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Tlc5940: Outputs in parallel together with one MOSFET for high voltage AND high current

Logan Logan
Prodigy 20 points
Part Number: TLC5940

I'm looking to use the TLC5940 to drive a 9W 12V RGB LED. While 12V supply itself would not require the MOSFETs in this design, I would like to use them to offload heat.

The LED needs around 250 mA per color channel, too much for a single output.

Even with 3-4 outputs in parallel, the heat load on the TLC5940 will be too high. Forward voltages are 5.5V for red, 7V for green, and 8V for blue. My rough calculations puts this at up to 3.5W for the TLC5940 when used on its own.

Can I achieve good operation by connecting 3-4 outputs in parallel to a single N-MOSFET, to reduce the Vout while providing a higher current?

Or would I need to use one MOSFET per output?

  • 0
    TI__Genius 14295 points
    Hi, Logan,

    Could you help to check below questions for our better understanding on your confusion:
    1. You're using TLC5940 to drive only one LED?
    2. Is it possibile to drop 12V to 9V?
    3. Have you considered using resistors instead of MOSFET?

    Thanks.
    Regards,
    Kenneth
  • 0 in reply to Kenneth Du
    Prodigy 20 points

    Kenneth, I'll do my best to answer your questions.

    1. It's one RGB LED, so I need to drive three channels. I would like to have a constant current driver with PWM control. I have a few unused TLC5940.

    2. The RGB LED is specified for 12V so I don't know if that would be a good idea. I also don't have as easy access to 9V as to 12V to drive the LED. While it may work for the red and green channels, it may work less well for blue. The RGB LED is common anode.

    3. Intressting idea, I haven't thought of this. I mainly only have 1/4W resistors on hand, but I guess that would work if I use a bunch of them. I do also have a few 10 Ohm 3W resistors, but I'm not sure if I can use these.

    For the resistor option, I have on the lower end some of 22, 39, 47, or 68 Ohm 1/4W resistors which may be suitable.

    If I for example push up to 57 mA per channel from the TLC5940 (using a 680 Ohm reference resistor), and each channel is connected in series with a 47 Ohm resistor, that resistor would drop the voltage a further 2.679 V and need to dissipate 0.1527 W at 100% duty cycle.

    With this solution I could use 4 channels from the TLC5940 for each color. For blue and green I could use 39 Ohm resistors in series to drop the voltage by 1.25 V. For red, which has an even lower forward voltage, I can use one each of 39 and 47 Ohm resistors.

    Could I even use two large 10 Ohm resistors in series connected to all four TLC5940 channels for a color, for a 2*2.28V drop?

    Using the approximate forward voltage of the red color:

    Red, option 1: 6.5V after LED, minus 4.902V (47+39 Ohm resistors, for up to 2.679+2.223 V drop), leaves approximately 1.6V at the TLC5940. For four channels at 57 mA it comes out to approximately 0.37W.

    Red, option 2: 6.5V after LED, minus 4.56V (10+10 Ohm resistors in series, to all four channels, for up to 2.28+2.28 V drop), leaves approximately 1.94V at the TLC5940. For four channels at 57 mA it comes out to approximately 0.45W.

    Am I understanding this correctly?

  • 0 in reply to Logan Logan
    TI__Genius 14295 points
    Hi, Logan,

    I'm OK with your calculation and understanding.

    Regards,
    Kenneth