AN4213 datasheet(17/42 Pages) STMICROELECTRONICS

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AN4213

Designing a TM PFC

Pin 4 (CS): input to the PWM comparator

Figure 7. Sense resistor

The current flowing in the Power MOSFET is sensed through a resistor, the resulting voltage

is applied to this pin and compared with an internal sinusoidal-shaped reference, generated

by the multiplier, to determine Power MOSFET turn-off. The Power MOSFET stays in the

OFF-state until the PWM latch is reset by the ZCD signal.

The sense resistor value can be calculated as follows:

Equation 29

Where:

•

Lpk

is the maximum peak current in the inductor, equal to 1.99 A

•

CSmin

= 1 V is the minimum voltage allowed on the L6562A current sense (given in the

datasheet).

For this project the selected commercial value for R

is 0.47

Ω.

Using this value and considering the maximum voltage V

CSmax

= 1.16 V allowed on the

L6562A (as given in the datasheet), the maximum peak of the inductor current can be

calculated as follows:

Equation 30

The calculated I

Lpkx

is the saturation inductor current and it is used for calculating the

winding number of the inductor and its air gap.

AM17321v1

INV

COMP

MULT

ZCD

GND

VCC

L6562A

Rsense

≤

502

min

Lpk

Lpkx

max

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