AN4496 datasheet(5/18 Pages) STMICROELECTRONICS

5 / 18 page

DocID026370 Rev 1

5/18

AN4496

ILED pin

1.2

High power factor modulation analysis

A solution to obtain the high power factor and low THD, as shown in Figure 2 on page 1, is

the AC coupling of the ILED pin with the rectified voltage. In this way the average of the

voltage VILEDavg is generated by the internal loop, which regulates the average output

current, while the waveform is modulated through the voltage divider by VRECT (see

Figure 3 B).

The average output voltage of an ideal single-phase full wave rectifier is:

Equation 3

Then the AC modulation is optimal when:

Equation 4

To ensure accurate regulation, the peak voltage on the ILED pin (VILEDpeak) must be smaller

than its maximum headroom voltage VILEDx (1.5 V). From Equation 2 and Equation 4 can be

estimated the maximum reflected voltage:

Equation 5

The effect of a reflected voltage greater of VRmax is the reduction of the output current when

VIN is lower than:

Equation 6

The equivalent input resistance of the ILED pin (RinILED) is 50 K when VILEDavg = 1 V and

the voltage divider must drive the ILED pin with equivalent resistance RAC << RinILED. The

phase rotation, at the input frequency, introduced from the capacitor CAC impacts on the

power factor, then the value of the capacitor must be:

Equation 7

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