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LP8758-E0: Inductance value below the minimum value

Yuto Sakai
Mastermind 6100 points
Part Number: LP8758-E0
Other Parts Discussed in Thread: LP8758

Hi,

In data sheet P.46 "inductor selection", it is stated that the effective inductance value should be 0.33 μH or more.
What is concerned when using an inductor smaller than 0.33 μH?

Best regards,
Yuto Sakai

  • 0
    TI__Mastermind 49171 points

    Hi Yuto,

      Thanks a lot for your interest in our PMIC device!  The major concern is the stability of the loop control provided by LP8758 inside design; also, DC output performance, like ripple, etc. will be impacted.

      Please let's know if you have more questions.

    Thanks!

    Phil

  • 0 in reply to Phil Yi
    Mastermind 6100 points
    Hi Phil-san,

    Thank you for your reply.
    I have an additional question.

    Q1.
    > the stability of the loop control provided by LP 8758 inside design
    Can I adjust the internal loop control with an external circuit and use an inductance value smaller than 0.33 μF?
    For example, add a feedforward capacitor etc.

    Q2.
    Is there measurement data of phase margin and gain margin for reference?
    Since measurement under specific conditions is acceptable, I want that data.

    Best regards,
    Yuto Sakai
  • 0 in reply to Yuto Sakai
    TI__Mastermind 49171 points

    Hi, Yuto-san,

    A1: Yes, adding external capacitance can help the loop stability.

    A2: No, we don't have the data of phase margin and gain margin; all data about the loop stability has been shown in datasheet in time domain as transient response. 

      FYI, all parameters in datasheet are simulated, tested and validated based one 0.47uH (+/-30%) inductance value.

      Please let's know if you have more questions.

    Thanks!

    Phil

  • 0 in reply to Phil Yi
    Mastermind 6100 points

    Hi Phil-san,

    Thank you for your reply.
    For A1, is the added capacitor an output capacitor (Cout)? Or feed-forward capacitors?
    If it is feed-forward capacitor, How shoud it be connected?
    Also, when the effective inductance value is less than 0.33 μH, how much extra external capacitor should be added?

    Best regards,
    Yuto Sakai

  • 0 in reply to Yuto Sakai
    TI__Mastermind 49171 points
    Hi, Yuto-san,
    We know add output capacitance (Cout) can help loop stability; but we have no test data to give comments on some extra capacitance for inductance less than 0.33 μH, since it depends on many conditions, like: load current value, profile (how the load changing in time and value range); working environment (temperature, etc.).
    When considering overall conditions, it's not recommended to use a less than 0.33 μH (0.47uH-30%) inductor.

    Thanks!
    Phil