AN4496 datasheet(14/18 Pages) STMICROELECTRONICS

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Designing a high PF wide range LED driver with the HVLED815PF

AN4496

14/18

DocID026370 Rev 1

2.5

Current sense resistor

Current sense resistor value is determined by the average LED current IOUT.

Equation 29

This formula is exact if:



The peak voltage on the ILED pin is smaller than the maximum headroom voltage for

all mains voltage range.



OVP protection is set 20% over the maximum output peak voltage.



Perfect transformer coupling.



The LED driver works in the TM mode for over an half of the semi-period.

For different designs, fine tuning may be needed, but once the final values are selected,

repeatability from unit to unit is excellent.

2.6

Primary inductance

The primary inductance Lp sets the working frequency.

For the best regulation is better to limit the minimum frequency at maximum mains voltage

(265 V) to 90 KHz, in this way when the voltage is around the peak of the semi-period (and

the instantaneous power is higher), the LED driver operates in transition mode (TM). In wide

range application, at minimum mains voltage (88 V), this frequency drops to the minimum

fmin  40 KHz.

Equation 30

The increase fmin reduces the primary inductance and the transformer can be smaller. In this

case KRs can be used to compensate divergence of IOUT from the case with optimal

frequency.

Real value used:

RS = 1 

Real value used:

LP = 1.5 mH

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