed-forward function) and brownout protection

This is second input to the multiplier block for 1/V2 function. A capacitor and a parallel

resistor must be connected from this pin to GND. They complete the internal peak-holding

circuit that derives the information on the RMS mains voltage. The voltage at this pin, a DC

level equal to the peak voltage on the pin MULT (pin 3), compensates the control loop gain

dependence on the mains voltage. The variation of internal current reference signal VCSX

versus the mains voltage level detected at MULT input (VFF = VMULT) is shown in the graph

in Figure 5. This is the ultimate signal which is compared with the error signal to output the

required gate signal ON time. Never connect the pin directly to GND but with

a resistor ranging from 100 K

 (minimum) to 2 M (maximum). This pin is internally

connected to a comparator in order to provide brownout (AC mains undervoltage)

protection. A voltage below 0.8 V shuts down (not latched) the IC and brings its

consumption to a considerably lower level. The IC restarts as the voltage at this pin goes

above 0.88 V.

This is one of the important features of the L6564 device where we can manage the

overload condition and it prevents excessive current to flow through the input

section at worst low mains operating condition. So as the mains voltage drops below

a programmed level, the IC stops as VFF pin detects the brownout level.

AN4339 Datasheet(PDF) 7 Page - STMicroelectronics