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LP3878-ADJ / Non linear start-up issue

S.Satoshi
Guru 21675 points
Other Parts Discussed in Thread: LP3878-ADJ, LP3878

Hi,

I found non linear start-up issue at LP3878-ADJ as attached. It can be seen with Cbypass=0.068uF though it's solved by increasing Cbypass to 0.15uF.

Can you give your comment why this behavior can be seen by using small Cbypass?

Please let me know if you need more information.

LP3878-ADJ Non linear start-up issue.pdf

Best Regards,

Sonoki / Japan Disty

  • 0
    TI__Mastermind 22610 points

    Sonoki-san,

    LP3878-ADJ has history for this start-up behavior.

    While I have not characterized with Cbypass >100nF, it has been reasonably confirmed that this behavior is not directly related to the value of the bypass capacitor. There is no known reason that 150nF would be any different/better behavior than 68nF, all other factors being the same.

    The issue of the 'step', or a 'lack-of-step' on Vout is tied to the start-up of the internal charge circuitry for the bypass capacitor, along with any residual voltage that may be stored on the bypass capacitor.

    It is also known that the bypass capacitor can become 'pre-charged' to some value if the /SD pin does not start at 0.0V. Depending on the level of the pre-charge, the step may disappear, or the entire rise time may disappear. (the most common complaint). This is why the data sheet says "...this is not intended to be used as a soft-start circuit..."

    There is no internal circuit to discharge Cbypass, so just taking the /SD low, or powering down the input voltage, will not remove any stored charge.

    That said, past investigations (2014) have shown that your behavior with Cbypass=68nF would most likely be considered 'normal' for this device.:

  • 0 in reply to Donald Jones
    Guru 21675 points

    Hi Donald,

    Thank you for your comment. I understand that this is known issue. I know that this is legacy device and so it may be difficult to comment the detail of operation, but I'd like to know why this step is disappeared by adding large Cbypass. Please ask this internally if you can. Or please give your guess...

    Best Regards,

    Sonoki

  • 0 in reply to S.Satoshi
    TI__Mastermind 22610 points

    Unfortunately this is older device, the original design team is long gone. There is no one else to ask.

    Based on the very limited internal schematic details that I have, I don't see any reason that there would be a change in behavior, other than the charge time.