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TPS7B85-Q1: Output capacitor ESR test condition and the test frequency

Zirui Su
Expert 5340 points
Part Number: TPS7B85-Q1

Hi

I would like to ask a question that the capacitor ESR requirement in TPS7B85Q1 SPEC is 1m Ω~2 Ω and want to know what is the test frequency? Need to know the test condition and the test frequency for validation, thank you!

  • 0
    TI__Mastermind 36045 points

    Hi Zirui,

    The ESR is located at the resonance of the capacitor.  It is the minimum impedance in the capacitor impedance curve.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Expert 5340 points

    Hi Stephen,

    Thanks and do you mean the 1m ohm to  2ohm tested at 1kHz frequency? Want to know the specific frequency value, thank you!

  • 0 in reply to Zirui Su
    TI__Mastermind 36045 points

    Hi Zirui,

    It is not 1 kHz, it is the actual resonant frequency of the capacitor.  Each capacitor will be different.  Usually there is a capacitor tool provided by the manufacturer that gives this information.  You will have to ask the capacitor manufacturer for the data if you cannot locate it online.

    As an example, here is the link to the capacitor tool from AVX.  You can use the drop down boxes to select the criteria for your specific capacitor, then you can locate the impedance plot across frequency.

    https://spicat.avx.com/mlcc

    For this example capacitor, the resonance is 5.26 MHz and the ESR value you would use is 4.7 miliohms.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Expert 5340 points

    Hi Stephen,

    Thanks for response and the question my customers are concerning is about the datasheet said that the ESR should between 1mohm and 2ohm but without frequency range description. So I really would like to know what is the requirement when ESR between 1mohm and 2ohm, Car oem need to know about the  frequency range when ESR between 1mohm and 2ohm. Here is the capacitor I have found and could you please comment it whether it is OK for TPS7B85Q1? Thank you.

  • 0 in reply to Zirui Su
    TI__Mastermind 36045 points

    Hi Zirui,

    The requirement in the datasheet pertains to the stability.  The stability is related to the bandwidth of the feedback loop.  So the frequency range will depend on the bandwidth of the LDO, however that is not published data.  If you use 10 MHz as a rule of thumb, that will give you an upper range of frequencies for most LDO's. 

    It is hard for me to see the ESR in your plot but it looks like it is 2 milliohms around 2 MHz.  If so, this meets the requirement in the datasheet of 1 milliohm minimum.  So this capacitor would be a good choice for this LDO.  Keep in mind that the effective ESR will be slightly higher due to board resistance, depending on how far away the capacitor is from the LDO.

    Thanks,

    - Stephen