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LM5121 UVLO

Kai Johnson
Prodigy 75 points


Is it possible/recommended to directly connect the UVLO pin to a microcontroller to switch the regulator ON and OFF, where UVLO = GND means regulator will be disconnected and UVLO > 1.2Vdc would been it is operational?

  • 0
    TI__Intellectual 1745 points
    Kai,

    Your question has been acknowledged, I just want to ensure you get the correct answer from someone more familiar with the part.
  • 0
    TI__Expert 7395 points
    Kai,
    UVLO is not intended to work that way. You should use a resistive divider to UVLO to drop down the voltage from VIN to set the turn-on threshold voltage. This is meant to keep the converter off during low VIN voltages to not overstress components if the design is not required to work at lower VIN.
    To enable with a Microcontroller from a GPIO, I think the best way would be to drive the SS pin low with the MCU GPIO to disable the device. Holding SS low will prevent the device from turning on. To turn-it on if VIN is present, just release the SS(open-drain output floating on GPIO) so that it can ramp up on it's own. The SS has an internal 10uA current source so the GPIO has to be able to sink more than that to over-ride that current source to prevent the SS cap from charging. For fast turn-off, you may need to have the GPIO driving a FET to pull SS low.

    Regards,
    John
  • 0 in reply to John Stevens
    TI__Expert 7395 points
    Kai,
    I talked further with the product team here. You can drive the UVLO as you've described, similar to an Enable pin. Either that or driving SS works to enable/disable with a MCU.
    Regards,
    John
  • 0 in reply to John Stevens
    Prodigy 75 points

    John & Christopher, 

    Greatly appreciate your prompt response! It seemed like this was feasible, but I wanted to double check. Thanks for your help!

  • 0 in reply to Kai Johnson
    TI__Expert 7395 points
    Kai,

    Glad to hear this answered the question. Yes, it sounds like the best method is to use the UVLO like an Enable. Using SS is an option, but more to stop switching as the SS pin can be pulled down by an external switch to shutoff, but you have to float to allow to turn on, which can be difficult. You can't enable by just pulling SS high-that is not allowed.
    UVLO is much easier-pull it low to disable, pull it high to enable-no problem.
    Let me know if you have any other concerns. Otherwise we will close this thread.
    Regards,
    John