TLC5940 High Current Shield Concept

My enthusiasm probably eclipses my ability, but I'm trying to make a stackable HP LED shield that can run (5) RGB Leds. I've read everything I can about this topic, understood about half, and guessed at the rest.

Parts haven't arrived yet to breadboard it, but I figure some keen eyes here might see something obviously wrong.

I'm using hex inverter ic's from the TLC5940 then four ULN2803's. I use only four channels per darlington array so I can safely push 250ma per channel. I'm thinking a different transistor IC might be better that can handle more current? 500ma would be nice.

Ideally, I'd like to be able to stack multiple shields up so the TLC5940's daisy chain together. I'd use an ATX power supply and hopefully be able to run a lot of LED's.

I wonder if I have it hooked up right....

Shouldn't those resistors be pull-ups?

As far as I can remember, the TLC5940 has open drain outputs. So it's either GND (sort of) or "nothing".

Excellent question. I think the 74HC04 has internal pull-up resistors, and I thought it might suffice. The resistors shown were based on comments I gleaned from a similar thread on here. They protect the TLC5940 from noisy things (my description).

I think the 74HC04 has internal pull-up resistors,

No it hasn't. Being TTL unconnected inputs will float high but that is not the same as a pull up resistor as the float time is very large.

Thanks. I've updated the layout with pull up resistors, and I ganged pairs of ULN2803's together so I can get 500ma per channel. This is fun!

I've a question. Do I need the current limiting resistors on the LED cathodes? I would think "yes" but I'm not sure if the chips are implicitly limiting the current themselves.

Otherwise, if I have 12v going to the LED's, I would need a whopper of a resistor. I calculate about 5 watts each.

Well, you will get 'natural' current limiting - by burning the chips and the LEDs :wink:

Compared to the TLC5940, the sink drivers are "dumb" and don't know about constant current operation.