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Original question:

UCC28C53-Q1: Webench design query

UCC28C53-Q1: Ucc28c53-q1

manoj kumatr
Prodigy 190 points
Part Number: UCC28C53-Q1
Other Parts Discussed in Thread: UCC28C53, UCC28C52

Tool/software:

Could you pls help with calculation for soft start from the primary (controlling Cs Pin) and secondary (zenor diode and capacitor, slow vout rise)?

We r looking for 500ms soft start time.

  • 0
    TI__Guru 73255 points

    Hi,

    The equation is as below:

    Css x dV /dt = Iss,

    Iss is estimated 30uA by average based on 2N2907 beta = 100, dV is between 1V and 4.5V, so = 3.5V, dt = 500ms = 500000us, then

    Css = Iss x dt /dV = 30 x(10^-6) x 500000 x (10^-6) / 3.5V =  4.3uF 

    Select Rss about 100k as start point.

    Note Iss is not a constant, so the above is for start point, and you need to make tune up from bench work.

  • 0 in reply to Hong Huang
    Prodigy 190 points

    HI Hong,

    Thanks for response, i have tried the above changes where as I'm observing the IC VDD drop and hence the output also.

    There are 200uF capacitor and Aux voltage is reg for 12V, we still seen the VDD drop.

    I have designed the Flyback (with UCC28C53Q1) and it is working in the current mode.

  • 0 in reply to manoj kumatr
    TI__Guru 73255 points

    Hi,

    UCC28C53 UVLO on and off hysteresis is quite small. You may need to change to a bigger hysteresis UCC28C5x, such as UCC28C52 which has bigger hysteresis between UVLO on and off.

    If you have to use UCC28C53, you would need to add external hysteresis to extend the hysteresis. For example, after VDD reaches 12V then turn on UCC28C53 then UVLO hysteresis is extended to much bigger.

  • 0 in reply to Hong Huang
    Prodigy 190 points

    Hi Hong,

    I am quite confuse , could you pls help me understand : How does UVLO hysteresis is related to short circuit protection?

    -Manoj. 

  • 0 in reply to manoj kumatr
    TI__Guru 73255 points

    Hi,

    Where did you mention short circuit protection? You have been talking about soft start. I am not sure why you mention short circuit protection here?

  • 0 in reply to Hong Huang
    Prodigy 190 points

    Hi ,

    My apologies : Correct sentence is:

    could you pls help me understand: How does UVLO hysteresis is related to Soft-start protection?

  • 0 in reply to manoj kumatr
    TI__Guru 73255 points

    Hi,

    Longer soft start time will make more energy transfer from VDD capacitor to soft start capacitor - this energy transfer makes VDD voltage drops so to make UVLO off if VDD drops to the UVLO off level. As the IC turn on is at VDD UVLO on with a voltage = Hysteresis before UVLO off, so bigger hysteresis can provide more energy to soft start before VDD reaches UVLO off. That is why hysteresis is part of the consideration to affect soft start time, or soft start longer time needs more energy and hysteresis can help to achieve this.