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LM5127-Q1: Output capacitor selection using quickstart calculator

shihui xu
Prodigy 100 points
Part Number: LM5127-Q1

Tool/software:

some questions about the output capacitor selection parameter:

1. What frequency does the ESR value here refer to?

2. How to calculate the ESR value when I put several different cap value of mlcc and e-cap in parallel?

3. Should the bulk e-cap put beside load side also be counted as the output capacitance here?

  • 0
    TI__Mastermind 22295 points

    Hi Shihui,

    Typically, we are using e-caps and ceramic caps, and the capacitance of e-cap is much larger than that of ceramic caps.

    In this design calculator, the ESR refers to the e-cap ESR as it determines the ESR zero. The ESR of the e-cap is almost constant over frequency.

    The bulk e-cap at the load side should be also counted if they are not very far away.

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    Prodigy 100 points

    Hi Feng,

    What if I don't use e-caps only MLCC (Since height limitation)?

    And how "far" should I consider this into calculation, we have some loads near the boost output  and some loads relatively far away.

  • 0 in reply to shihui xu
    TI__Mastermind 22295 points

    Hi Shihui,

    If MLCC only, the ESR is typically very low.

    "Far" means more ESL and ESR. You should calculate the impedance of the output caps (including ESR, ESL) and see which is dominating the zero. Here is an example.

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    Prodigy 100 points

    Hi Feng

    Yes the ESR of is typically very low around its resonant frequency, but it also varys by frequency. That's why I want to know the frequency of ESR in the Quickstart form.

    When you say "Far" means more ESL and ESR, you probably mean that I also need to take the ESR and ESL cause by PCB traces into account when I  calculate the impedance?

  • 0 in reply to shihui xu
    TI__Guru 65170 points

    Hello shihui,

    Far means in this case when the caps are not connected through a high impedance connection. If they are connected with a relatively low impedance, independent where they are placed on the board, they count for the ESR value. Please be aware that the ESR of ceramic caps is normally low enough that the resonant frequency is well above the crossover frequency and does not really influence it.

    You might check this with the quickstart calculator and different ESR values in the low mohm range.

    Please be aware that the quickstart calculator shall help to start a design with ok values, it does not replace detailed tests in the lab, but normally it should give you values that result in a stable design as long as the layout rules are as well kept in mind.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 100 points

    Hi Brigitte,

    So the ESR here on the form only affects the ESR zero and has nothing to do with the output ripple? Do I only need to concern with the ESR near the crossover frequency (for this cell)?

  • 0 in reply to shihui xu
    TI__Mastermind 22295 points

    Hi shihui,

    The design calculator ignores ESR influence to the output voltage ripple.

    You are correct, take the ESR at crossover frequency for this cell.

    Best Regards,

    Feng