HVLED807PF datasheet(21/36 Pages) STMICROELECTRONICS

21 / 36 page

HVLED807PF

Device description

Doc ID 023464 Rev 5

21/36

The capacitor CLED has to be chosen so that its voltage VILED can be considered as a

constant. Since it is charged and discharged by currents in the range of some ten µA

(IREF=20 µA typ.) at the switching frequency rate, a capacitance value in the range 4.7-10

nF is suited for switching frequencies in the ten kHz. When high power factor schematic is

implemented, a higher capacitor value should be used (i.e. 1 µF-10 µF).

The average output current IOUT can be expressed as:

Equation 1

Where ISEC is the secondary peak current, TONSEC is the conduction time of the secondary

side and T is the switching period.

Taking into account the transformer ratio N between primary and secondary side, ISEC can

also be expressed as a function of the primary peak current IPRIM:

Equation 2

As in steady state the average current ICLED:

Equation 3

Which can be solved for VILED:

Equation 4

where VCLED=R* IREF and it is internally defined (0.2 V typical - see Table 5: Electrical

characteristics).

The VILED pin voltage is internally compared with the CS pin voltage (constant current

comparator):

Equation 5

Combining (1), (2) (4) and (5) the average output current results:

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