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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.

  5. #5
    Flashaholic
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    Default Re: LED Drivers IC: Sink current using PCA9685?

    I suppose you could bypass on-board resistors and use off-board ones. I suggest not blobbing solder over the resistors, just short each out with a fine solid or stranded wire e.g. #30. This mod is reversible if need be.

    You can drive MOSFETS, or biploar PNP or NPN transistors for higher current, up to amps per LED if needed. Exact circuit depends on what you want, each has its own requirements.

    You will still need series resistors with LEDs with external driver unless it is constant-current, which is more complexity.


    Dave

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

    Assuming this is the board we're talking about, I see what looks like 220 ohm resistors in series with the PWM outputs. That would explain your lower than expected sink current. If you placed a 330 ohm resistor in parallel with the 220 ohm one you'd get around 130 ohms which is close to Dave H's suggested value.

  7. #7
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    Default Re: LED Drivers IC: Sink current using PCA9685?

    By my calculation, 220 ohms would give around 12mA which was the case.

    Series resistors are tiny quad SOIC packs with close pin spacing...will depend on OP's soldering skills and facilities, and components on hand. By the look of it the pins are spaced less than 1mm .

    To change them on board would require removal (hot air) and either replacement with a similar part with same footprint, or small chip resistors; a lot of work. Or, you could solder a 330 ohm part directly on top, which is tricky.

    I my case doing soldering of this type it was using a microscope or at least large magnifier, #30 wire, tweezers, very fine solder, and fine-tipped soldering iron (or two) at 600-700F. You might get away with less.

    Dave

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