Datasheet

+
+
D3
C7
C9
C10
D4
D8
D9
V
BUCK
V+
+
-
+
-
V
BUCK
V
BUCK
AC-RMS 2
VF-CAP
V
V
2
+
+
D3
C7
C9
C10
D4
D8
V
BUCK
V+
+
-
+
-
2
V
BUCK
2
V
BUCK
LM3448
www.ti.com SNOSB51C –SEPTEMBER 2011–REVISED MAY 2013
V
BUCK
supplies the power which drives the LED string. Diode D3 allows V
BUCK
to remain high while V+ cycles on
and off. V
BUCK
has a relatively small hold capacitor C10 which reduces the voltage ripple when the valley-fill
capacitors are being charged. However, the network of diodes and capacitors shown between D3 and C10 make
up a "valley-fill" circuit. The valley-fill circuit can be configured with two or three stages. The most common
configuration is two stages which is illustrated in Figure 22.
When the “input line is high”, power is derived directly through D3. The term “input line is high” can be explained
as follows. The valley-fill circuit charges capacitors C7 and C9 in series when the input line is high (see
Figure 23).
Figure 23. Two stage Valley-Fill Circuit when AC Line is High
The peak voltage of a two stage valley-fill capacitor is:
(1)
As the AC line decreases from its peak value every cycle, there will be a point where the voltage magnitude of
the AC line is equal to the voltage that each capacitor is charged. At this point diode D3 becomes reversed
biased, and the capacitors are placed in parallel to each other (see Figure 24) and V
BUCK
equals the capacitor
voltage.
Figure 24. Two stage Valley-Fill Circuit when AC Line is Low
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