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UCC28730: Vs Specification/Function

Brett Nourrcier
Intellectual 905 points
Part Number: UCC28730

Hello - we are designing a 15W, 12Vout, DCM flyback using the UCC28730 controller.  

The Vs pin is specified at -0.75V to 7V.  

I am wondering about the -0.75V spec.  Typically the aux voltage feeding Vs will be significantly less than -0.75V.

On some of the TI reference designs (e.g., TIDM512A), a Shockley diode and a Zener diode are used to clamp the Vs voltage but, unless i am missing something, this doesnt limit the negative voltage to -0.75V. Also, the datasheet implies that the aux voltage is sampled while the mosfet is on. I am pretty sure the aux voltage is negative during that sample time. 

Can you please provide guidance on the spec?  And also on how the input (run) and input (stop) values are determined?

Thanks!

Brett

  • 0
    TI__Expert 7080 points

    Hi Brett,

    Thanks for your interesting in TI's part.

    VS pin is internal clamped to ~-0.25V , so you don't need to worry about it during the normal operation. just don't impose another negative voltage on it is ok.

    The purpose of schottky diode and zener diode is to limit the current out of VS pin(Ivs) not exceed 1mA in the condition of very high input voltage .

    The input run and stop calculation equation please refer to datasheet section 5.1.6 . the VS current threshold at run and stop are 225uA and 80uA separately.

    Thanks. 

  • 0 in reply to Jaden Ning
    Intellectual 905 points

    Hey Jaden - thank you for the quick response.  When you say "just don't impose another negative voltage" what do you mean?  The negative voltage will be imposed during the MOSFET ON time.   And just so i am clear, you are saying it is ok to exceed the -0.75V spec on the VS pin? 

    Thanks!

    Brett

  • 0 in reply to Brett Nourrcier
    TI__Expert 7080 points

    Hi Brett,

    VS pin will be internal clamped to -0.25V when mosfet turn on . so don't worry about it will exceed -0.75V. what I mean is don't  add external additional circuits to VS pin that maybe let it lower than -0.75V . 

    Thanks.

  • 0 in reply to Jaden Ning
    Intellectual 905 points

    OK.  I think i understand.  Thank you.