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LM3404 Folha de dados(PDF) 11 Page - National Semiconductor (TI) |
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LM3404 Folha de dados(HTML) 11 Page - National Semiconductor (TI) |
11 / 18 page Modified Application Circuit Design Example 4 • V IN = 48V (±20%) • Driving 3, 4, or 5 HB LEDs with V F = 3.4V • I F = 500 mA (typical application) • Estimated efficiency = 82% • f SW = 500 kHz (typ app) The inductor, R ON resistor, and the RSNS resistor are calcu- lated for a typical or average design. • V OUT = 3 x 3.4V + 200 mV = 10.4V • V OUT = 4 x 3.4V + 200 mV = 13.8V • V OUT = 5 x 3.4V + 200 mV = 17.2V Reduce switching frequency for the typical application to about 500 kHz to increase efficiency. Calculate t ON, tOFF & RON • V OUT = 13.8V • V IN = 48V • I F = 500 mA • t D = 220 ns • η = 0.85 • f SW = 500 kHz t ON ≊ 705 ns R ON ≊ 179 kΩ (use standard value of 182 kΩ) Calculate Inductor Value • I F = 500 mA • Δi L = 250 mA (target) • R ON = 182 kΩ • L = 100 µH Calculate Δi L with L = 100 µH (VIN = 48V, VOUT = 13.8V) Δi L = 241 mA (all combinations) Calculate Switching Frequency Example 4 Switching Frequency V IN (V) V OUT (V) f SW (kHz) Three Series LEDs 36 10.4 374 48 10.4 412 V IN (V) V OUT (V) f SW (kHz) Three Series LEDs 60 10.4 435 Four Series LEDs 36 13.8 430 48 13.8 497 60 13.8 537 Five Series LEDs 36 17.2 454 48 17.2 558 60 17.2 620 Calculate R SNS • V OUT = 13.8V • V IN = 48V • I F = 500 mA • t D = 220 ns • η = 0.85 • L = 100 µH R SNS = 488 mΩ Calculate Average Current through LED Example 4 Average LED Current V IN (V) V OUT (V) I F (A) Three Series LEDs 36 10.4 0.507 48 10.4 0.507 60 10.4 0.507 Four Series LEDs 36 13.8 0.500 48 13.8 0.500 60 13.8 0.500 Five Series LEDs 36 17.2 0.493 48 17.2 0.493 60 17.2 0.493 In the reduced frequency application you can see that there is a difference of 14 mA between the low and high of the av- erage current. If the original t ON circuit was used (no PNP transistor) with the switching frequency centered around 500 kHz the difference between the high and low values would be about 67 mA. 11 www.national.com |
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Descrição semelhante - LM3404 |
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