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INT- TPS40200-HT soft start & shutdown delay

Mahendra Patel1
Expert 3560 points
Other Parts Discussed in Thread: TPS40200

Hello team,

TPS40200 datasheet has given an example on soft-start operation on page26 of datasheet.

It uses 0.047uF of capacitor and delay calculated is 0.95ms. I notice there is an additional for output to rise to expected level because of output cap.

My query is, what minimum value of capacitor can be used on SS pin to have least delay due to soft-start? What are other ways to achieve fast output response?

Also, pulling SS pin below 150mV puts device in shutdown mode. What is the delay in this case (from the instance SS pin goes below 150mV and switching stops)?

Regards,

Mahendra Patel

  • 0
    TI__Mastermind 35185 points

    Hi Mahendra,

    In most cases the SS pin will start from 0V so there is no change in delay if the SS pin is pulled below 150mV. Theoretically I do not think there will be a limit to how small the SS capacitor can be.

    Practically the limit would be from how much output capacitance there is and the amount of current needed to charge it up to the output voltage. If the SS period is to short the inrush current will be very high and can trigger the hiccup current limit. The amount of inrush current to charge the output capacitors could be estimated by I = C * dV/dt. Also include any load current that may be present on the output.

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    TI__Expert 3560 points

    Hello Anthony,

    Thank you for your reply.

    You answered my question on cap value on SS pin and delay for start up.

    However, my other question is,

    If controller is running (Vin>4.25V) and if SS pin is pulled low below 150mV, what would be the delay for shutdown operation?

    I understand that output capacitor needs to be discharged, but apart from that what is the response time of controller to stop switching?

    Regards,

    Mahendra Patel

  • 0 in reply to Mahendra Patel1
    TI__Mastermind 35185 points

    Oh I see, I misunderstood the second question.

    When SS is pulled below 150mV and switching is forced to stop I expect the only delay to turn off would be propogation delay. Also switching would stop because the TPS40200 would begin trying to regulate to the SS voltage instead of the reference once SS goes below 1.4V.

  • 0 in reply to Anthony F
    TI__Expert 3560 points

    Thank you Anthony!

    Regards,

    Mahendra Patel