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ISO1044DEVM: output capacitor usage

Ann_Lien
Genius 10640 points
Part Number: ISO1044DEVM
Other Parts Discussed in Thread: ISO1044

Hello expert,

May we know the usage for L1, C4/C5/C6? As we know, L1 is for common choke, filter usage. And C5/C6 are CANH/CANL output capacitance. 

Best regards,

Ann Lien

  • 0
    TI__Genius 10640 points

    attached circuit here.

  • 0 in reply to Ann_Lien
    TI__Mastermind 49445 points

    Hi Ann,

    Thanks for reaching out.
    Both the common-mode choke and CAN bus capacitors (both common-mode and differential capacitors) are meant for EMI noise filtering. They are effective for reducing emissions from device and also for suppressing external noises from affecting ISO1044. There are even other ways to achieve such filtering / noise suppression like using TVS diode, MOV, GDT among others. The actual components needed depends on the EMI tests that need to performed and on their passing requirements levels.

    Please find below a CAN reference design that implements various protection components for ESD, Surge and EFT and tests all of individually and in combination. A lot of test data is provided to show how these protection components enable a CAN device to meet various EMI requirements. The same protection components will also be applicable for ISO1044, although the actual test levels achieved can be different.

    https://www.ti.com/tool/TIDA-00629

    Let me know if this answers your question, thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Genius 10640 points

    Hello Rao,

    Thanks for your reply! May I know how can we calculate the capacitance value? Thanks a lot!

    Best regards,

    Ann Lien

  • 0 in reply to Ann_Lien
    TI__Mastermind 49445 points

    Hi Ann,

    The overall capacitance affects the actual datarate that you can achieve on CAN bus. Please note that one has to take into account the capacitance due to other protections components like TVS, MOV and others. Hence, there is restriction on how high one can go on these capacitance values, usually in 10s of pF or maybe a few 100s of pF for slower datarates.

    The capacitors alone might not be sufficient to meet EMI requirements, they only slow down the transients to some extent but it does help overall. Other components like TVS have a larger role in clamping voltage on the bus and protecting the device.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Genius 10640 points

    Hello Rao,

    Thanks for your reply :)

    Best regards,

    Ann Lien