TLC5940, multiplexed RGB LEDs (10x16), and transistor selection

Other Hardware LEDs and Multiplexing
1

Hey all.

I've searched around the forums, but can't really get a good idea of what to do in terms driving the Common Anode LEDs to achieve a proper multiplexed RGB LED matrix.

I'm designing a PCB with an ATMega328 on it (basically an arduino clone) with three TLC5940's on it in order to create an RGB LED matrix. I've gotten the wiring of the TLC's down pretty well in my head (have yet to order them), but one thing I'm trying to figure out is how to drive a transistor array in order to create the proper matrix effect. Some of the RGB LED controllers I've seen produced by Seeedstudio, ITeadstudio, etc. all seem to use a a single-chip transistor array (Darlington) to drive the outputs, but, of course, their chips are next to impossible to find.

I want the TLC5940 to drive the Red/Green/Blue cathodes with a transistor array driving the anodes, with at least a 8 row/16 column, or ideally a 10 row/16 column capability.

So here's a breakdown of my questions:

  1. Is what I'm trying to do possible with the TLC5940's, in terms of sinking the R/G/B cathodes?
  2. If so, what kind of transistor array should I use to drive the common anodes? How should I time it with the TLC5940's to work properly? Is there any other component I'll need to make them jive together?
  3. Most importantly... is that chip available from Digikey?

I'm a relative newbie when it comes to these things, and multiplexing both makes complete sense and is completely lost on me at the same time.

Thanks in advance for your help... if you need clarification or schematics, I can try to provide. :slight_smile:

2
  1. Is what I'm trying to do possible with the TLC5940's, in terms of sinking the R/G/B cathodes?

Yes see my projects:-
http://www.thebox.myzen.co.uk/Hardware/Mini_Monome.html
and
http://www.thebox.myzen.co.uk/Hardware/Hexome.html

  1. If so, what kind of transistor array should I use to drive the common anodes?

I used p-channel FETs but you can use PNP transistors if you like.

How should I time it with the TLC5940's to work properly?

You build the multiplexing into the refresh software to drive the TLC5940.

Most importantly... is that chip available from Digikey?

Yes they have transistors and FETs, lots of them in fact.

3

Grumpy_Mike:

  1. If so, what kind of transistor array should I use to drive the common anodes?

I used p-channel FETs but you can use PNP transistors if you like.

Alright... so I'd want ten P-channel FETs that could drive around 150mA per channel? The LEDs are rated at 20mA max amperage, but I'm assuming that I wont be running all of them at full blast all of the time. How would I trigger the FETs? With a shift register like a 595?

Will this setup be fairly bright?

Thanks for the answers. :slight_smile:

4

You want the FET to take the current for all the LEDs on at the same time, that is your design figure. Most FETs will do this.
Yes you can turn the FETs on with a shift register or demultiplexer or port expander or anything else you like.
The brightness will depend on the current and the ratio of on to off time in the multiplexing, you can make it brighter by putting more than 20mA through the LEDs. This is ok because they are being pulsed, look at the pulsed rating of you LEDs it is normally five to ten times the continous rating.