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Original question:

TPS53679: Slva882

TPS53679: Slva882

GREG TING
Prodigy 60 points
Part Number: TPS53679

Hi Carmen,

I have another question.

From the equation (4), I know how many capacitance per phase I should use, but I do not know how much RMS input current per phase ?

Because how to decide amount of MLCC need to take those condition into consideration.  

  • 0
    TI__Expert 4060 points
    Hi Greg,

    You can use Equation 1 to calculate the total RMS input current for the design.

    When choosing the ceramics for CIN you find the total capacitance needed to handle the total RMS current and the total capacitance to keep the VIN ripple where you want it. Then you choose the higher number and distribute the caps accordingly across the phases.

    In the app note the input RMS current was 20A and the 22µF MLCCs were rated to 5Arms (from vendor datasheet) so I would only need 4 input caps. But, from Equation 4, each phase needs 22µF to keep VIN ripple within spec. That increases the count to 6 caps which goes up once capacitor derating is factored in. If each cap was only rated for 2Arms then I would need at least 10 caps across the six phases before looking into derating effects.

    Please let me know if you have any other questions.

    Thanks,

    Carmen
  • 0 in reply to Carmen Parisi
    Prodigy 60 points
    Hi Carmen,

    Thank you for your feedback.
    Let me clarify my understanding.
    According to input RMS current, need 20A/5A=4pcs 22uF MLCC.
    According to equation 4, need 1pcs 22uF MLCC per phase, total 6pcs for 6 phase.
    So i choose the higher number of MLCC is 6pcs for CIN.
  • 0 in reply to GREG TING
    TI__Expert 4060 points
    That's correct Greg! Don't forget to take capacitor derating into account either since with a 12V bias the 22uF caps will really be closer to 12uF depending on the manufacturer. That's why in our datasheets we typically recommend 3x22uF, 0805 per phase.

    Cheers,

    Carmen