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LM5118: LM5118 unstable output at low temperature

Lee Sommerville
Prodigy 10 points
Part Number: LM5118

Hi,

 

We are having a problem with an existing product in our factory. The product has been in production since prior to 2014. Recently our yields have fallen to less than 50%.

I have tracked our problem down to the LM5118. At low temperatures (-40 degrees C) we are seeing the output of the supply becoming unstable, oscillating between approximately 3.5V and 7.5V at around 400Hz on a circuit that is designed to output a fixed 5V. Unfortunately I cannot share the design on a public forum as the product is not EAR99.

I have probed the UVLO signal and verified the unit is not shutting down due to overcurrent.

I have been unable to probe the feedback signal at low temp due to access in the chamber.

Our design feeds the output 5V into the VCCX input. I had a technician lift the VCCX pin and short it to ground. Two units that reliably failed at low temperature are now passing. Another unit did fail, but this unit had a lot more handling with conformal coating being stripped and parts being replaced, so I am not sure of the significance of this failure.

I had the LM5118 swapped out with devices from a different Lot #, going back to the original design (i.e. we didn’t lift the VCCX pin). This did not resolve the issue, so it does not appear to be a batch issue.

Our contract manufacturer has verified the BOM used on the board, we have stripped conformal coating and washed the board.

Does anyone have any idea on what could possibly be going wrong?

 

Any input would be greatly appreciated.

 

Cheers,

 

Lee

  • 0
    TI__Guru 58521 points

    Hi Lee,

    Thank you for posting to the E2E forum.

    It would be good if you can share the schematic and also the input / output parameter via "send privat message".

    (This is available if you put the mouse pointer over my name)

    Thanks,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 10 points

    Hi,

    Is there anyone at TI that can take over this case from Stefan?

    Unfortunately I have been held up with getting NDA's in place.

    Now that I am looking to re-engage on this, it appears Stefan is out on vacation for the next two weeks.

    Because the product in question is not an EAR product, I am unable to share information over a public forum. If someone new at TI is assigned to this case, can you please provide me with an email address for data transfer purposes.

    Cheers,

    Lee

  • 0 in reply to Lee Sommerville
    TI__Guru 65170 points

    Hello Lee,

    I will take over from Stefan, Please accept my friendship request and send it via private message.

    BTW, you mention that the yield is reduced, but the problem is not dependent on the used device date code. Did you change anything in your testing during production?

    Best regards,
    Brigitte