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Original question:

UCC28780

UCC28780EVM-021: UCC28780

Jung-hui Cheng
Prodigy 150 points
Part Number: UCC28780EVM-021
Other Parts Discussed in Thread: UCC28780

We isolated the current to other extra circuits, VDD waveform became

However, failed to get output voltage and tried to measure VREF pin of UCC28780. VDD and VREF became 

VDD changed and VREF could not reach 1V. Any suggestion?

  • 0
    TI__Mastermind 31930 points

    Hello Jung-Hui,

    In your first screen-shot, the VDD rises in a more normal time interval, but then VDD drops down to a regulation level after switching starts. 
    It stays there for about 150ms, then falls below the UVLO turn-off threshold and restarts.
    I suggest to probe the output voltage to see what is happening there. 

    In your second screen-shot, VREF is more like 0.5V in amplitude.  It never gets to 5V, so switching never starts and VDD bleeds down to the turn-on threshold repeatedly. 
    It appears that VREF is overloaded.  Because there is 0.5V, it is not shorted to GND, but it does look like too much load on VREF. 

    This did not happen in your first screen-shot, because VDD appears to be regulated for a time (reflected from the output).  So I presume VREF was okay then.
    Something happened when the VREF measurement was probed that overloaded VREF.  Please investigate this.

    Regards,
    Ulrich