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TPS7A84A: Ref Design Schematic Review Request

Part Number: TPS7A84A

Hi Team,

I am working on a reference design board for a high-speed device and would like to use the TPS7A84A to provide my supply rails. I will be taking 3.3V from a lab supply and supplying 2 rails for my device. Depending on the DUT loaded, the supply rails will need to be either 2.5V/2.5V or 1.2V/1.8V, respectively, so I have chosen to implement this flexibility with the ANY-OUT feature.DS560ARXXX_HCB_LDO.pdf

I have referred to the TPS7A84EVM as a reference, but could use some expert help to double-check whether I have missed anything before going into layout.

Can you review the attached schematic?

Thanks,

Michael

  • Hello Michael,

    I will respond to your question within 2 business days.

    Thanks,

    - Stephen

  • Hi Michael,

    The designs look good overall.
    The other schematic sheets are not shown, but please make sure to add local decoupling capacitors from Vdd to RTN of each digital IC.
    To help protect against EMI, you should have GND of the linear regulators on layer 2, directly underneath the LDO circuitry.
    You can use Altium to run a Power Distribution Network analysis of your final layout, when you have that, to review steady state IR drop and current density.

    If there was anything I would modify in your schematic, it would be to increase the voltage rating on the 47uF capacitor.

    The datasheet assumes a 50% voltage derating for the capacitors (see section 8.1.1).
    The voltage bias you have will be 3.3V (input) and 2.5V (output), not including tolerances.
    It is not clear to me if the datasheet intent was to derate applied voltage vs rating (2V max on a 4V part) or review manufacturer specs for actual cap loss.
    In any event, with 2.5V or 3.3V applied, we lose more than 50% of the initial capacitance according to Murata's website.

    I reviewed several options on Murata's website, the manufacturer of the 47uF cap.
    There may be options in other manufacturers, but I only reviewed Murata's website because they made the capacitor you originally chose in your design.
    There are no better options at this time in an 0805 sized package. 
    If you choose a 1206 sized package from Murata, the 47uF, 6.3V rated value has favorable curves to these datasheet recommendations.
    Here are the curves for GRM31CR60J476ME19K.
    I also reviewed Altium Vault and it looks like this component is already in the design database.

    Thanks,

    - Stephen

  • Hi Stephen,

    Thanks for the expert feedback!

    We have a combo of 1uF and 0.1uF for our retimer IC that will be placed closest to the supply pin vias and GND.

    I think we have room to move up to a 1206 size cap per your recommendation so that we have a better quality 47uF cap. Thanks a bunch as well for making the extra effort to check availability in the Vault.

    Regards,

    Michael