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TPS7A03: output cap inrush

Fred Cheng
Expert 5310 points
Part Number: TPS7A03

Hi team,

customer encounter inrush current too much( input current is limited)  that output can't start up successfully,

we want to reduce the output cap,

but they have two 1uF, one is close to our LDO , the other is close to far end RTC loading,

if I have to choose one, which one can I remove to reduce the inrush current?

  • 0
    TI__Mastermind 18466 points

    Hello Fred, 

    We have an FAQ detailing inrush current. 

    While it's true that you can decrease the COUT to decrease inrush current, a COUT, with a minimum of 0.5uF effective capacitance, needs to be connected in proximity to the LDO Output to maintain stability. 

    Is the power supply limited?

    How long is the trace connecting from the LDO to the load? 2uF Capacitors connected to the output should not be causing an unreasonably large current. 

    Does the large inrush current issue continue with a .1uF capacitor connected in parallel with the 1uF capacitor closest top the LDO?

    This start-up plot shows a 15mA inrush current with a 1uF output cap.

    This star-up plot shows a 145mA inrush current with a 22uF output cap.

    Best,

    Juliette

  • 0 in reply to Juliette
    TI__Expert 5310 points

    Hi Juliette,

    Output inrush is not large but input current is inevitablely limited.

    there’s a 1kohm in seires with the input of LDO which limit the input current .

    so back to the question, you prefer I remove the far end output cap instead of the the close to our LDO, right? 

  • 0 in reply to Fred Cheng
    TI__Mastermind 18466 points

    Hey Fred,

    Yes, you need to keep at least 0.5uF capacitance close to the output. Please ensure that the capacitor you have chosen does not de-rate past this value. 

    Best,

    Juliette