TLC5940 Vout rating on OUTn pins

[sinns]

Hi, I've recently been doing some work with the TLC5940 and I'm a little confused by the Vout rating of 17Vs in the datasheet...

If I have a bunch of LEDs in series with a Vdrop of 3.1Vs per LED at 20mAs: I supply 24Vs to the positive side of the 7 LEDs in series, I get a total Vdrop of 3.1V X 7 = 21.7V. This means I have 2.3V @ 20mA on the OUT pin, which is easily less than the 17V max.

If I set the channel to 0% duty the result is the same (2.3V at the OUT pin) presumably, since the TLC isn't perfect, there is still a small amount of current present.

I could (by this reckoning) put 300 LEDs in series with a supply voltage of 930Vs, but this screams "You are missing something!". Can I really control 930Vs of LEDs on a signal channel with the TLC in theory?

Question is, what *am* I missing here or am I not missing anything? :smile:

 

[sinns]

Seems I have (I think!) solved this issue. So for the benefit of anyone else who scratched their head over it, here is what I think was missing:

The forward voltage (VF) of an LED is related to the forward current (IF); as the current drops the forward voltage drops too (looking at some data-sheets it seems it drops to a minimum of around 2-2.6V). This is the key missing factor.

When the LED string is on everything is fine and the TLC OUTn pin is within spec (in my example above). The issue is when the LED string is off. It seems that in the off state the channel isn't really off, it is actually current limited to a maximum of 1mA (down from the 20mA on-current). With the drop in current the 'dissipation' of the voltage (through the LED) drops too, this in turn raises the voltage at the OUTn pin.

In my example (with 7 LEDs) the VF would drop to 2.6V per led raising the OUTn voltage to 24 - (2.6 * 7) = 5.8V which would be ok for the TLC5940 (provided 5.8Vs on the channels does not exceed the maximum mW dissipation rating of the chip). In the extreme example of 300 LEDs the 5940 would be turned into popcorn.

So, you can have a higher LED driving voltage than the TLC's rated maximum V on the OUTn pins, but you have to be careful to understand what happens when the channel is 'off' to ensure you have enough headroom in the power so as not to exceed 17Vs. Of course connecting many LEDs in series generates other design issues, but now at least I understand what should happen in theory and perhaps someone else will find this useful.