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LP5907: Frequency Domain

Julian Chen
Expert 2550 points
Part Number: LP5907

Hi E2E,

We use LP5907 to design (3.3V→3.0V) and put COUT = 10uF, we can find the 16K noise @ frequency domain.
BUT, when we modify COUT to 1uF, have no noise @ frequency domain.

We can modify COUT to 1uF, but could you let us know why have a noise when we use COUT=10uF.

(Customer want to a Explanation)

BTW, we disable others power except LP5907. And we try to change others component in LP5907 circuit, and find that had to modify COUT to 1uF can fix this issue.

  • 0
    TI__Mastermind 33885 points

    Hello Julian,

    I am reviewing this question.
    I should have a reply within 2 business days.

    Thanks,

    - Stephen

  • 0
    TI__Mastermind 33885 points

    Hi Julian,

    It looks to me like the resonance you are seeing is from the 10uF capacitor.
    At the most basic level, the self resonance of a capacitor is:

    f = 1/(2*pi*C)

    f = 1/(2*3.14159*10u) = 15.9KHz

    Can you increase your frequency to 200KHz?  You may be able to see the effects of 1uF around 159KHz.

    If I have resolved your question, please click the "Resolved" link.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Expert 2550 points

    Hi Stephen,

    Thanks for your reply,


    Today I discuss it with customer, they said...
    1.  just only <0.5mA can find the noise.

    2. 3% fail, After A-B-A Swap, fail item follow IC not board

    So, I have 2 question:
    1. Does self resonance of a capacitor related with  output current?

    2. Because fail item follow IC, Is it not stabilliy if we use 10uF output CAP?

  • 0 in reply to Julian Chen
    TI__Mastermind 33885 points

    Hi Julian,

    I believe the issue has to do with stability of the LDO, possibly by the input filter interacting with the linear regulator.

    I've sent you an email, but to recap, please try shorting out the ferrite bead in a failing unit and reply back with the results.
    I would also like to see plots of the input voltage, output voltage, and enable pin voltage to make sure nothing is happening at those nodes.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Mastermind 33885 points

    Hi Julian,

    Per my recent email to you, it looks like the issue has to do with the output capacitor not meeting the ESR and possibly maximum capacitance value for stability, per the datasheet recommendations. 

    Thanks

    - Stephen