AN3142 datasheet(24/44 Pages) STMICROELECTRONICS

24 / 44 page

Designing a fixed-off-time PFC

AN3142

24/44

Doc ID 17005 Rev 3

A complete description is given by the diagram in

Figure 14 and 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:

Equation 48

The voltage on the MULT pin is used also to derive the information on the RMS mains

voltage for the VFF compensation.

The suggested procedure to properly set the operating point of the multiplier is now

described. First, the maximum peak value for VMULT, VMULTmax is selected. This value,

which occurs at maximum mains voltage, should be 3 V or thereabouts in wide range mains

and less in the case of single mains. The sense resistor selected is Rs = 0.12

Ω and it is

described in the pin #4 section. According to the L6563S datasheet and to the linearity

setting of the pin, the maximum voltage accepted on the multiplier input is:

Where ILpk and Rs have been already calculated, 1.66 is the Multiplier maximum slope,

reported on the datasheet.

From

15 the maximum required divider ratio is calculated as:

Equation 49

Figure 14.

Multiplier characteristics family for

VFF =1 V

Figure 15.

Multiplier characteristics family for

VFF=3 V

Multiplier Char ac ter istics @ VFF=1 V

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.1

1.2

0 .1

0 .2

0.3

0.4

0 .5

0.6

0.7

0 .8

0.9

1.1

VMU LT (V)

4.5V

3.5V

Uppe r voltage clamp

3. 0V

2. 6V

5.0V

4.0V

5.5

VC OMP

Multiplier characteristics @VFF = 3V

10 0

20 0

30 0

40 0

50 0

60 0

70 0

00 .5

1.522.5

33.5

VMU LT (V )

2.6V

3.0V

4.5V

4.0V

3.5V

5.0V

5.5V

Upper voltage

VCOMP

MULT

VMULT

max =

(15)

Vac

265

VAC

max

MULT

−

⋅

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