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All   >>  LED Lighting   >>   LCD Backlighting Applications   >>   Questions
 

Your LED Constant Current Source Scheme AND8109/D is very interesting for my backlighting applications, spesially the circuit in Figure 4. First I will use LM317T so give 1.5A. I will use only one 1W, 0.35A white Golden Dragon (LUW W5AM) LED in each leg. My question is: Can you set up the equation for resistor Rsense. How big are it? And can you also set up the equation for resistor Rbase for MPS2222. How big are it? How many legs can LM317T supply? I thank you for your attention.

When the circuit operates properly and all the LEDs are running, the three sense resistors have about 1.25 V across them, which turns the transistor switches ‘on’. Assume your 12V input voltage, your R base resistor value would be 100k and R sense resistor value is10 Ohms. (R sense power dissipation is 1W at 320mA LED current so 2W resistor would be needed). Lm317 can source max. 1.5Amp. ,therefore four different strings can be connected to its output. Answered by:
ON Semiconductor
2014-01-14 12:09:42.195