darlingtondriverled
I feel this is going to be a stupid question-
Since the TLC5940 is sinking current, I take it you can't hook up the outputs to the inputs of the ULN2004A to drive something more demanding?
You can build your own common-anode driver out of discretes like this:
A forward-biased diode at the zener (D2) cathode will let you use 1 zener and a bunch of standard silicon diodes for lower cost and easier availability.
Remember that discrete devices are available in arrays, which will greatly simplify breadboard or perfboard construction. However, this will reduce your available power significantly. If you're doing it on a PCB, it's perfectly fine to do it with discretes (as long as you have room.)
With that circuit R1, D2, and Q1 all dissipate minimal power, so can be SOT-23 and 0805/0603, or low power arrays.
Since this is a constant-current sink IC, no resistors are needed in series with the LEDs.
The current is programmed using a reference resistor connected to one of the pins that sets the reference current used to drive the LEDs, and it is the driver chip's responsibility to make sure the correct voltage and current is applied to the LED. In practice, I have found this works well.
You could add a resistor in series, but this defeats the purpose of using this chip. It will increase your part count, increase your minimum VDD requirement, and if you need to increase VDD, then you will waste additional power.
Below is the figure used to determine your reference current for each resistor from the datasheet:
Best Answer
You can use pull-ups on the output of the TLC5940 and this can form a voltage output device depending on what current is programmed for the sink. Say you programmed 10mA and the supply was 5V. If you used a 500 ohm pull-up resistor, the output would be close to 0V i.e. not driving the ULN2004A. If you programmed the current to be 1mA, the output would be about 4.5V and plenty to turn on the ULN2004A.
You've just got to remember the output is inverted.
If you found an PNP darlington array (common point to positive rail) then it wouldn't be inverted.