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Original question:

TPS7B7702-Q1: Time dealy for EN pin toggling

TPS7B7702-Q1: Input capacitance rule

Masayuki Uchihara
Intellectual 1686 points
Part Number: TPS7B7702-Q1
Other Parts Discussed in Thread: TPS7B7702

Hello team,

I have question about the selection of TPS7B7702's input capacitance value. According to datasheet, the recommend value is 10uF but depended on the application. Now we are likely to change the value. Is it possible to change smaller capacitance than 10uF? Do you have any concern to select smaller capacitor?

I am awaiting your reply.

Thanks,

Masayuki Uchihara  

  • 0
    TI__Mastermind 23361 points

    Hi Uchihara-san,

    When you plan to use both output channels from TPS7B7702, the concern with smaller input capacitor is due to the reason that a short condition happening on one output channel may create a dip on your input supply and this may trigger the other output channel into reverse current protection mode. Using 10uF or bigger input capacitor can reduce the chance of false trigger.

    Here is the related section I copied from datasheet.

    Depending on what's the current limit you set on each output channel through the resistors, you may reduce the size of the input capacitor if you have a relatively smaller current limit. However, if false trigger is a problem for your application, you will need to decide the input capacitor size by running short circuit test on your boards to find out the impacts from shorting one channel to the output of the other channel. As 10uF is a suggested value for most of the application that can prevent the false trigger, it may or may not be the best ones for your application.

    Regards,
    Jason Song

  • 0 in reply to Jason Song
    TI__Intellectual 1686 points

    Hi Jason, 

    How about TPS7B7701? we also know the rule for 1ch this LDO. How do I calculate acceptable smaller value for resistor? 

    Regards,

    Masayuki

  • 0 in reply to Masayuki Uchihara
    TI__Mastermind 23361 points

    Hi Uchihara-san,

    For the single channel version of TPS7B7701, we would recommend to have a 10uF decoupling cap at input as datasheet suggested. The input capacitor is to help lower the impedance of the power supply to the LDO. You can potentially reduce the size of the capacitor if you have a very low impedance supply. 


    Regards,
    Jason

  • 0 in reply to Jason Song
    TI__Mastermind 23361 points
    Hi Uchihara-san,


    This is a follow-up, do you have any questions regarding my last reply about the input cap selections?

    Regards,
    Jason Song
  • 0 in reply to Jason Song
    TI__Intellectual 1686 points
    Hi Jason,
    Could you tell me how much value for battery impedance is low ?

    Regards,
    Uchihara
  • 0 in reply to Masayuki Uchihara
    TI__Intellectual 1686 points
    Hi Jason,
    In my understanding there are some good point to decrease battery impedance to add the input capacitor.
    - The affection of external noise should be decreased.
    - The output voltage should be stable if the LDO has load transient.
    - The input ripple voltage should be decreased

    Do you have other good point to add the input decoupling capacitor for the LDO?

    Thanks,
    Uchihara
  • 0 in reply to Masayuki Uchihara
    TI__Mastermind 23361 points
    Hi Uchihara-san,

    If the LDO is very close to the battery, most likely the source impedance from the battery should be low. But in application, there could be a long wire from the battery to the input pin of the LDO, the added inductance from the wire could potentially increase the impedance at certain frequency. The added 10uF will help with this case.

    Your understanding on the good points of having an input capacitor are generally correct. There is one point I would like to bring up. The noise of LDO is considered to be self-generated ones and primarily from the internal reference voltage. Input cap will help with PSRR, not noise.

    Regards,
    Jason Song