I'm using the TLC5940 as a control source for a Luxeon.
This is the circuit I've come up with:
I've breadboarded and it seems to work just fine (I think the 220[ch937] values on R4/5 should be lower though, it doesn't leave much voltage across R2).
The right hand part of the circuit comes from the Circuits for using High Power LED's Instructable http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/
. The left hand part (Q3, Q4, R4, R5) is a current mirror that was conceived with help from the Eng-Tips forums http://www.eng-tips.com/viewthread.cfm?qid=268810
. The challenge here was to separate the PWM part of the TLC's functionality from the constant-current part, and this was achieved by mirroring the current. R2 converts the mirrored current into a ~+5V voltage input for the Instructable circuit.
All grounds should be connected together, including the TLC5940's. +V1 should be 5V, +V2 can be anything up to the maximum that the MOSFET Q2 can take. For more info on Q2 and for setting R3 see the Instructable.
Thoughts? I know it's a bit more elaborate than it could be, but I needed to be able to run the other TLC5940 pins at ~20mA for ordinary LEDs, and I couldn't figure out how to set the current mirror ratio reliably so just left it at 1:1.