I tried it, and it doesn´t work.
The vref out is mainly used together with a switch or potentiometer, as it only outputs 20mA.
Do you have this working on your end?
AndrewStone:
Often constant current drivers cannot pull down as far as a transistor. I have had similar problems with the M5451 chip. Check out the TLC5940 datasheet (or test it with a multimeter) to see how far it can pull. If it cannot pull down far enough it won't register as a logic 0. A solution is to feed the TLC into the base of a PNP transistor to get a hard pull down.
I am probably misunderstanding something, but since I am able to dim normal LEDs, as well as powerLEDs (using a circuit based on the LM317 chip which also uses a pull-up resistor), I would expect it to be able to pull down far enough?