AN3142 datasheet(23/44 Pages) STMICROELECTRONICS

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AN3142

Designing a fixed-off-time PFC

Doc ID 17005 Rev 3

23/44

Where:

●

ILpk: it is the maximum peak current in the inductor, calculated as described in 4.2

●

Vcsmin = 1.0 V, it is the minimum voltage admitted on the L6563S current sense (on the

datasheet).

Because the internal current sense clamping sets the maximum current that can flow in the

inductor, the maximum peak of the inductor current is calculated considering the maximum

voltage Vcsmax admitted on the L6563S (on the datasheet):

Equation 45

The calculated ILpkx is the value at which the boost inductor must not be in saturation and it

is used for calculating the inductor number of turns and air gap length.

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 get a sinusoidal voltage reference. The multiplier

can be 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).

max

pksat

Vcs

pksat

Ω

rms

ISW

⋅

()

⋅

Ω

MULT

COMP

OFFSET

)

(

⋅

−

⋅

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