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TLC5941: output voltage during ON state

Rene Wassenburg71
Prodigy 50 points
Part Number: TLC5941

Hi there,

I am working on a design where i need 16 PWM signals. For this I use the TLC5941.

Instead of driving LEDs, I have connected a resistor R to every output, which are then connected to 3.3V.

The outputs of the TLC5941 are also connected to 16 (logic level) N-channel mosfets.
This results in 16 mosfets with individual pull up resistor R on the gate, which can be individually pulled low by the outputs of the TLC chip. The mosfet acts like an inverter and I have power circuitry behind it. This part is not relevant for this discussion.

I understand the TLC is intended for regulating current through LEDs, so I ‘tricked’ the IC as following:

I have set the output current of the TLC outputs to 5mA (Rset = 10k). The 16 'load' resistors are 1k each.
With 1k resistors and 3.3V power supply, the output current per channel can’t exceed 3.3mA.

According to figure 5, page 10 of dataheet, I can expect an output voltage of ~0V when the output is switched on.
In other words, I can switch the mosfets on/off with these signals.

I have built up my prototype and this works great.
However…after building up 10 similar circuits I start to see that in some circuits 1 or 2 outputs of the TLC do not switch to 0V…
On these ‘faulty’ outputs I measure Vout(low) = 1.5V or even higher, which results in a mosfet being permanent on.

This is the part which I do not understand.

So..I have 3 questions:
1) How can Vout(low) be greater than 1.5V in this case?
2) do I misunderstand figure 5 of the DS?
3) I assume the outputs are current limited and not current regulated. Is this actually true?

I hope to get an answer soon.
Thanks 

  • 0
    TI__Expert 3190 points

    Hi, Rene,

    (1) The voltage should be similar with the curve below. If you use 3.3V LED power with 5mA constant current, the output current cannot achieve 5mA, so the output voltage should be smaller than the knee voltage 0.5V, which should be meet the shutdown requirement of the MOSFET.

    (2) Yes, you are right, the large resister connected to 3.3V VLED, the lower output voltage.

    (3) Yes, you are right, the output current are limited as the curve below.

    Let us know if you have more questions, thanks!

    Best regards!

    Shawn

  • 0 in reply to Shawn Ding
    Prodigy 50 points
    Thanks for your reaction Shawn,

    I am not able to see the image you posted, it looks like a broken JPG.
    Of course I am still interested in the image.

    Nevertheless, I am happy that you agree with my design approach, and the fact that this setup should work.
    However, as I wrote before, on some outputs it does not work and I get output voltages up to 2V instead of ~0V.

    So, I tried decreasing my current set resistor. This did the trick. The original value was 10k; I replaced it with 2k.
    In terms of concept nothing changes, I just increased the current limit per input from 5mA to approx 15mA.
    Now the 'malfunctioning' outputs behave correct as well.

    So..I have two new questions:
    1) What is the maximum value for the current set resistor?
    2) Is my initial choice of 10k actually out of spec?

    I am curious for the answer...
    Thank you in advance,
    Rene
  • 0 in reply to Rene Wassenburg71
    TI__Expert 3190 points
    Hi Rene,

    1) The Imax must be set between 5 mA and 80 mA. The output current may be unstable if Imax is set lower than 5mA. that maybe the reason that the output voltage is wrong.

    2)And from the equation 6 (page 16), the range of the resistor is from 488 ohm to 7.812k ohm.

    Thanks and best regards!

    Shawn.
  • 0 in reply to Shawn Ding
    Prodigy 50 points
    Hi Shawn,
    This explains my findings. I was obviously working out of spec for the current set resistor.
    Now that it's set to 2k, the issue is resolved.

    Thanks for helping me understand this behaviour!
    Rene
  • 0 in reply to Rene Wassenburg71
    TI__Expert 3190 points
    My Pleasure, Rene!

    Shawn