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TPS63060 / Extending the Soft Start Time

S.Satoshi
Guru 21685 points
Other Parts Discussed in Thread: TPS63050, TPS63060

Hi,

My customer plan to extend the soft start time with added diode and capacitor introduced in SLVA553. Can you please give your comment for following question?

  • This application report states that this additional circuit has not affected the control loop though it's included in feedback loop. Can you give me further explanation why it has not affected the control loop?
  • Can you give us the formula to calculate the soft start time with C5, R3 and D1 in attached?
  • Can the schottky diode be used as D1? Please let me know if it create an adverse result.

Best Regards,

Sonoki

  • 0
    Guru 21685 points
    Hi,

    I have asked the same question and then TPS63050 was suggested.
    e2e.ti.com/.../401640

    But the 8.4V need to be input this time, so I'm asking this again. Your comment will be appreciated.

    Best Regards,
    Sonoki
  • 0 in reply to S.Satoshi
    TI__Expert 8420 points

    This circuit does not significantly affect the control loop. However I would consider using a diode with junction cap in the typical range of ~ 2pF as not critical.

    The series capacitance of C5 and the diode act as a feedforward cap, I would keep this series capacitance lower than 5pF to avoid oscillation problems. For the same reason I would not use a shottky diode, it has a higher junction capacitance.

    As said in the previous post, there are no formula to calculate the SS time,  I would suggest to start the experiments with the values of the app note and then tune them.

  • 0 in reply to Sabrina
    Guru 21685 points

    Hi Sabrina,

    Thank you for your response. Please let me ask three more thing.

    1. Please let me know the reason why you recommend lower C5 than SLVA553 for TPS63060? 
    2. I know that the feedforward capacitor extend start up time and the equation is stated in SBVA042. Do you think this equation can also use for this SS circuit?
      • http://www.ti.com/lit/an/sbva042/sbva042.pdf

    3. The series capacitance of C5 act as feedforward cap at only start-up due to the diode. Is my understanding correct?

    Best Regards,

    Sonokki

  • 0 in reply to S.Satoshi
    TI__Expert 8420 points
    I do not recommend lower C5, I am talking about series capacitance: C5 in series with the junction capacitance of the diode.
    I would keep it lower than ~5pF by selecting a diode with low junction capacitance.

    I would not use this equation to calculate the SS, as it does not take in account the diode. In my opinion, the best way is to try out the circuit and then adjust the values of C5 and R3 accordingly.

    During start-up you have a open-loop, the capacitance ( C5 in series with the junction cap of the diode) starts to act as FF only when the loop is closed.
  • 0 in reply to Sabrina
    Guru 21685 points
    Hi Sabrina,

    Sorry for my misunderstanding. I understand that the junction capacitance of the diode keep lower than ~5pF. And they start with C5 and R3 with default value in SLVA553 and adjust them depend on their application.

    Best Regards,
    Sonoki