Picking iREF for TLC5940NT

Hi. Does the resistor I put at IREF depend on the voltage supplied to the LEDs, or should it always be 2K ohms?

I intend to daisy chain 24 TLC5940NT chips to control 128 RGB leds. Right now I’m testing only 3 chips, connected to the arduino’s 5V, and 16 RGB leds connected to an adapter with 12V/5A.
With the the 2K resistors the ICs get extremelly hot! Measured with a multimeter I get total current of around 1A when I set the LEDs to white, which makes sense (16320mA). But if I change the color of some to red, and others to blue, the current shoots to several amperes (instead of dropping). If I use 5.6K resistors the ICs don’t get as hot, but the LEDs lose a lot of brightness…

So… Should IREF be 2K? If so, any idea why the ICs are getting so hot, and why the maximum current at anytime isn’t under 1A? Should I remove the 10K pull-up resistor?
Thanks in advance!

Does the resistor I put at IREF depend on the voltage supplied to the LEDs

No.

Should I remove the 10K pull-up resistor?

What pull up resistor?

But if I change the color of some to red, and others to blue, the current shoots to several amperes (instead of dropping).

That indeed is odd. It could be that the meter does not respond well to the PWM signal. Where are you measuring it? The amount of heat the chip generates does indeed depend on the voltage supplied to the anodes of the LEDs.

Hi Mike, thanks for the fast reply.

Here (http://tlc5940arduino.googlecode.com/svn/wiki/images/breadboard-arduino-tlc5940.png) it is suggested that a 10K resistor should be put between +5V and the BLANK pin (so the LEDs won't blink when the reset is done). I've read elsewhere that this could fry the first IC. That's what I meant by "pull-up" resistor; that's probably not the best way to call it, but I'm not an expert in this electric stuff :)

I've been reading your instructions about decoupling the ICs, and will give that a try tomorrow! I'll put 0.1u F capacitors between the VCC and the ground on each chip, but you also suggest putting some 100u F capacitors every other IC... Where exactly do I plug those? Between the +5V and ground?

I've also calculated that my power dissipation should be a bit over 2.5W (because of the 12V), so I'm thinking about adding 3 or 4K resistors on the IREFs... I can live with the lower brightness, if that will lower the temperature. Lowering the voltage on the LEDs would work too, right? Unfortunately, since I want to connect 128 LEDs, I'll need almost 8A of current, and I don't think I can find a 5V adapter that can deliver that. Right now I intend to test 64 LEDs (3.84A) with my 12V/5A adapter, and if everything works I'll try to find an adapter that outputs enough current...

Thanks again for your help!

Oh, and I measured the current by putting the multimeter in series between +12V and line that feeds all the leds. Current behaves as expected with 5.6K resistors in IREF (max current is achieved when all LEDs are white, and lower when I change colors), but with 2.2K I get that weird behavior where the current goes well over 3A (didn't do much testing cause when I saw the current spiking I was afraid to burn the ICs or the multimeter).

It sounds like you are having stability problems at high currents. This is only to be expected with no supply decoupling capacitors. Things will be more stable with more decoupling. Yes put the large capacitors to power and ground as well. With those sorts of currents I would put a large cap on each chip as well. You can always use two power supplies and split them to halve the LEDs each. Remember to connect the grounds together as well.