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Original question:

TPS54550: High frequency noise observed at PH pin due to resistor at Boot pin

TPS54550: PH pin failure mode - low dc resistance

mark chimene
Prodigy 20 points
Part Number: TPS54550

we have the tps54550 in a new design - of 4 boards all were initially operational but  one is now failing with a very fast soft start and overcurrent "hiccup" oscillations.  the ss/ena pin is open I this design.  the load aside - I am measuring a simple dc resistance to ground on the PH pin of 15 ohm for a failed unit  - with other boards at 1k, and a suspect one at 35 ohm.  question - is there a failure mode/indicator of the 54550 that would cause the PH pin dc resistance to ground to reduce  as described please.

  • 0
    TI__Guru**** 191445 points
    Mark,

    TPS54550 does not have any extensive history of failures when used properly. TPS54550 has an internal pull down for charging the BOOT capacitor, that may possibly be failing. More likely is the external low side FET. Can you post your schematic? Can we see the start up waveforms?
  • 0 in reply to JohnTucker
    Prodigy 20 points

    howdy john

    so there is no presumed fail mode of the 54550 pin 14/15 that could pull down the rail.  I have included a snip of the schematic and a scope plot.

  • 0 in reply to mark chimene
    TI__Guru**** 191445 points
    Mark,

    The most common failure would be a short from PH to VIN from a shorted high side FET. The internal FET has by far the largest portion of the die area. A really destructive event could possibly cause damage down to the substrate or flash metalization from PH to a top side GND exposed area. In that case, I would expect short to include pins 1. 2. 11. 14 and 15. Device encapsulation may also show signs of damage. However you stated previously that the device is attempting to start and is entering hiccup mode over current, so I suppose the hide side FET is not shorted.

    Your waveforms and schematic did not post. Can you try it again? Also there is an external email thread on this topic. We can take this off the forum if you like. Contact David or Alex directly
  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points
    Mark,

    You can contact me directly at j(dash)tucker(at)ti(dot)com. I will close this thread for now.