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Thread: WS2812B High Side Switching (IRF9350 replacement)

  1. #1
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    WS2812B High Side Switching (IRF9350 replacement)

    Hi,

    I'm working on something at the moment, which will be using 16 WS2812B LEDs, running from a battery. As such, keeping power consumption down is a priority.
    Assuming the WS2812b's are susceptible to the power draw shown in the 2811's when idle (https://www.pjrc.com/teensy/td_libs_OctoWS2811.html toward the bottom of the page), I'll want to be able to switch power off to these entirely.

    I'm following the given circuit example, but as this project is to go on PCB, I'm intending on keeping it all SMT and need a replacement fo the IRF9350 shown in the example. Some of the specifications conufse me a little though. I'm just getting to grips with n-types.

    I've got a few questions:

    1) Would something like this be suitable? http://uk.farnell.com/nxp/nx2301p/mo...t23/dp/1894738
    Otherwise, can anyone recommend a suitable replacement MOSFET?

    2) In the circuit diagram shown, I suppose that whilst the circuit is on (LEDs are powered) , I'll be sinking 5v / 1k = 5mA of current straight to ground. Am I free to increase that resistor to something more like 10 or 20k in order to decrease power consumption?

    3) Lastly, can anyone explain why values for drain current, threshold voltage etc, often seem to be given negatively for p-types, but not always?

    Thanks in advance,
    Iestyn.
    Last edited by Cosford; 04-06-2015 at 03:27 PM.

  2. #2
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    Unfortunately I can't speak too much about the properties of MOSFETS…my knowledge of the underpinnings of circuitry is pretty limited, but from what I can tell the MOSFET you linked isn't logic level so I don't think it will work for your application.

    Here's a slice of a circuit I did a while ago to do the same thing you're trying to do. A Teensy pin controls a small transistor (a MMBT3904LT3G) to convert the Teensy's 3.3V signal to 5V, then the transistor controls a logic level P-channel MOSFET (an AOD4185) that supplies the power to the LEDs. The MOSFET I used can switch a lot of Amps (my LED matrix was 24x16 so I needed something fairly big) so it's probably overkill for your application, but hopefully it can get you started in the right direction.

    Click image for larger version. 

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  3. #3
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    Thanks for your reply.

    From what I can tell, the NX2310P I linked has a Vgs of -4.5v. My understanding (may be wrong) is that this means the mosfet will let current flow through it if the voltage presented to the gate is more than 4.5v less than the source? With the circuit diagram below, when pulling the control pin high, the N-channel transistor should pull the gate to ground, and hence the Vgs would be -5v, meeting the requirement for the MOSFET?

    Click image for larger version. 

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  4. #4
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by Cosford View Post
    1) Would something like this be suitable? http://uk.farnell.com/nxp/nx2301p/mo...t23/dp/1894738
    Looks like it ought to work, but not a huge amount of design marging. 16 LEDs can draw about 0.9 amps if all are on fully white. That's about half of its spec. It's on resistance is 0.1 ohm, so power (I*I*R) would be 81 mW, which is a bit much but not too bad for SOT-23 package.

    2) In the circuit diagram shown, I suppose that whilst the circuit is on (LEDs are powered) , I'll be sinking 5v / 1k = 5mA of current straight to ground. Am I free to increase that resistor to something more like 10 or 20k in order to decrease power consumption?
    20K is probably fine.

    It will take time for the mosfet gate voltage to rise, and the LEDs to consume energy stored in the decoupling capacitors. Not much time, but Teensy is very fast. You might put some code in to make sure the LEDs stay off for some minimum length of time before you try turning them back on.

    3) Lastly, can anyone explain why values for drain current, threshold voltage etc, often seem to be given negatively for p-types, but not always?
    Different manufacturers follow different documentation conventions and practices, even when they're making virtually identical products.

  5. #5
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    Great. Thanks for that.

    A quick search turned up this part too; http://uk.farnell.com/taiwan-semicon...t23/dp/1864586

    Up to 4A, and with 0.045Ohm on resistance, should give plenty of headroom and reduce power consumed on it to 36mW. That sounds better?

  6. #6
    Hello everyone. First of all tell that I am a complete novice in electronics, so sorry for the possible nonsense.

    I'm trying to build one of those Teensy powered LED strips system, with 100 LEDs or so. The device is powered by battery and would like to feed the LED strips directly with the voltage of a Lipo and use only a little Step-Up converter (Lipo>>5V) to power Teensy.

    To stop the residual consumption of the WS2812, I would like to have one of these MOSFET On/Off switches to turn off the stripes when not in use.

    However I find it difficult to find a suitable, high amperage, through hole mounting, ultra-low “logic” VGS(threshold), MOSFET in the channel P flavor.

    Could a more conventional “low-side channel N switch” configuration work? Why all the circuit examples use a high-side switch?

    Could the circuit in the drawing work? (My plan is to turning On/Off the stripes with output Low/input high impedance in GPIOA)

    ¿Can something explode if I, by accident, send data to the WS2812 through the green line in the picture when Q2 is off?

    Are there any examples of a simplistic, suitable for beginners, switch circuit but using the most readily available Channel N, through hole, MOSFET?
    Thanks in advance

    Click image for larger version. 

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