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TPS40303: SS pin oscillating

Part Number: TPS40303

TPS40303 team,

A customer designed a board with TPS40303 to support up to 21A load.  The Overcurrent isn't expected to be a factor since it is programmed at about 55 to 60A with R = 15.4kOhm.

They tested first with current well below 21A, probably 5A, and all was fine.  They increased the current to about 10A, and it ran fine for 2 to 3 minutes then shut off.  Since then, the regulator can't bring the rail back up.  See attached plots of the SS pin on the failed board.

My suspicion is that the high side FET was damaged somehow, but before they go resoldering, does this type of pattern mean anything to you?  You'll see the HDRV scope capture below as well.


SS on failing board

SS on failing board

SS on failing board

SS on Good board (before failure)

HDRV pin on failing board with no load.

  • Hi ,

    Peter /Lishuang will respond to you soon.




    The SS pin and HDRV waveforms you shared are consistent with the TPS40303 tripping the over-current protection.  The SS pin is discharged, and then charged 7 times to provide a time-out between restart attempts to allow the FETs to cool down between restart attempts.

    The shorter ramp voltage every 8th cycle on the SS pin is a restart attempt cut short by the over-current.

    Before you replace the high-side FET, I would recommend you check:

    1) SW voltage with respect to VDD

    The high-side current limit is triggered based on the drop from VDD to SW during the high-side FET on-time.  SW should be rising very close to VDD during each on-time.  

    2) If the SW is not rising very close to VDD, check HDRV and boot to make sure the high-side driver circuit is working correctly.

    Your HDRV waveform only looks like it's rising to 5V, but that could be aliasing due to the resolution.  If the SW pin is staying at 0V and the HDRV pin is rising to ~5V, I would suspect the low-side FET is damaged and shorted to ground more than the high-side FET, but both could easily be damaged.

    If you do find that the FETs were damaged, I would recommend paying very close attention to switch node ringing and the potential for voltage stress on the Power FETs during switching once they are replaced and you can restart the output.