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LM5118 instability at cold

John Vranes
Intellectual 590 points
Other Parts Discussed in Thread: LM5118

Hello,

At cold temperature (but still within the recommended operating temperature of the part), the output of LM5118 starts to exhibit the behavior in the attached screenshot.  This mainly occurs in full buck mode and at little to no load on the output.  This circuit is designed for a nominal output of ~21.5V and Fsw of 500kHz.  There is a short period that can last ~100us where the switching is occuring (when the output rises really fast) followed by a long period of no switching.  During the period of no switching, the output is prone to very poor response to a load transient.

Is this normal behavior for this device?  I have only observed this behavior on one device, so it could be a one-off bad chip, but are there potential design issues with supporting components that have been known to cause this?

Thanks

  • 0
    TI__Mastermind 21720 points
    Hello John,


    Just to clarify, during the short period of 100us you are seeing the output oscillation you see about? Or is that a separate issue?

    Can you share a schematic?

    Out of how many parts are you seeing this failure?

    What temperature are you going down to? Do you see the issue if you go colder? How are you cycling the temperature? Are you just pulling it down to cold or is it possible that condensation could be on the board?

    Thanks,

    -Garrett
  • 0 in reply to Garrett Roecker
    Intellectual 590 points

    Hi Garrett,

    Thank you for the reply.  I'm not sure I completely understand your clarification, but let me try to clarify anyways.  The ~100us is how long the steep rises last.  Then the switching stops during the downslopes.

    This is first observed between about -30 and -35 degrees and is observed all the way down to -45 deg (haven't tested it beyond that).  The board sits at room temp indefinitely and it brought down to cold when the behavior is observed.It occurs when going from room temp (i.e., sitting at room temp indefinitely) and brought down to cold.  Some ice can be seen in the temperature chamber, so condensation is present when bringing it back up.

    This has only been observed on only one part out of several (i.e., dozens of parts), so I'm inclined to believe it's a bad part.  Is there some sort of minimum load requirement that is only happening to be causing an issue on this part?

    Thanks,

    John