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UCC2818 : SOFT START & RESPONSE TIME UNTIL PWM OFF

alpha
Expert 2530 points

Hi

What is trigger to starting soft start?
Does it start when Vcc is turn on or device is enabled(EN=high)?
How much is response time to PWM off when EN is going to low ?

Best regards.

  • 0
    TI__Mastermind 34465 points
    Soft start is initiated when the voltage on VCC pin rises to its UVLO point.
    An OVP/EN signal shuts down the PWM almost immediately (less than 100nS)
  • 0 in reply to John Griffin1
    Expert 2530 points


    Hi John

    Thank you for your reply.

    Our customer measured SS pin,EN pin and Vout when EN was turn on as follow.

    SS pin is raised between 1.388s but Vout is raised very fast.
    And also you mentioned soft start is initiated when the voltage on VCC pin rises to its UVLO point but if it is true , I think Css has been already charged.

    Question
    Why SS pin can be raised slowly?

    Green : SS pin

    Blue : Vout

    Aqua : OVP/EN

    Best regards

  • 0 in reply to alpha
    Expert 2530 points
    Hi John

    In my understanding Soft start is started by charging to Css and Css is starting to charge when EN is high.

    I think Waveform in the above is indicated proper operation.

    If your answer is correct , I think Css is starting to charge when Vcc was UVLO point.

    Please support and give your comment.

    Best regards.
  • 0 in reply to alpha
    Prodigy 20 points

    Hi,

    I can confirm that  Css starting to charge when  EN pin is high, not when Vcc is in turn-on threshold.

    In this case there is not soft-start.

                                        

      No soft-start; no delay time from Vcc UVLO and                         Correct soft-start, delayed time from Vcc UVLO and EN 

      EN.                                                                                                

      Green: PWM

      Aqua:  E/A 

      Best Regards

      Ivo