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TPS790: slow start up

kevin p
Intellectual 470 points
Part Number: TPS790
Other Parts Discussed in Thread: TPS799

Hello team,

I use  TPS79901DDCR in my design.

I want  to slow start to reduce inrush current on the power supply when power on,

I add RC on the EN pin.

the R=20k, C=1UF，the input power is 5V, output  is 4V,I want to delay ~6ms  to start up when power on,

Could you help to review and give your comments,

Could  you give your solution to slow start up when power on?

Thank you,

kevin

  • 0
    TI__Genius 13085 points

    Hey Kevin,

    Given that you are using the adjustable version of TPS799, there are two ways to reduce in-rush current:

    1. Reduce the amount of output capcitance if possible
    2. Add a feed forward capacitor (connected to Vout and FB).
      1. We have an app note on the tradeoffs to keep in mind when using an feedforward cap and section 2.1 addresses startup. 

    There are a couple of things you can do to help mitigate the Vin droop caused by in-rush current (assuming that is your problem)

    1. As you are doing, delaying the enable signal will not reduce the amount of in-rush current however it can help your input supply start at a higher voltage and may be able to keep Vin from drooping so low that the LDO turns off. 
    2. Increasing the amount of input capacitance will help to provide more current to charge the output capacitor while keeping the input supply more stable. 
      1. In general a good starting point is to have an equal amount of capacitance on the input as the output. However, more may be needed on the input if the input supply still can't provide enough current. 

  • 0 in reply to Kyle Van Renterghem
    Intellectual 470 points

    Hi Renterghem,

    Thank you for your support.

    Sorry, I havn't find the  app note  . Could you help to send the app note  to me.

    If I want to adjust the startup timing of  the LDO when power on,

    Can I add RC on enable pin simply or add precisely control the enable delay timing?

    Could you help to review and give your comments.

    Thank you.

    Kevin

  • 0 in reply to kevin p
    TI__Genius 13085 points

    Hi Kevin,

    Sorry for the bad link I don't know why it isn't working. I've recopied it below. 

    As for delaying the EN signal, yes many people delay the EN signal using an RC but that will only delay when it turns on. It will not reduce the amount of in-rush current. So you'll need to determine if delaying the startup will help by allowing the input supply to stabilize at a higher voltage so when it droops it does not cause issues in your system.

    If delaying the startup time is not enough to address the issue, you'll need to try and reduce the in-rush current using the feedforward cap as described in the app note. 

    http://www.ti.com/lit/an/sbva042/sbva042.pdf