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UCC28C43-Q1: UCC28C43-Q1

Alice Dong
Prodigy 30 points
Part Number: UCC28C43-Q1

Hello,

I am looking for some support using the COMP pin to disable the PWM IC. I use the P-MOSFET to pull COMP to GND as suggested in the datasheet. The voltage of COMP initially stabilized at 3.38V. Then P-MOSFET is turned on, COMP pulled to GND through the P-MOSFET. Now the voltage of COMP drops to ~1V. To keep the P-MOSFET ON, the source-to-gate threshold shall be lower than 1V. The question is, to keep COMP pulled to GND, what's the suggested gate threshold for the P-MOSFET? 

Thank you,

Alice 

  • 0
    TI__Genius 14570 points

    Hi, Alice,

    Where is VF3 in your waveform?

    I think the VGS need be negative to fully turn on P-FETs because the source is 0V when the P-FETs is turned on.

    Regards,
    Sean

  • 0 in reply to Sean Xu
    Prodigy 30 points

    Hello Sean,

    VF3 is the waveform of COMP. Now COMP is connected to the source of P_MOSFET. Of course, Vgs is negative. When disable is called at 15ms. BJT is ON, Vgs is -3.38V at the moment. P-MOSFET then turns ON. COMP drops from 3.38V to around 1V. Now the issue comes, if my gate threshold is not low enough (say -2V), my P-MOSFET will back to OFF state and disable failed. I am not sure if you get my point. 

    Alice

  • 0 in reply to Alice Dong
    TI__Genius 14570 points

    Hello, Alice,

    I saw the problem. It needs a negative voltage to maintain P-FET off, but the circuit can't provide negative voltage. Do you think you can try the below circuit? Use N-FET and pull up to VCC from the Gate? But VG need be reversed to disable output.

  • 0 in reply to Sean Xu
    Prodigy 30 points

    Hi Sean

    I cannot reverse the VG. Here is the reason. This flyback converter shall be used as auxiliary/backup power supply which means I don't have any other power supply to hold VG high at normal running conditions. How about I ask in a different way, any other better methods to disable the PWM IC? 

    Thank you,
    Alice 

  • 0 in reply to Alice Dong
    TI__Genius 14570 points

    Hi, Alice,

    You may reverse VG. Can you do this?