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Original question:

UCC28780: ucc28780- AGAIN SWS STUCK AT 2V

UCC28780: ucc28780-proof of grounding is in the soldering iron

robin gangopadhya18
Mastermind 6296 points
Part Number: UCC28780

Ulrich: This is worth reporting here.

So we had  REF come up to 5V. It did not happen with .1uF . We put an extreme value: 1uF

Then we see VREF to 5V

RUN still looked low.

So in order to get fixed probes, I though of soldering short stub at the VREF interconnect. 

Power down VBULK.

Get the stub & solder: bam, we see a small arc right at the point where the iron touched the stub! 

After that the chip was dead.

Painful proof of our soldering iron being grounded.

Lesson: with high impedance circuits all around, we got to wait before running with a soldering iron in hand on this assembly.

In process of changing the controller - again.

robin

PS: Most likely 2 shorts we reported before must have come about in a similar manner. Bringing in soldering iron too quickly to a pad we suspected did not have good solder wetting...

  • 0
    TI__Mastermind 40120 points

    Hello Robin,

    I'm sorry for your bad experiences... school of hard knocks. I went to the same school.

    Anyway, REF should have come up with only 0.1uF. Actually it shouldn't matter what value is on it from a power-up point of view.
    A filter cap of 0.1uF minimum is recommended for stability and load transient support.
    It is possible that the REF cap was shorted somehow, and replacing it cleared the short.

    Adding wire stubs to clip probes on is something I do a lot.  My boards bristle with little stubs.
    Avoid soldering these stubs to capacitor terminals... any torque on the stub from the probe will break off the cap termination.
    Also when adding stubs, solder them in securely with a big blob.  Lightly soldered wires will fatigue and break off, then the probe will drag them across the board and short out any and all maximum damage situations.  Murphy's Law. 

    Use a 3~5W resistor to bridge across the Cbulk pins to discharge Cbulk first, then across VDD points to discharge it. Doing VDD first will just charge it back up again while there is volts on the bulk.

    Regards,
    Ulrich