AN2782 datasheet(22/39 Pages) STMICROELECTRONICS

22 / 39 page

Designing a fixed off-time PFC

AN2782

22/39

Doc ID 14763 Rev 2

Figure 14.

Multiplier characteristics family

A complete description is given in Figure 14, 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:

Taking this into account, the following is the suggested procedure to properly set the

operating point of the multiplier.

First, the maximum peak value for VMULT, VMULTmax is selected. This value, which occurs at

maximum mains voltage, should be 3 V or nearly so in wide range mains and less in case of

single mains. The sense resistor selected is Rs = 0.117 Ω and it is described in the

paragraph concerning pin 4 of this section. The maximum peak value, occurring at

maximum mains voltage, is:

where ILpksat and Rs have been already calculated, and 1.1 V/V is the multiplier maximum

slope, as given in the datasheet.

(60)

(61)

Multiplier characteristic

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

0.00.2 0.40.6 0.81.0 1.21.4 1.61.8 2.02.2 2.42.6 2.8 3.0

VMULT (pin3) (V)

V COMP (pin2) (V)

Upper Volt. Clamp

2.5 V

3 V

3.5V

4 V

5.75 V

4.5V

5 V

MULT

min

max

pksat

max

VAC

VMULT

⋅

Vac

265

117

VMULT

max

⋅

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