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LP38513-ADJ: feed forward capacitor to slow inrush

steven heitbrink
Prodigy 230 points
Part Number: LP38513-ADJ
Other Parts Discussed in Thread: LP38502-ADJ, TPS7A85A, TPS7A83A

 I am looking at using a LP38513-ADJ or a LP38502-ADJ to provide a 4V supply from a 5V source, and I would like to try to use a feed forward capacitor to slow the inrush current on my design. For reasons that are out of my control I need to put 100uF of low ESR capacitance in my design. Unfortunately I’m concerned about the short rise time (therefore large input current) pulling down my 5V power supply.

I was wondering if using a larger feed forward capacitor would be a good way to limit this. In my simple breadboard test I’ve been able to increase the rise time to somewhere around 400us. I was wondering if this isn’t a horrible Idea, and what the best method is to ensure that the power supply is stable. Do I need to  physically build the circuit and inject a small voltage into the feedback path, like you would to measure the open loop gain of a switching converter? 

  • 0
    TI__Expert 6825 points
    Hi Steve,

    This is a valid way to implement a slower startup, but there are some problems with increasing the feed forward capacitor. Take a look at this app note detailing the pros and cons of using a feed forward cap www.ti.com/.../sbva042.pdf. Due to some of the limitations of using the feed forward cap it would be better to use a device that has a soft-start control circuit built in. You could look into using the TPS7A83A or the TPS7A85A as alternatives to these devices.

    Regards,
    Mark
  • 0 in reply to Mark Sellers
    Prodigy 230 points

    Hi mark,

    Thanks for the response. I guess the  TPS7A85A is also a good part. I guess I have to decide if we want to use a QFN in this application though. Another requirement I have is that I need to run be able to dissipate 1 Watt, and possibly operate up to 70C.  I guess my other option is to put some sort of soft switch between this LDO and its supply. 

    Thanks For the Help, 

    Steve H