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LM117QML: Short circuit protection/stress

Michael Ford
Prodigy 160 points
Part Number: LM117QML


During testing we had a short on output of regulator. Input was supplied by a 1A current limited supply that would go to 15V if current did not limit. In this case it would have come up to 2.6V @1A (for ~100mSec before shutting off) based on Pspice simulation of device.  That is 2.6W versus the absolute max rating given of 2W. So it would be considered stressed by those numbers and even more so if include program deratings. But datasheet also says the device is short protected. Is shorting the output with 1A input current limit damaging?

Thanks,

Mike

  • 0
    TI__Mastermind 23361 points

    Hi Mike, 

    Is this done on the bench or purely with the PSPICE simulator? 

    Regards, 

    Jason Song

  • 0 in reply to Jason Song
    Prodigy 160 points

    We are trying to determine if we need to replace a real part on a real board that had a short on the output with a 15V input supply that had a 1A limit and overcurrent shutdown. Because I needed to do a stress evaluation I needed to know the voltage across the device in that condition.

    A Pspice simulation puts the voltage across the device at just under 2.6V with output shorted and 1A input current. I did not see a way to determine that voltage from the datasheet alone.

  • 0 in reply to Michael Ford
    TI__Mastermind 23361 points

    Since we have moved the conversation to direct email, I am closing the thread here. 

  • +1 in reply to Jason Song
    TI__Mastermind 23361 points

    Hi Michael, 

    You may use this plot for the estimation of the maximum power together with the PSPICE simulation. 

     LM117QML has been released for a long time and the datasheet was done in a very different format and some of the wording is indeed a little confusing comparing to the newer LDOs. We don't usually define the maximum power an LDO can dissipate, instead, we use the maximum junction temperature and for this LDO, the recommended maximum junction temperature is 125C (150C max), and the maximum junction temperature can be estimated as Tambient + power x Rja, the Rja here is 39C/w. 

    I know you were concerned about the reduce the reliability, but since this short condition only lasts for a very period of time comparing to the life of the device, I would consider this as low risk. We normally suggest reliability degrading when the part is constantly above or near the absolute max conditions. 

    Does it make sense? 

    Regards, 

    Jason Song