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TPS65400: RCLOCK_SYNC pin capacitor usage question.

ahmet ersoy
Prodigy 20 points
Part Number: TPS65400

Hi,

While I was studying TPS65400 EVM Schematic, I could not find any explanation about C46 calculation that is connected to RCLOCK_SYNC pin.

Why do we use it and how do we calcualte it?

http://www.ti.com/lit/ug/slvuac4/slvuac4.pdf

Best regards,

  • 0
    TI__Mastermind 19080 points
    Hi Ahmet,

    I think it's used for filtering the high frequency noise on RCLOCK SYNC pin. A few pF is enough for this.


    Regards,
    Sam
  • 0 in reply to Sam(MPAA) Zhang
    Prodigy 20 points
    Thank you for the answer but, I want to learn how to calculate it.
  • 0 in reply to ahmet ersoy
    TI__Mastermind 19080 points
    Hi Ahmet,

    I don't think there is a equation you can refer to directly to calculate it...
    During normal operation, on RCLOCK SYNC pin there should be a DC voltage. Internal circuit take this voltage as a reference to generate a clock signal. A small capacitor here can help to attenuate the high frequency noise on this DC voltage. As current configuration, 4.7pF capacitor and 374k resistor forms a pole at ~90kHz, which means any noise with a higher frequency than that is attenuated.
    Actually, even without this capacitor, the part should also work well. A lot of EVM designs for other parts don't have this small capacitor, such as TPS65261-EVM.
    Hope this can help.


    Regards,
    Sam