AN2259 Datasheet, PDF (8/38 Pages) STMicroelectronics – High intensity LED driver using

2 INPUT CAPACITOR SELECTION
AN2259
Iin = ^{Io ⋅ Vo}
Vin ⋅ η ⋅ 0.7
For the high frequency part (ignoring output current ripple), we have:
Iripple =
Dav −
2 ⋅ Dav²
η
+
Dav²
η²
⋅ Io
where Dav is the average duty cycle.
We will use the average duty cycle assuming that the voltage on the capacitor changes from
the peak to the minimum voltage linearly.
Dav = ^Vo
Vav
Vav = ^{Vpeak + V min}
2
Iripple = Dav − ^{2 ⋅ Dav2} + ^Dav2 ⋅ Io
η
η²
The equivalent series resistance of an aluminum capacitor has different frequency
characteristics. There is a coefficient associated with different frequencies. Typically, for 120Hz,
Kfl=1; for frequency greater than 10 kHz, Kfh=1.5.
Icap =


Iin
2


+


Iripple
2


 ^Kfl   ^Kfh 
Therefore, the ripple current rating of the capacitor has to be greater than Icap
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