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LP3856 Vin Droop

Jake Kim
Intellectual 680 points
Other Parts Discussed in Thread: LP3856

Hi TIer

I have experienced VIN drop when I control SD pin low to high.

Output current is only around 3mA.

Who can explain why it is occurred?

  • 0
    TI__Mastermind 22610 points

    Charge transfer from Cin to Cout when the output is enabled would cause this.

    The voltage on 10uF (Cout) changed 3.8V in what appears to be about 150us, so typically ...

       I = (C * (dV / dt)) = (10uF x (3.8V / 150us)) = 0.253A

    In reality, the current could go as high as the LP3856 short circuit current limit (Isc), which would be about 6A typical.

    In your case it would appear that there is some limitation on the Vcc_5V supply.

  • 0 in reply to Donald Jones
    Intellectual 680 points

    Thanks your prompt answer.

    If it is possibly cause, how can I fix it?

    Do you have any idea? hope to see the solution

  • 0 in reply to Jake Kim
    TI__Mastermind 22610 points

    Usually the bulk input supply would expected to pick up the slack and prevent this from happening

    A larger value input capacitor should help.

    Without all the details, I would suggest adding 47uF in parallel with the existing Cin. This could be just about anything ... Tantalum, Aluminum, ceramic.

    It may turn out that more might be needed to get to accepatble performance, or less might be 'good enough'.

    It's unlikey that you will get it to totally go away.