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LM3444 Datasheet(PDF) 9 Page - Texas Instruments |
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LM3444 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 27 page = D = tON tON + tOFF = tON x fSW VO VIN L2 R3 C11 Q2 Q3 D10 ICOLL 4 6 7 8 R4 PGN D ISNS GAT E COFF LM3444 VBUCK C12 VVF-CAP = 2 VAC-RMS 3 LM3444 www.ti.com SNVS682B – NOVEMBER 2010 – REVISED NOVEMBER 2011 A three stage valley-fill circuit performs exactly the same as two-stage valley-fill circuit except now three capacitors are now charged in series, and when the line voltage decreases to: (2) Diode D3 is reversed biased and three capacitors are in parallel to each other. The valley-fill circuit can be optimized for power factor, voltage hold up and overall application size and cost. The LM3444 will operate with a single stage or a three stage valley-fill circuit as well. Resistor R8 functions as a current limiting resistor during start-up, and during the transition from series to parallel connection. Resistors R6 and R7 are 1 M Ω bleeder resistors, and may or may not be necessary for each application. BUCK CONVERTER The LM3444 is a buck controller that uses a proprietary constant off-time method to maintain constant current through a string of LEDs. While transistor Q2 is on, current ramps up through the inductor and LED string. A resistor R3 senses this current and this voltage is compared to the reference voltage at FILTER. When this sensed voltage is equal to the reference voltage, transistor Q2 is turned off and diode D10 conducts the current through the inductor and LEDs. Capacitor C12 eliminates most of the ripple current seen in the inductor. Resistor R4, capacitor C11, and transistor Q3 provide a linear current ramp that sets the constant off-time for a given output voltage. Figure 15. LM3444 Buck Regulation Circuit OVERVIEW OF CONSTANT OFF-TIME CONTROL A buck converter’s conversion ratio is defined as: (3) Copyright © 2010–2011, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM3444 |
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