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LM3410 Datasheet(PDF) 11 Page - Texas Instruments |
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LM3410 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 44 page L = ¨¨ © § 2'iL VIN ¸¸ ¹ · x DTS VOUT - KVIN D = VOUT VOUT x ILED K = VIN x IIN '¶ K VOUT VIN = VOUT - VIN D = VOUT 1 - D 1 1 = c D = VOUT VIN ¨ © § ¸ ¹ · = 2'iL DTS 'i L = ¨¨ © § L VIN ¸¸ ¹ · ¨¨ © § 2L VIN ¸¸ ¹ · x DTS t L i L i ' S T S DT ( ) t L I L VIN L V V OUT IN - LM3410 www.ti.com SNVS541F – OCTOBER 2007 – REVISED AUGUST 2010 Figure 8. Inductor Current (3) The Duty Cycle (D) for a Boost converter can be approximated by using the ratio of output voltage (VOUT) to input voltage (VIN). (4) Therefore: (5) Power losses due to the diode (D1) forward voltage drop, the voltage drop across the internal NMOS switch, the voltage drop across the inductor resistance (RDCR) and switching losses must be included to calculate a more accurate duty cycle (See Calculating Efficiency and Junction Temperature for a detailed explanation). A more accurate formula for calculating the conversion ratio is: (6) Where η equals the efficiency of the LM3410 application. Or: (7) Therefore: (8) Inductor ripple in a LED driver circuit can be greater than what would normally be allowed in a voltage regulator Boost & Sepic design. A good design practice is to allow the inductor to produce 20% to 50% ripple of maximum load. The increased ripple shouldn’t be a problem when illuminating LEDs. From the previous equations, the inductor value is then obtained. (9) Where 1/TS = fSW (10) One must also ensure that the minimum current limit (2.1A) is not exceeded, so the peak current in the inductor must be calculated. The peak current (Lpk I) in the inductor is calculated by: Copyright © 2007–2010, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3410 |
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