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LM3444MAX Datasheet(PDF) 9 Page - Texas Instruments |
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LM3444MAX Datasheet(HTML) 9 Page - Texas Instruments |
9 / 30 page VVF-CAP = 2 VAC-RMS 2 + + D3 C7 C9 C10 D4 D8 VBUCK V+ + - + - 2 VBUCK 2 VBUCK + + D3 C7 C8 C9 C10 D4 D5 D6 D7 D8 R7 R8 D9 R6 VBUCK V+ + + D3 C7 C9 C10 D4 D8 D9 R7 R6 R8 VBUCK V+ LM3444 www.ti.com SNVS682C – NOVEMBER 2010 – REVISED MAY 2013 Figure 12. 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 VBUCK to be lower than if there were no valley-fill circuit, and adds passive power factor correction (PFC) to the application. 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 (Figure 13) when the input line is high. Figure 13. 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 (Figure 14), and VBUCK equals the capacitor voltage. Copyright © 2010–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM3444 |
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