Datasheet

Device description HVLED815PF
20/34 DocID023409 Rev 5
The capacitor C
LED
has to be chosen so that its voltage V
ILED
can be considered as
a constant. Since it is charged and discharged by currents in the range of some ten µA
(I
REF
= 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 I
OUT
can be expressed as:
Equation 1
Where I
SEC
is the secondary peak current, T
ONSEC
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, I
SEC
can
also be expressed as a function of the primary peak current I
PRIM
:
Equation 2
As in steady state the average current I
CLED
:
Equation 3
Which can be solved for V
ILED
:
Equation 4
where V
CLED
= R * I
REF
and it is internally defined (0.2 V typical - see Table 5: Electrical
characteristics on page 9).
The V
ILED
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:
Equation 6