AN4149 datasheet(26/43 Pages) STMICROELECTRONICS

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L4984D biasing circuitry

AN4149

26/43

DocID023523 Rev 2

3.2

Current sense resistor

Pin 4 (CS): The pin #4 is the inverting input of the current sense comparator. Through this

pin, the L4984D senses the instantaneous inductor current, converted in a proportional

voltage by an external sensing resistor (RS). As this signal crosses the threshold set by the

multiplier output, the PWM latch is reset and the power MOSFET is turned off. The threshold

is defined by:

Equation 75

where:

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

offset).

–km=0.23 (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 pin MULT (pin 3).

The sense resistor value (RS) can be calculated as follows. For the 350 W PFC it will be:

Equation 76

Where:

–ILpk is the maximum peak current in the inductor, calculated with

–Vcsm in = 0.84 V is the minimum value of the L4984D current sense reference

clamp (VCSclamp in the datasheet).

According to the result, three parallel resistors of 0.33

Ω with 1 W of power rating have been

selected.

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

inductor, the maximum peak of the inductor current will be calculated considering the

maximum Vcsclamp admitted on the L4984D:

Equation 77

The calculated ILpkx will be the value at which the boost inductor shall not saturate and it will

be used for calculating the inductor number of turns and air gap length. The power

dissipated by RS is given by:

Equation 78

)

(

MU LT

COM P

OFF SE T

⋅

−

⋅

pkx

rms

ISW

⋅

()

2 =

⋅

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