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UCC28780EVM-002: Vdd Fault

TI-CSC
Genius 10400 points
Part Number: UCC28780EVM-002
Other Parts Discussed in Thread: UCC28780

Hello E2E Experts,

Good day.

I am currently looking for help with my UCC28780EVM-002. It seems I am getting a fault delay while measuring at Vdd, and am getting 0 V of output instead of 20 V. We are specifically measuring at C15 to see the voltage at Vdd. What do I do in this case to reset/fix the board? I have attached a picture of what we are seeing at Vdd at the moment as well as a screenshot from the datasheet showing the fault delay example.

Thank you in advance for yr support.

Regards,

CSC

  • 0
    TI__Mastermind 30420 points

    Hello CSC, 

    Thank you for your interest in the UCC28780 ACF controller and its EVM. 

    Unfortunately, the screen shot of a fault delay example did not make it into your posting.  However, I did find Figure 53 in the datasheet which appears to match your description. 
    This fault delay pertains to an OPP fault, where the 160-ms timer protects against excessive overload above the OPP threshold by inserting a 1.5s shutdown interval between each 160-ms operating window for as long as the OPP fault exists. 

    In your screenshot above, the VDD cycling is continuous, so it is not indicative of any faults which involve a 1.5 second shutdown.  
    One thing you may notice in Figure 53 is that the VDD up-slopes are rapid and the down-slopes are relatively slow.  In this case, the Idd current drawn during the 1.5s ionterval is Ifault, which is only about 265uA.
    In your screen shot above, the opposite is true: a slow rise followed by a rapid fall of VDD.  This indicates that Idd is high after VDD reaches the 17.5-V turn-on threshold, and Irun is about 10X Ifault.  This shows that your controller is attempting to start up, but cannot finish the job before VDD drops to UVLO.  
    This situation may arise from any one of several possible causes.  It could be as simple as insufficient input voltage for start-up, or something more complex.

    I recommend that you follow the debug procedures in this App Note: https://www.ti.com/lit/pdf/slua982 to identify and solve the problem. 

    Regards,
    Ulrich