AN3112 datasheet(23/36 Pages) STMICROELECTRONICS

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AN3112

Designing a fixed-off-time PFC

Doc ID 16820 Rev 3

23/36

The power dissipated by Rs is given by:

Equation 46

According to the result, for example four parallel resistors of 0.47

Ω with 1 W of power rating

can be selected.

Pin 3 (MULT): The MULT pin is the second multiplier input. It is connected, through a

resistive divider, to the rectified mains to obtain a sinusoidal voltage reference. The multiplier

is described by the relationship:

Equation 47

Where:

●

VCS (Multiplier output) is the reference for the current sense (VCS_OFFSET is its

offset).

●

k = 0.45 (Typ.) is the multiplier gain.

●

VCOMP is the voltage on pin 2 (E/A output).

●

VMULT is the voltage on pin 3.VFF is the second input to the multiplier for 1/V^2

function. It compensates the control loop gain dependence on the mains voltage. The

voltage at this pin is a DC level equal to the peak voltage on the MULT pin (#3).

A complete description is given in the diagram in

Figure 14 and Figure 15 which shows the

typical multiplier characteristics family. The linear operation of the multiplier is guaranteed

within the range 0 to 3 V of VMULT and the range 0 to 1.16 V (typ.) of Vcs, while the

minimum guaranteed value of the maximum slope of the characteristics family (typ.) is:

rms

ISW

⋅

()

⋅

Ω

MULT

COMP

OFFSET

)

(

⋅

−

⋅

Figure 14.

Multiplier characteristics family for

VFF =1 V

Figure 15.

Multiplier characteristics family for

VFF=3 V

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