High Brightness LED driver question

Community Projects
1

I’m working on a PCB design with a few of the OSRAM Dragon LRW5SN High Intensity LEDs that will be hook up to an Arduino to control the blink rate. My first prototype uses a Sipex SPX29300T 3A LDO regulator to power 3 LEDs in parallel. These particular LEDs operate at 2.1-3v and can draw up to 1A each. This setup works as far as powering the LEDs to a high brightness, but the LDO quickly goes in to thermal shutdown mode. I tried putting in some resistors to limit the current for each LED, but the LDO still eventually overheats. I’ve looked at several LED driver IC datasheets, but I’m not sure how these are supposed to work in general. Do the drivers have an adjustable voltage output or do they just pulse the voltage provided on the input pin? I’ve seen most try and provide a constant current, but wouldn’t that still lead to thermal issues? If someone could point me to a driver or circuit that will keep these LEDs on and not overheat that would be a great help!

Thanks!

Jeff

2

LED drivers are constant current devices, putting whatever voltage is needed across the LED to get the desired current.

The voltage regulator you mentioned is a straight forward linear regulator, dissipating power = whatever current you had (?3A?) x voltage difference (input - output). If the current is anything more than “small”, it’ll get hot and need a heatsink. W/o one I wouldn’t be surprised to see it shutdown.

Look how it’s done on this item ;

https://www.sparkfun.com/products/11850

http://dlnmh9ip6v2uc.cloudfront.net/dat … AL8805.pdf

ps = putting LEDs in parallel is “icky”. Each LED will have a different V vs I curve and so unless very closely matched, current hogging by 1 LED is likely.

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OK, so using the LED drivers then, a 12v input wouldn’t fry a 2v LED because it will adjust the duty cycle to keep the current at the level defined by the rest of the circuit?

4

jeffcraighead:
OK, so using the LED drivers then, a 12v input wouldn’t fry a 2v LED because it will adjust the duty cycle to keep the current at the level defined by the rest of the circuit?

In effect, yes. It's a switch mode power supply whose output voltage is varied to get a desired current. Exactly what's varied, the pulsewidth or switching frequency or ... to get the output voltage needed for that current is device/manufactuer dependent.