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TPSM84A21: Is there a recommendation for an input filter for TPSM84A21/22?

Dor Aharony
Prodigy 30 points
Part Number: TPSM84A21

Hello,

I have multiple regulators off a single 12V rail off a server. The switching freq of the 12V input is unknown and can vary from server to server. Are there any recommended input filter values for use with this regulator?

Thank you,

Dor

  • 0
    TI__Mastermind 18370 points

    Shalom Dor,

    The TPSM84A21 is designed to operate from an input voltage supply range between 8 V and 14 V.
    This input supply should be well regulated and able to withstand maximum input current and maintain a stable voltage.
    The resistance of the input supply rail should be low enough that an input current transient does not cause a high enough drop at the supply voltage that can cause a false UVLO fault triggering and system reset.
    If the input supply is located more than a few inches from the TPSM84A21, additional bulk capacitance may be required at the input pins.
    A typical recommended amount of bulk input capacitance is 47 μF - 100 μF.

    Hope this helps.

    Regards,

    Yitzhak Bolurian

  • 0 in reply to Yitzhak Bolurian
    Prodigy 30 points

    Shalom Yitzhak, I am missing something here. Wouldn't the switching current at the input of the TPS induce noise on other regulators on the same 12V rail? I understand it has low input resistance, but is the input to the TPS isolated from its output? Are there any articles by TI explaining this relationship? Thanks again,

  • 0 in reply to Dor Aharony
    TI__Mastermind 18370 points

    Yes. There is some switching noise at the input, but it is minimal. 
    Ideally the input current is mostly DC. 
    You could add a Ferrite bead and a cap to filter this switching noise further.
    It all depends on the max ripple/noise requirement.
    You may or may not need it.

    The input of TPS is not isolated from it's output.
    But even in isolated DC/DCs there is still some induced noise at the input. 
    This is not necessarily related to isolation.

    If you want, you can add a filter (ferrite bead and extra cap). 
    Later, you can always decide not to use it. (Replace the bead with a short and not populate the cap).

    https://www.ti.com/lit/an/slta013a/slta013a.pdf?ts=1625250877333&ref_url=https%253A%252F%252Fwww.google.com%252F

    Regards,

    Yitzhak