AN4496 datasheet(3/18 Pages) STMICROELECTRONICS

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AN4496

ILED pin

The ILED pin voltage (VILED) is the constant current (CC) loop reference. This voltage

divided by two is used as the reference for the MOSFET's peak drain current during the CC

regulation. An external capacitor is used for filtering the ILED pin current with an appropriate

time constants (

ILED >> 1 / 2 *  * fIN).

In this configuration the peak of the drain current remains constant during the semi-period

but the tON of the primary MOSFET increases when the instantaneous input voltage

decreases and with the mains near the zero-crossing the MOSFET remains in the ON state

until the mains voltage becomes enough to source the peak drain current.

Then near the zero-crossing the mains current has a peak as in Figure 3 A.

Figure 3. ILED voltage and input current waveform

1.1

DC analysis

With a DC input voltage (VIN) and the device working in the transition mode (TM), the

voltage on the ILED, VILED, is:

Equation 1

This voltage changes in function of mains voltage to maintain constant the output current

and VILED reaches the maximum value at the minimum mains voltage.

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