LM3444_15 Datasheet, PDF (10/27 Pages) Texas Instruments – AC-DC Offline LED Driver

LM3444
SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015
Feature Description (continued)
V+ _D3
V_BUCK V+ _D3
C7 ⁺
R6
+
D9
C7
C10
D8 R8
D4
+ _C9
R7
D5
D4
D6
C8
D9
D8
R8
D7
+
C9
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V_BUCK
R6
C10
R7
Figure 9. 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.
7.3.3 Valley-Fill Operation
When the input line is high, power is derived directly through D3. The term input line is high is explained as
follows. The valley-fill circuit charges capacitors C7 and C9 in series (Figure 10) when the input line is high.
V+ _D3
V_BUCK
+
C7 ⁺ V_BUCK
2
-
D8
+
D4
V_{BUCK +}
2
C9
-
C10
Figure 10. Two Stage Valley-Fill Circuit When AC Line is High
The peak voltage of a two-stage valley-fill capacitor is given by Equation 1.
V_VF-CAP
=
V_AC-RMS
2
2
(1)
As the AC line decreases from its peak value every cycle, there is 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 (Figure 11), and V_BUCK equals the capacitor voltage.
10
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