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LM3414: Risk of latchup from DIM pin pullup?

David Dean
Expert 2315 points
Part Number: LM3414

I am using the LM3414 in a system where the LM3414 is powered by the system input voltage, and the DIM pin is cconnected to a microcontroller which is powered by a switching regulator.  The LM3414 has a weak pullup on DIM up to VCC, which may be active when power to the microcontroller is not yet established.  Basically - the system voltage will rise above the LM3414 UVLO before the switching regulator is enabled so there will be some period where the DIM pin will be putting a non-zero voltage on a microcontroller pin when VCC is zero.

In general, is there a risk that this could harm the microcontroller, either by causing latchup or any other issues?  Is the 30uA pullup so little current that it will not do any harm?  Or would you recommend having some sort of protection against this scenario?

  • 0
    TI__Mastermind 28655 points
    Hello,

    This is more related to the micro used and if it can handle 30 uA pull up on the pin being used. If you are driving that pin high in your application you could also add a pull down resistor to limit the voltage when there is no load on the pin.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Expert 2315 points
    Yes, I acknowledge that it's more of a question on the micro side, so I guess it's more of a question of "how do you usually see the LM3414 used". But the pulldown seems reasonable - 10KΩ would limit voltage on the pin to 0.3V when the micro is unpowered, and add a load of 330uA when the pin is driven high @ 3.3V. Not ideal, but better safe than sorry, I guess.

    Maybe you can give me advice on the micro question? I've read conflicting information about injection current and latchup - some sources say "limit it to 1mA and you'll be fine", others say "never go above zero or your microcontroller will explode".
  • 0 in reply to David Dean
    TI__Mastermind 28655 points
    Hello,

    It really depends on the pin you're dealing with. I would find out from the manufacturer. You may even find it in the datasheet. Some pins will also hold low when first powering on to prevent issues.

    Regards,