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[on:]

I'm designing a circuit that powers a 1W 'Luxeon' style high-power LED from an LT1302 that boosts a small voltage to 5v (this is required for other reasons I won't go into). My plan originally was to use 4 parallel 1/4 watt resistors to bring the current down to about 200ma (maybe 250). There are four of them mainly to give me some breathing room with the watts/heat dissipated (since I could swap out the 1/4 watt for 1/2 watt and so on). To turn the LED on I'm using a BD681(NPN darlington power transistor).

From everything I've read most people here recommend using a constant current device (like an LM314 in constant current mode) to drive the LED but I'm curious if I could forgo this since I'm using the LT1302. The datasheet doesn't specifically say that it guarantees a constant current though it hints at load balancing which should give me a steady enough current.

I'm also curious if someone could explain to me why the constant current device is so critical. If I just hooked up the LED in a similar configuration but instead of the LT1302-5 I used an LM7805 with power from a wall adapter, what issues could I run into?

In that light, when is the constant current device overkill?

[Reply #1 on:]

I'm designing a circuit that powers a 1W 'Luxeon' style high-power LED from an LT1302 that boosts a small voltage to 5v (this is required for other reasons I won't go into).

Boosting the small voltage to a large voltage results in a smaller current. Hardly the approach needed to drive a constant (high) current LED.

An LED looks like a short circuit to the power supply. A constant current power supply means that the LED won't suck all available current. Remember, current is pulled, not pushed.

[Reply #2 on:]

Indeed, those properties are inversely proportional, however, the DC/DC booster is rated to supply the necessary current I need.

I did some more research and wikipedia had some interesting insights: http://en.wikipedia.org/wiki/LED_circuit

If I understand what the author is saying correctly I am able to use a constant-VOLTAGE device successfully (as opposed to a constant-CURRENT device) but more care will need to be taken to ensure the current is stabilized, specifically with resistors.

In the end I would prefer not to add the cost of a LM317 (which I can't use anyways because of the high dropout) but I may opt for something like the LM2941 which allows me to remove the transistor I'm using to enable/disable the light.

[Reply #3 on:]

In that light, when is the constant current device overkill?

I consider constant current drive for leds only overkill when working with standard 20ma type leds as simple current limiting resistors with a constant voltage source is much cheaper and the normal device variations will not result in device damage.

 When dealing with 'power' leds, those rated at 1 watt or higher, then constant current drive is the best method if you wish to use the leds at their rated maximum output. Proper heatsinking and controlled constant current is the only way to insure against device destruction when using 'power' leds.

Lefty