TLC5947: Power management forum

Luís Filipe Ferreira Pereira

Part Number: TLC5947
Other Parts Discussed in Thread: LP5024, LP5867, LP5811, LP5810, LP5866T

I need to control 24 LED's intensity individually, TLC5947 seemed a good ideia but later on I found out some LED's take over 30mA (50mA to be exact).

So I thought about adding a mosfet and even a PNP transistor to control greater currents.

I set the reference resistor on the TLC5947 to 10K, which limits the current to greater values than my PNP transistor takes to get saturated.

By choosing the LED current limiting resistor carefully to its specific limit, the schematic below seems to be working just fine.

I can control the PWM dutycycle to each PNP and so i get 24 diferent intensities, each LED with a diferent maximum current (we the PWM to be at 100% dutycycle).

But seems quite odd not having a limiting resistor on the PNP base, or a base pullup to ensure to turns off every time.

But the schematic its working fine, there's no current at all flowing if I set a PWM to 0% dutycycle;

And although the 10k Reference resistor could lead to ~5mA on each output of the TLC5947, the fact that I'm limiting the emmiter current, ends up limiting the base current intake as well, in the end I get base currents around 0.2mA so achieve ~25mA on the LED.

What am I missing here? Should this work reliably?

Thank you!

Cheers, Luís Pereira.

2 months ago

0 Jared Zhou 2 months ago

TI__Genius 11970 points

Hi Luís Pereira,

Actually, I am not sure what's your problem here?

But for your application, I think if you only need to control 24 LEDs and need the output current up to 50mA. I think LP5024 maybe a better solution compared with TLC5947.

Of course TLC5947 is a fine solution, but I think adding an external FET might be Redundancy.

BR, Jared

0 Luís Filipe Ferreira Pereira 2 months ago in reply to Jared Zhou

Prodigy 20 points

Thank you for your anwser.

The LP5024 you suggest, has a current limit of 35mA per channel, my LEDs get to 50mA. And if i get it right, if I use any TLC directly (without) a transistor, then my maximum current in any channel will be the same.

In my 24 led matrix, I actually have 6 types of LEDs, with 6 diferent maximum currents, hence 4 LEDs of each kind.

Within each group group of 4 equal LED's i want to have different intensities at same time.

I was looking for ways to avoid the transistors, and I found these options:

LP5867 - 6 channel, limit 50mA per channel (0.3A current total) -> I could use 6 of these (1 per each set of 4 equal LEDs, there would be 2 unused outputs on each TLC5867 =\)
LP5811 - 4 channel, limit 50mA per channel (0.2A current total) -> I could use 6 of these (1 per each set of 4 equal LEDs)

The LP5811 seems promissing, but there's information here that I don't quite get and I fear i might get an IC that can't set a diferent intensity to each output, at same time:

LED configuration: Matrix or paralel?
Topology:Linear or boost?

Can you help me here?

Thank you!

Cheers, Luís Pereira.

0 Jared Zhou 2 months ago in reply to Luís Filipe Ferreira Pereira

TI__Genius 11970 points

Hi Luís Pereira,

Please refer the below comments.

1. For the LP5024, actually the max output current means the recommend operation condition, if you don't care the accuracy or any spec, I think LP5024 can realize it. But this condition (50mA output current) need to be verified by yourself to see if can meet your requirements.

2. For LP5811 and LP5810, I think if you can accept 6pcs LP5811. It will be a vey nice solution. Could you please tell me the max forward voltage of the 6 types of LEDs? Then I can tell that if you should use the linear version LP5810 or boost version LP5811.

3. For the matrix topology, I think our current matrix LP5866T can satisfy your 50mA and 24 LEDs requirements using one chip. LP5867 can't meet the 50mA output current requirement.

If you don't mind, please free to contact me via email (jared-zhou@ti.com) and we can talk about this project in detail, then I can help to find the best TI solution for you. Thanks!

BR, Jared