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  • TI Thinks Resolved

TPS756: Input Capacitor

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Replies: 5

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Part Number: TPS756

Hi Team, we just want to make a clarification regarding the input capacitor. Can we just use a single capacitor for bypassing and help on not drooping? And is there a guideline on selecting the parallel capacitor in case a single capacitor is not allowed.

Thanks.

Regards, 

Mark

  • Hi Mark,

    Ceramic capacitors or polymer type capacitors have very low ESR and are good choices for this design challenge.
    You may use a single capacitor if the current transient does not cause the voltage drop to exceed the UVLO threshold.
    Use the standard capacitor equation to estimate the drop.

    • The drop due to capacitance will be I=C*dV/dt.
    • The drop due to ESR will be Isurge * ESR = drop.

    The ESR drop will be seen immediately, followed by the capacitor droop until the source supply can kick in and provide additional current.

    The delay on the source supply will be due to its own feedback loop and limited bandwidth.

    If a single capacitor is not possible, then the common guidance is to use different capacitors a decade apart in parallel with each other.
    So 1uF, 0.1uF, etc. 

    Thanks,

    - Stephen

  • In reply to Stephen Ziel:

    Sir,

    As we have finalized our board schematics with only a bypass capacitor (GRM219R61H105MA73D) at the LDO input (TPS75633), I have the following questions for you:

    1. If the input to the LDO is provided by a battery, still is there a chance of input voltage drooping due to large transient current? If still a droop occurs, please answer below question.

    2. The normal current requirement of my board from the LDO is 300 mA. Only Sometimes the total current requirement from LDO goes to 400 mA. So how much voltage droop at the input I may expect as per the capacitor equation you have mentioned in your reply.Note that only a bypass capacitor exists at input.

    Your reply will help me to use a parallel capacitor if it is definitely/compulsorily required.

    Regards,

    Ameet Kumar Haware

  • In reply to Ameet Haware:

    Hello Ameet,

    FYI - the 1uF capacitor (GRM219R61H105MA73D) is listed by Murata as "not for new designs" so you may wish to look for an alternate component.
    This capacitor has a nominal ESR of 10 milliohms, and your current draw delta is only 100mA, so the total drop due to ESR will be very small: 0.1A * 0.01 ohms = 1mV.  I would ignore this.  The capacitive droop depends on how long the source supply takes to respond to the request in 100mA additional current, as well as how fast the ramp rate is for the extra current.  If the current pulse is a slow rising pulse, the capacitor may be sufficient as the battery may react fast enough to provide the additional current.  If the 100mA current pulse is fast, the battery may not react in time and the capacitor may need to increase.  A 1uF capacitor on the input may or may not be enough.  You will have to test this if you do not have additional data on the battery to go by.

    Thanks,

    - Stephen

  • In reply to Stephen Ziel:

    Thanks for your reply sir.

    I have one last question to you:

    1. The datasheet of TPS75633 says to use 0.22 uF-1uF bypass capacitor at its input. So is it possible to use a capacitor higher than 1 uF, which will help in both bypassing and not drooping.

    The reason for asking you this question is because we have freezed the schematics of the board.

    We can change it only if it is compulsorily required.

  • In reply to Ameet Haware:

    Yes, you can add a larger capacitor.
    On page 18 of the datasheet, it specifies "If this droop causes the input voltage to drop below the UVLO threshold, the device will turn off.  Therefore, it is recommended that a larger capacitor be placed in parallel with the ceramic bypass capacitor at the regulator's input."

    Thanks,

    - Stephen

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