*HELP* TLC5940 Driving both Common Anode / Common Cathode Leds on same configuration.

Thread Starter

Federico Cardana

Joined Nov 10, 2017
4
Hello Everyone,
first of all I wanted to thank you all for your help and time responding to my help request.

The situation:

I'm building a LED interface testing boards. And I have to test LED boars that can have various numbers of LED ( from 4 to 25 ) so I thought to use the following structure:

Arduino --> TLC5940s ( multiple chip chained to reach enough lines to drive Leds. In my case this number is 25 so I will connect 2 chips to have 32 lines [16 +16 ] --> Led lines.

The Problem:

TLC5940 is a chip that drives Leds with constant current sinking which works for common anode Leds but I have to test boards with common cathode leds and I have to test some boars with both CA Leds and CC mixed configuration boards.



Is it possible to create this solution?

I would need a configuration with Arduino + 2 chained TLC5940 chips that on each output line can drive both CA Leds and CC Leds on same configuration and I can program the chip base on my needs when I connect different boards to be tested by the system.

I know I need some hardware design to implement in the system and I would like to know how to do it.

These are the three configurations I need:

Common Anode:

Common-Anode.jpg

Common Cathode:

Common-Cathode.jpg

Mixed configuration: CC & CA

Mixed-Configuration.jpg


Thank you all so much for helping me out with this!

Best,
Federico





 

crutschow

Joined Mar 14, 2008
23,494
Bad news. :(
If you have both common-anode and common-cathode LED arrays, I believe you will have to design two constant-current circuits.
One that can sink multiple currents and one that can source multiple currents.
 

Thread Starter

Federico Cardana

Joined Nov 10, 2017
4
Hi, Crutschow, thank you so much for your help.

So, I thought to re-design the system like this. Please, tell me if you think I'm on the right path to make it work as I wished for.

Arduino ==> TLCs ( 4 total ) coupled in chain 2 X 2. So I have two Arrays of 25 output lines each: 1 for CA and 1 for CC Leds.

I read a post about scouring constant current from output lines of an TLC5940:
https://forum.allaboutcircuits.com/threads/tlc5940nt-alternative-but-with-current-sourcing.22776/

The say it is needed to use NPN + resistor network for each output lines. ( In my design I put a red circle where I'll insert transistors + resistors ).

So, now I think to create the following circuit.

Do you think it is a good solution for my purpose ?

Thank you!

Federico

Screen Shot 2017-11-11 at 5.42.27 PM.png
 
Last edited:

crutschow

Joined Mar 14, 2008
23,494
If you want the output source current to match the sink current from the TLC5940 I suggest you use a current mirror circuit (below) that requires just one component (a dual transistor).
As you can see from the LTspice simulation, the LED (D1) current closely follows the TLC simulated sink current to ground (I1).
(The bottom plot shows the two traces overlaid).

For good accuracy, a current mirror requires matched transistor so I used the dual transistor DMMT3906W-7-F (which is basically two 2N3906 transistors on one chip) and can be purchased here for U$0.37.

upload_2017-11-11_11-3-20.png
 
Last edited:

Thread Starter

Federico Cardana

Joined Nov 10, 2017
4
Thank You! So the solution for TLC5940 circuit to drive CC Leds is the following right?

Is not needed any resistors in the circuit you posted? Can i regulate maximum output current with the value of R-Iref connected to the chip ?

And If I need 16 output lines from single TCL5940 chips I have to use 16 DMMT3906W-7-F, one for each outputs, right?

Thank You,
Federico

FullSizeRender.jpg
 

crutschow

Joined Mar 14, 2008
23,494
So the solution for TLC5940 circuit to drive CC Leds is the following right?
Right.
Is not needed any resistors in the circuit you posted? Can i regulate maximum output current with the value of R-Iref connected to the chip ?
No resistors needed.
Yes the LED current will be essentially the same as the source current from the TLC.
And If I need 16 output lines from single TCL5940 chips I have to use 16 DMMT3906W-7-F, one for each outputs, right?
Right.
 

Thread Starter

Federico Cardana

Joined Nov 10, 2017
4
Last question: In the case I mentioned in my first thread about testing mixed board with CC & CA leds, I will connect CA Leds to the outputs array from TCL standard configuration and CC leds to array outputs from the last configuration I designed with Dual NPN Transistor chips.

And the two arrays will share same VCC and Same GND. Right?

Thank you so much for your time and your help!

I really appreciated it and I hope I will give back someone else in the forum with my help :)

Best,
Federico
 
Top