Hi.
Quick question. If I connect the arduino to 1 TLC5940, and then connect the TLC5940 to 2 ULN2803, will I be able to control 16 channels using pwm? I am considering using this setup to control 5 3w rgb leds (potentially more, by daisy chaining other TLC5940s), but have no idea if this will work...
In other words, the TLC5940's outputs have 20ma constant current, is this any good to control the darlington transistors in the ULN2803? Will I be getting pwm in the ULN2803's outputs?
Thanks! 
It should work, but the TLC5940 has open-collector outputs, so you'll need to add pull-up resistors to the ULN2803 inputs. Which is easy, since they're so handily arranged: just put a 9-pin SIP resistor network next to the inputs, and tie the "common" pin to Vcc.
One word of caution: if you're going to have a lot of the LEDs on at high brightness for a high percentage of the time, you need to check the ULN2803's "total package" rating. Even though it's rated at something like 600mA per channel, you're not supposed to have all 8 channels running at full power simultaneously.
Ran
Even though it's rated at something like 600mA per channel,
It's actually 500mA per channel but the power dissipation means that that the total current being drawn at any one time on a single chip is restricted to about 650mA.
The advantage that the 5940 has, with constant current outputs, is negated by using a Darlington as an output driver.
You could address the power dissipation problem with a heatsink, but you'd (quickly?? eventually??) run into a problem because there's only one ground pin to sink all the current through. And, probably, a limited path through the silicon that could get zapped by a peak even if the average current was very low.
The constant-current driver is only one "advantage" of the TLC5940: for faxi's application, it acts as a handy source of many independent PWM outputs.
Ran