This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28780EVM-021: UCC28780

Part Number: UCC28780EVM-021
Other Parts Discussed in Thread: UCC28780

The measure resistance on VREF pin is 28 ohms.  We think UCC282780's VREF was damaged internally as all other external components connected to VREF are OK.. What condition would cause the damage? 

EVM module's VDD stays around 45ms of ramp-up period where VREF is 0V. Is 150ms of VDD ramp-up period for our design OK ? 

  • Hi Jung-hui,

    If impedance on VREF pin is 28ohm , I agree with you that the REF pin is damaged ,  REF is a "internal" using pin ,It didn't connected to "external" circuits. so it rarely be damaged in the past. unless by the electrostatic damage since the maximum allowed voltage for REF pin is 7V.   right now you may have to replace the damaged IC with a new one.

    Usually, we don't care about the VDD ramp-up time.  this ramp-up time is decided by VDD caps volume , HV start mosfet threshold . and the sum of  internal (1k)and external(121ohm ) resistor.  a little long or short ramp-up time will not impact the behavior of UCC28780, just a different start up time.

    If you find a VDD UVLO hiccup , it means some faults has happened after VDD reaches to ~17.5V, what we have to do is find out which protection been triggered.  here are two application notes which can help you to solve the start up issues.

    https://www.ti.com/lit/pdf/slua982

    https://www.ti.com/lit/pdf/sluaaf9

    Based my experience , I prefer to firstly capture low side FET Vds , transformer primary winding current , and VDD in the scope , there signals can tell us more information.

    Thanks.