I am using the UCC28780. The waveform of VDD appears in the startup time as shown below. The RUN pin has not been set high. The schematic used is the same as TIDA-01622.
Hello Ulrich，
The waveform after retesting is shown in the figure. The yellow is the waveform of VDD, the green is the waveform of the HVG pin, the blue is the waveform of Vref, and the purple is the RUN pin (not set high).
The fault may be a fault without "delay action". How to check and solve the fault?
thanks for your help.
Hello User4731692,
The second set of waveforms that you posted are normal except for no RUN or PWML pulses. This situation would be true during the 1.5s interval between start-up retries.
I request that you concentrate on the special case where the VDD drops suddenly, as in your first waveform:
If this drop recurs every 1.5s and you can trigger on it, you may be able to capture something happening on RUN or PWML. If this is the main clue to the problem, I think some sudden overload on VDD is causing a fault where there is no delay to action, or 1 to 3 pulses of PWML.
At these slow sweep speeds (100ms/div), your scope may not be able to resolve a few microseconds of RUN or PWML signals. But at a fast sweep (10us/div) there may be some signals that can be captured. Please see if this drop happens every ~1.5s, and try to trigger on it to make a fast sweep and see what's going on.
As an alternative, you can trigger on REF going high each VDD-UVLO cycle, with a 10us/div sweep. Maybe 4 out of 5 times there are no signals, but 1 out of 5 times there should be a RUN signal whenever the IC attempts to restart after the ~1.5s delay.
Regards,
Ulrich
