TLC5941 with Parallel CA leds

Pete Dudash

Other Parts Discussed in Thread: TLC5941

I made some area lighting by cascading three TLC5941s and using common-anode RGB leds. In order to produce more light in different directions, I connected 4 leds in parallel. I connected the common-anode to 5V, and connected all reds together, greens together, and blues together. Then the red went to one TLC5941 output pin, and green to another output, and blue to another output. The IREF pin is connected to a 1K resistor to ground, and my dot correction is set to 63 (39mA).

I have been having a few issues with leds not working all the time. For example, individually all R, G, and B work. But when I try to make yellow, one led is green but the other three are yellow - the red doesn't work for that particular led. I even had one led quit working altogether.

My question is, what is the proper way to connect CA RGB leds in parallel? Do I need to add a series resistor to each leg of each RGB led? Do I need to adjust the dot correction to different values for the red, green, and blue leds?

over 10 years ago

0 Mike(DC/DC) Wang over 10 years ago

TI__Intellectual 1820 points

Hello,

Yes, if 4 leds connected in parallel, one ballast resistor is required in series with each leg of LEDs. Since you are control the sum of four LEDs current, the difference of forward voltage of each led may cause some led turn on, and some still be off state.

Regards

Mike

0 Pete Dudash over 10 years ago in reply to Mike(DC/DC) Wang

Prodigy 60 points

That makes sense. I was thinking that since the TLC is a constant current device, it would be okay. But I didn't account for slight variations in the forward voltages. For anyone else curious, this is my derivation of the resistor values:

LED Specs	Current	Fwd Voltage	Resistor
R	20 mA	2V	(5-2)/0.02 = 150Ω
G	20 mA	3.2V	90Ω
B	20 mA	3.2V	90Ω

The TLC can output a max of 80mA per output. Since I have 4 leds per output, each led can have 20mA. I subtract the Vfw from Vcc (5V) to get the voltage drop across each resistor. Then I divide by the current to get the resistor value.

It was a very obvious oversight. Thanks for clearing that up for me!

0 Mike(DC/DC) Wang over 10 years ago in reply to Pete Dudash

TI__Intellectual 1820 points

Hello,

I think the resistor should be a little lower, since tlc5941 need some voltage(~0.8V) at output pins to maintain a constant sink current.

E.g, for red led, (5-2-0.8)/0.02=110ohm

Thanks

Mike

0 Pete Dudash over 10 years ago in reply to Mike(DC/DC) Wang

Prodigy 60 points

Thanks for the tip. That's a piece of important info when designing with these ICs. I must have missed that in the datasheet. Could you tell me where you found that?

0 Mike(DC/DC) Wang over 10 years ago in reply to Pete Dudash

TI__Intellectual 1820 points

Hello,

It's in fig5, page10 of datasheet: Output current VS Output voltage curve.

Mike

0 Pete Dudash over 10 years ago in reply to Mike(DC/DC) Wang

Prodigy 60 points

Thank you very much for your help. I am sure adding resistors to each leg will resolve my issues. And thanks for pointing me to the diagram in the datasheet.

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TLC5941 with Parallel CA leds