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lmz23610 ESR min

JOHN CARRARA
Intellectual 485 points
Other Parts Discussed in Thread: LMZ23610

Customer question:

What is the minimum ESR for proper function.

Equation 8 is an approximation.

"For applications with dynamic load steps; the following equation provides a good first pass approximation of COUT

for load transient requirements."

  • 0
    Intellectual 485 points

    Adder from customer:

    I noticed that Evaluation has CFF across RFBT with additional RFB_LP while data sheet doesn’t mentioned CFF and RFB_LP? What is the purpose of RFB_LP? What is the purpose  of CFF? And why is evaluation board are using two RFBT?

  • 0 in reply to JOHN CARRARA
    TI__Genius 16025 points
    Hi John,

    It depends on how much output voltage ripple you are accepting of. For 15mOhms and less, you will need more Cout and if you have more than 15mOhms, you'll need less Cout.

    That said, I would start with 15mOhms and then increase or decrease it based on your application.
    I would then check the loop response of the Part using a bode plot or load transient test and adjust my Cout to achieve the best compensation.

    The Cout equation in equation 8 indicates the minimum Cout you will need.

    Cff is used to further help compensate the part. This will become necessary when design constraints prevent you from achieving the necessary Cout and ESR combination to have a stable design.

    The most efficient way to have a complete design that can be tweaked and simulated is to use WeBench. Simply navigate to the LMZ23610 product page and fill out your conditions in the Webench applet on the middle right of the page.

    Rfbt_lp is simply an injection resistor that carries a value of 10ohms to 100ohms. It is only used for the purposes of a bode plot to convert the signal coming from the bode analyzer into a ripple voltage to be injected into the FB pin. You do not need it and it will not affect operation as long as it's value is less than 100 ohms.

    Thanks,
    Anston
  • 0 in reply to Anston Lobo
    Intellectual 485 points
    Customer has a follow on:

    Is ESR based on total effective ESR of all capacitor or each capacitor?

    Again, thanks for the help.
  • 0 in reply to JOHN CARRARA
    TI__Genius 16025 points
    Unless specified it is Total effective ESR.

    Thanks,
    Anston
  • 0 in reply to Anston Lobo
    Intellectual 485 points

    Customer follow-on question:

    Which one to rely on data sheet formula or Web Bench or use combination of both? Web Bench only has output of OSCON cap with no ceramic capacitor and input of OSCON as well with no ceramic at all. The data sheet recommended have 47nF in parallel to capacitor for high frequency on both input and output as well 47uF ceramic in parallel on output capacitor.

                   To clarify that it recommended tantalum capacitor in parallel with ceramic on data sheet but still need it for Aluminum Polymer capacitor since that Aluminum polymer capacitor is ultra low ESR which give overall impedance of 6mΩ which web bench recommended maximum of 8mΩ.

    Thanks in advance.