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LM27313 soft start

Mark Whittington
Intellectual 480 points
Other Parts Discussed in Thread: LM27313, LM3017

Is there a recommended way to implement a soft start functionality on the LM27313?  I'm looking to limit the in-rush current upon start up.

Thanks.

Mark

  • 0
    TI__Genius 16025 points
    Hi Mark,

    Let me forward your question to the appropriate forum.

    Thanks,
    Anston
  • 0 in reply to Anston Lobo
    TI__Genius 16025 points

    Also, since this is a boost, there is no easy way to do this unless you use an LM3017.

    This boost has an additional disconnect switch between input and the inductor and can prevent sudden in-rush currents.

    Thanks,
    Anston

  • 0 in reply to Anston Lobo
    Intellectual 480 points

    Anston,

    Where do I find the link to where you have put my question?  

    Also, I tried implementing a circuit similar to what is referenced in the pdf here and was able to reduce the required turn-on current.  I think the solution works but I wanted to get some sort of confirmation that the LM27313 doesn't have some special issue with using this type of soft start.

    Thanks.

    Mark

  • 0 in reply to Mark Whittington
    TI__Expert 6245 points

    Hello Mark,

    The method shown in the PDF that you reference shows how to slow how quickly the LM27313 will try to ramp voltage on the output from input voltage to the output voltage that you are designing for. Anston has pointed out that during the initial period during which output voltage ramps from zero to the input voltage is not controlled by the LM27313.  This current runs through rectifying diode, shown as D1 in section 8.2 of the LM27313’s datasheet, and is uncontrolled.  If you need to limit current during this initial period, a device which can provide true shutdown is needed such as the TPS6107x – which provides true shutdown internally – or the LM3017 which uses an external FET. 

    When using this technique with the LM27313, care should be taken not to exceed 6 V on the FB pin. For the TPS6107x, this is not a concern since output voltages provided by the TPS6107x are low so do not damage that part.  You are most likely interested in an output voltage above 6V. 

    Regards, Robert