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TPS763: How to select output capacitor ?

Hyun woo Kim1
Intellectual 460 points
Part Number: TPS763
Other Parts Discussed in Thread: TPS7A24

Dear team, I'm hyunwoo-kim.

There is how to select output capcitor in 14page of datasheet.

there is esr range of capacitor but there is no specific frequency.

As you know , Capactior has various ESR value as frequency so, I think I have to know specific frequency for correct selection output capacitor.

please refer to the below graph, and it is recommened capacitor (TPSC475K035R0600) in TPS763 datasheet.

it has a ESR between 01.ohm and 2.9ohm from 100Hz to 100MHz.

(1) What is the specific frequency to select ESR between 0.3ohm and 10ohm ?

(2) If there is specific frequency or Range , Why do i have to check that frequency ?

Best and Regards,  

  • 0
    TI__Mastermind 44790 points

    Hi Hyun Woo,

    When we simulate ESR for stability we assume the minimum and maximum value independent of frequency. Note: the bandwidth of the error amplifier is less than 100Khz so this is where the ESR is more important. 

    The datasheet shows the following stability plots:

    I hope this helps.

  • 0 in reply to JCHK
    Intellectual 460 points

    Dear JCHK

    I understand what you mean.

    if you make EV board to evaluate LDO design , you might choose output capacitor and check ESR.

    (1) Which frequency range do you check to satisfy ESR spec ? bandwidth of error amplifier what you previous comment ?

    Best regards,

  • 0 in reply to Hyun woo Kim1
    TI__Mastermind 44790 points

    HI Hyun Woo,

    If you have an EVM, you can look at the load transient response to see whether or not the LDO is stable. As far as frequency, of concern, the ESR of the capacitor should be looked at up to 100Khz (i.e. the minimum ESR below 100KHz). You can this in the PSRR plot:

    Does this make it clearer?

    Please let me know.

  • 0 in reply to JCHK
    Intellectual 460 points

    Dear JCHK

    thank you for your reply. 

    (1) In sum , If the capacitor has ESR range by frequency , It can be checked ESR around bandwidth of error amplifier. right ?

    (2) If yes, How can I check bandwidth of error amplifier ? You showed me PSRR graph and marking it but it is not clear to find in graph because there is no any clue. (Just , it's marked in graph by your hands)

    Best regards, 

  • 0 in reply to Hyun woo Kim1
    TI__Mastermind 44790 points

    HI Hyun Kim,

    For question#1. I  don't have actual data on the bandwidth of error amplifier, but when your seen PSRR roll-off, that is around the bandwidth of the error amplifier so maybe around 50KHz?. 

    For Question #2, it is the same. When you see PSRR roll-off, that gives you a hint as to the bandwidth of the error amplifier. 

    If you want a device that is inherently stable with ceramic capacitors, have a look at our TPS7A24. 

    Also here is a link to an app note on comparing an older LDO and ESR stability vs the TPS7A24.

    I hope this helps.

    Regards,

  • 0 in reply to JCHK
    Intellectual 460 points

    Dear JCHK

    In sum,  checking ESR of capacitor at around bandwidth of error amplifier and the easiest checking way is see the roll-off of PSRR graph.

    Right ?

    Best regards,  

  • 0 in reply to Hyun woo Kim1
    TI__Mastermind 44790 points

    Hi Hyun Kim,

    Yes. That is a good way to estimate the bandwidth of the error amplifier and a good way to estimate whether or not your ESR is within the range. 

    Going forward, I am trying to get this clarified better on datasheets.

    Regards,