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Thread: LED Drivers IC: Sink current using PCA9685?

  1. #1

    Help LED Drivers IC: Sink current using PCA9685?

    Hi, all
    I'm using a PCA9685 with a pinout board like Adafruit's. I'm trying to run a bunch of LEDs at 20 mA, and the chip is supposed to be able to source 10 mA and sink 25 mA. I understand that to have it Source, connect the LED between the PWM and GND pins and sink it between V+ and PWM.
    I've tried both ways and using a multimeter and a red LED I get 12.5 mA. Why is it not 25 in Sink? Also, why do I get 12.5 on Source mode as well?

    Is there any idea of it? so if possible a simple explanation would be nice, thanks in advance.

    Enclose PCA9685 data source as Reference: https://www.apogeeweb.net/productdet...W-118-t5Bl0pmz

  2. #2
    Flashaholic
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    Default Re: LED Drivers IC: Sink current using PCA9685?

    What value of series resistors are you using? If you're not, they need to be there. Driver outputs can sink up to 25mA but they are not constant-current outputs as with some chips. See page 29 of the datasheet.

    Also, what is supply voltage to the chip and LEDs?

    Dave

  3. #3
    Flashaholic
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    Default Re: LED Drivers IC: Sink current using PCA9685?

    With 5v supply and a red LED, series resistor would be approximately:

    (5.0 - 1.8 - 0.5)/0.020 = 135 ohms. Anything close should work.


    Dave

  4. #4

    Default Re: LED Drivers IC: Sink current using PCA9685?

    Thanks for all the responses guys, they've been super helpful!

    My intention for this project was to use the datasheets' values at 20 mA as that's their typical test point. So without getting extra FETs, this is the highest current I can get from each pin? It sounds like I should be sure at least to run on current sink mode to not damage my chip then.


    So the chip on itself has no resistors which I need for my current project, but it's too small to actually prototype with and I don't know how to code with I2C, so I got the Adafruit version that's a breakout board with libraries. But this one already has resistors on it and won't let me reach above a certain current depending on that resistor. Would it be a totally stupid idea to try and put solder over the resistors on the breakout to create a short, and put my own resistor on the prototype board to achieve the current I'm looking for? Or how would I go about using FETs to increase current, please? Sorry if these are stupid questions.

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