XCore Exchange

Hi Guys,

I would like some suggestions for a led drive ic or circuit. Basically a buffer that is short tolerant and able to drive a led with 25ma current. Preferably not open collector output as I would like the ability to also do simple RS232 from the same port if possible.

Thanks in Advance

Matt

I've used a simple constant current source like the one here:

http://en.wikipedia.org/wiki/Current_source

in the past. I simulated it in SPICE, and it works quite well: changing the load from 1R to 5R changed the current from 0.045429007A to 0.045409291A, which is negligible. It will protect against a short; the current with a 0.001R load is 0.045433927A.

Here is the SPICE circuit:



You will need to design it for 25 mA, of course; I just set the input at 1.0V for the simulation. I'd use a simple voltage divider rather than a zener to set the base voltage, for your application.

But usually you dont need a constant current source for your LEDs, cause your supply is most likely regulated and you are not driving like 5W power LEDs. So just use 1 resistor and 1 transistor to do it.

And as far as i know a MAX232 takes a open collector signal no problem.

Or, use a 74HC04 as a buffer. You get four of them in the same package.

True, those are very popular. You can also add a cap to pulse the LED. IR LEDs can take up to 1A peak. Good TV remotes discharge a small cap in to the LED to reach such peaks for very short times. It increases the range drastically.

take a look at TV-B-Gone at ladyada's site: http://www.ladyada.net/make/tvbgone/design.html