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SNVS570L – JANUARY 2009 – REVISED MAY 2013
VALLEY-FILL CIRCUIT
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. Figure 18 illustrates a two and three stage valley-fill circuit.
V+
D3
V BUCK
V + D3
V BUCK
C7
+
D9
R6
C7
+
D6
D9
R6
D8
R8
C10
D8
R8
C10
D4
+
C9
R7
D4
D5
C8
D7
C9
+
R7
Figure 18. Two and Three Stage Valley Fill Circuit
The valley-fill circuit allows the buck regulator to draw power throughout a larger portion of the AC line. This
allows the capacitance needed at V BUCK to be lower than if there were no valley-fill circuit, and adds passive
power factor correction (PFC) to the application. Besides better power factor correction, a valley-fill circuit allows
the buck converter to operate while separate circuitry translates the dimming information. This allows for dimming
that isn’t subject to 120Hz flicker that can be perceived by the human eye.
VALLEY-FILL OPERATION
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 (see Figure 19 ) when the input line is
high.
V BUCK
V+
D3
+
C7
+
V BUCK
2
-
D8
+
C10
D4
V BUCK
2
+
C9
-
Figure 19. Two Stage Valley-Fill Circuit When AC Line is High
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