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TCAN1043A-Q1: VSUP recommended capacity

oaku.hiroshi
Prodigy 190 points
Part Number: TCAN1043A-Q1

Tool/software:

Hi

The recommended circuit "9.1.1 Typical Application" and "10 Power Supply Recommendations" recommend 100nF, but I have designed 100nF in series immediately next to the current Vsup power supply. (To prevent short circuit failure due to primary failure) Therefore, the capacitance will be 50nF, but should I design it to be 100nF even when using it in series?

  • 0
    Prodigy 190 points

    What are the concerns about capacitance below 100nF?

  • 0
    TI__Mastermind 27320 points

    Hi Oaku-san,

    100nF is only the recommended value, it's not required to be exactly 100nF. The main use of this capacitor is to suppress voltage ripple during the switching of power supplies so you can choose this value based on your application's requirements.

    I'm not sure if you put this capacitor in series with the power supply will work as the same function, for a DC voltage input, it should expect a surge in current at the instant when the capacitor and the battery are connected, then current immediately drops to zero because the capacitor is fully charged up. 

    Would you share the schematic to help me understand how this series capacitor is placed?

    Regards,

    Sean

  • 0 in reply to Sean Guo
    Prodigy 190 points

    Hi

    Sean-san

    Thank you for your reply.

    I've attached a simple circuit diagram.

    I understand that 100nF is recommended and there is no need to adjust it, but is that correct?

    I think the attached circuit diagram gives you an idea of ​​what it is, so I would like to receive your reply again.

  • 0 in reply to oaku.hiroshi
    TI__Mastermind 27320 points

    Hi Oaku-san,

    Sorry I misunderstood your connection way, yes that is fine, you can put two capacitor in series. 

    And as you said thel capacitance will be 50 nF with two 100 nF in series, I would recommend to replace with larger capacitors like 0.2uF so it will still be 100nF in total.

    Regards,

    Sean

  • 0 in reply to Sean Guo
    Prodigy 190 points

    Hi

    Sean-san

    Thank you for your reply.

    Your previous reply was "100nF is only the recommended value, it's not required to be exactly 100nF." Does that mean it's better to use more than 100nF?

    Which is correct?

    If the latter is correct, I have a 1200uF electrolytic capacitor installed on the same line of VSUP, but can that be interpreted as being okay because it's more than 0.1uF?

  • 0 in reply to oaku.hiroshi
    TI__Mastermind 27320 points

    Hi Oaku-san,

    Generally we recommend at least 100nF for decoupling, more than that value is also OK. Since your are using two capacitors in series, in order to keep the total capacitance still be 100nF, you may need to use two 200nF.

    For the 1200 uF electrolytic capacitor, it is OK to have that bulk capacitance on the VSUP line, but I still recommend to keep the ceramic 100nF in parallel for decoupling high frequencies,  and place it near the VSUP terminal, as close as possible to the pin.

    Regards,

    Sean