Datasheet

Vsw
n u VLED
VOS
V
SN
V
MOS_BV
V
AC_PEAK
TPS92311
www.ti.com SNVS811B –MAY 2012–REVISED MAY 2013
Figure 24. Snubber Waveform
where
• n is the turn ratio of the transformer. (17)
(18)
At the same time, sum of the snubber clamp voltage and V
AC
peak voltage (V
AC_PEAK
) must be smaller than the
MOSFET breakdown voltage (V
MOS_BV
). By re-arranging terms, equation in below can be used.
(19)
In here, snubber clamp voltage, V
SN
= 250V is recommended.
Output Capacitor
The capacitance of the output capacitor is determined by the equivalent series resistance (ESR) of the LED,
R
LED
and the ripple current allowed for the application. The equation in below can be used to calculate the
required capacitance.
(20)
Assume the ESR of the LED stack contains 7 LEDs and is 2.6Ω, AC line frequency f
AC
is 60Hz.
In this example, LED current I
LED
is 350mA and output ripple current is 30% of I
LED
:
(21)
Then, C
OUT
= 480μF.
In here, a 470μF output capacitor with 10μF ceramic capacitor in parallel is suggested.
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