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LM25085-Q1: LM25085 dead

Gonzague GENET
Prodigy 30 points
Part Number: LM25085-Q1
Other Parts Discussed in Thread: LM25085, LM5085

Hi everyone

We develop a board with a DC/DC converter based on LM25085

This board is powered by a 24V

Sometimes (1/100), during power on or power off operation, the LM25085 become dead. The component seems to be hot, but not burning, and the output is stucked to 0V

We investigate Vin, to determine if it go above absolute max rating, but we notice anything in this way.

Any clues ?

What could cause such a failure state ?

bests regards

  • 0
    TI__Genius 11040 points

    Hi Gonzague,

    My initial thoughts are that the Absolute Maximum ratings cannot be exceeded, and the VCC voltage should not be pulled negative.

    If any pin exceeds the Abs Max ratings, this can result in damaged units:

    Also, VCC should not be allowed to go negative:

    Could you also share your schematic and layout files?  I can also review these documents for any potential causes of the damaged units.

    Best Regards,

    Katelyn Wiggenhorn

  • 0
    TI__Mastermind 39980 points
    Please can you post your schematic? I would be happy to review for you.

    Thanks.

    David,
  • 0 in reply to Gonzague GENET
    TI__Mastermind 39980 points

    I don't see anything that would cause a failure, except...I would rule out possible over voltage on the Vin to VCC pin as previously mentioned by Katelyn.

    Please can you try placing a schottky very close to Gnd and VCC (Anode to Gnd and cathode to VCC pin) of the device.

    if Vin drops quickly, the VCC cap can fly to a negative voltage. If the input recovers quickly, the voltage on the VCC cap can go out of regulation causing an over voltage on the pin (between Vin and VCC). The Schottky prevents VCC from flying to a negative voltage on a fast negative Vin transient or a fast On/turn off/Turn on cycle. From you description, I am not clear if you are turning off and then on, quickly, this could cause an issue?

    Hope this helps?

    David.

  • 0 in reply to David Baba
    Prodigy 30 points

    Thank you David for your help.

    The Vin is supplied by an external AD/DC converter (24VDC- 700W). I noticed that the supplied rise very slowly (about 100ms).  Also, there is in fact 20 identic boards supplied by this AC/DC converter, with this same DC/DC converter schematic.

    On others models we had, the input capacitor is bigger (10uF) and we do not have this problem.

  • 0 in reply to Gonzague GENET
    TI__Mastermind 39980 points
    Hello Gonzague,

    Usually, if there are many boards that are working and one board that is not, then figuring out what is the difference between them gives good visibility into what is causing the issue. It appears that the differences in Cin may have something to do with the failure on the board?

    The input capacitor value and placement is critical to reliable operation of the converter. Please make sure you have a higher value ceramic input cap (uFs) for your power components, close to MOSFET drain and DIode Anode. Also it is recommended to have a cin bypass cap for the LM5085 (~100nF) close to Vin to Gnd of the device.


    Hope this helps?

    David.