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UCC2897A: Vout ramp up rate change

Rocky Chen
Expert 6775 points
Part Number: UCC2897A
Other Parts Discussed in Thread: PMP8790,

Hi

For PMP8790 test result, when 60V VIN at no load, the 3.3V output ramp up rate will become slow when Vout come to ~2.7V. While, there is not this kind of curve for 18V Vin or adding some load (0.3ohm). 

Could you help advise what reason caused this different ramp up curve? Could we make it more consistent ramp up rate for 60V no load?

https://www.ti.com/lit/ug/sluuac2/sluuac2.pdf?ts=1602233866889&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FPMP8790

Thanks!

- Rocky

  • 0
    TI__Intellectual 2640 points

    Hi Rocky! 

    Thank you for reaching out and for your interest in the UCC2897A and the PMP8790! I want to get the board designer's insights here, I'll get back to you when I hear back from him. 

    Thanks again for getting in touch, I look forward to talking more soon! 

    Aidan Davidson

  • 0 in reply to Aidan Davidson1
    TI__Intellectual 2640 points

    Hi Rocky! 

    I spoke with the engineer that designed the board, here are his insights: 

    There are two circuits that affect the startup of the circuit: the integrated soft start inside the UCC2897A device (controlled by C17) and the circuit comprised of D7, R24, and C21 that helps to prevent overshoot. 

    His theory is that the change in slope during the no-load startup occurs at the intersection between the two startup circuits' operations. If you remove the second circuit (D7, R24, and C21) you may have a smoother startup but at the price of potential output overshoot under light load conditions. 

    Another approach would be to tweak both C17 and C21 to match the slopes of the two segments. 

    Thanks again for getting in touch with us, I hope this information helps! 

    Aidan Davidson