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LMZ1200x Uoutput > Uinput permitted by power down?

Michi S
Intellectual 265 points

Hello TI-Team,

is it permitted that the input voltage falls below the output voltage (Uoutput > Uinput) by power down - for example there is a heavy load at the input voltage by other electronic parts and only a little load at the output of the switiching regulator LMZ1200x? (Is there for example an external diode of the output to the input necessary?)

Best regards

  • 0
    TI__Genius 10115 points

    I have attached comments from the designer.

     

    "Yes, the body diode of the high-side FET limits how high Vout can go above VIN.

     

    Other devices may specify this differently, but all step down switchers with FET switchs have a diode from the output to the input, therefore limiting the internal SW  node to a diode drop above VIN.

     

    Now, discharging the output through the VIN-SW diode: when current flows in the body diode, it also cause substrate current to flow, and when this happens, there's a potential for something else to "latch up". Unfortunately this has not been characterized beyond 200mA per latch up requirement. Therefore, what we know is: if the discharge current is much less than 200mA, this is believed to be harmless. If the discharge current is >200mA, we do not guarantee anything. The device could be damaged before output gets dragged to zero.

     

    A Schottky diode between VIN and SW would reduce the current into the chip and this is the recommended solution.

    "

    Regards,

    Marc

  • 0 in reply to Marc Davis-Marsh
    Intellectual 265 points

    In the redesign of the board we will add a schottky diode from the output to the input - we hope/think this will solve this problems/effects.