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LM3410: Physical disconnection high and return/feedback side connections

Jared Wolff
Prodigy 120 points
Part Number: LM3410


Hey Ti Folks,

I'm working on a design that is a bit unique. Right now, we have the LM3410 situated on an MLB. There are several strings of LEDs in parallel. Half of those strings can be physically disconnected as part of the design of the product. So, VCC_LED_WARM and VCC_LED_WARM_RET are disconnected from half of the LEDs depending on the state of the device. See below for my references.

I've added both 12V TVS on the positive and 5V TVS on the return. I was mainly concerned about the 3V limit on the FB pin so I added an additional Zener that could handle the max power seen during an ESD event.   (not handled by the already existing TVS diode)

I do have full control over the LED driver and I know when the device is disconnected (as that could cause some potential damage). We are driving the LEDs at a fraction of what their maximum currents are so I'm less worried about that. 


I know it's out of the scope of a typical design, but I wanted to make sure there weren't any gotchas. We are running 40 strings total at 10mA each. 

Thanks!

Jared

www.jaredwolff.com

  • 0
    TI__Mastermind 28655 points
    Hello,

    If half of the strings are disconnected the remaining half are connected? If so the current will just go through those strings instead. Are you concerned about reconnecting the LED string and what it does to FB? You can clamp FB with a diode to ground and a zener as well. It needs to have no current flow at 0.190V which is what it's trying to regulate at. How are you connecting and disconnecting the LED string? Are you reducing the current when half of the LEDs are removed? Just trying to see what your application is doing.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 120 points

    Hey Irwin

    One half does remain connected. When it is disconnected, the device is not usable. Only when both "pieces" are connected would the firmware ever switch on the LED driver.

    I'm mostly concerned that

    #1 we're doing something outside the norm for the driver
    #2 we're exposing the top and fb pins to the outside world (potential ESD, etc)

    The halves are connected via a male/female magnetic connector.

    Cheers

    Jared

  • 0 in reply to Jared Wolff
    TI__Mastermind 28655 points
    Hello Jared,

    This is out of the norm for a design that I have seen. I would need to see a lot more detail on what you are doing to understand.

    If you are exposing the FB pin to the outside world I would definitely protect it. You may even want to have a way to make sure any built up charge is dissipated before reconnecting. You may also want to have a ceramic capacitor across the electrolytic for added ESD protection. This design requirement should make sure that none of the pins exceed ABS MAX ratings. There is test equipment that can be purchased or rented to simulate ESD discharge that you could use to test your design when you come up with a solution.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 120 points

    I'd love to share more information. Unfortunately it's a public forum. So that's restricting me a bit.

    You may even want to have a way to make sure any built up charge is dissipated before reconnecting.

    On the feedback portion of the circuit? I don't have any control on the "dead"/"disconnected" side. I'm thinking a MOSFET that's enabled until the driver itself is turned on.

    You may also want to have a ceramic capacitor across the electrolytic for added ESD protection.


    I have an electrolytic at the input. Is that where you're suggesting? (C18 on the diagram I originally attached) Or do you mean on VCC_LED_WARM_RET signal? (or on both VCC_LED_WARM_RET and VCC_LED_WARM?)

    Thanks for all your assistance on this!

  • 0 in reply to Jared Wolff
    TI__Mastermind 28655 points
    Hello,

    An electrolytic capacitor will not protect against ESD, I would add a small ceramic in parallel with it. Diode clamps can work instead of the MOSFET as well. Feedback is 0.19V so you don't want any impedance around that voltage (over all operating conditions including temperature. I meant on the dead piece. If it could discharge through a safe path as it was approaching the board.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 120 points
    I'm still confused where you're referring to place the ceramic cap. At the input of the main inductor? I'm not sure how that helps with ESD.

    As for the clamp, got it. As mentioned I already have a 2V zener on the main board side. I can certainly add one to the board that gets disconnected.
  • 0 in reply to Jared Wolff
    TI__Mastermind 28655 points
    Hello,

    Across C19, it may not be necessary but an electrolytic capacitor will not help with ESD.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 120 points
    Ahh gotcha. C19 is ceramic 33 uF. I'll add a few more footprints there for good measure.