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LM5175-Q1: Please help to advise input capacitance

user4979989
Intellectual 2800 points
Part Number: LM5175-Q1

Hi Sir,

The operation condition is 6V-20V input, 8.5V output, 2.1A output current.

Current design is 68uF(POSCAP) and 3*10uF/1206/35V MLCC in input side.

Please help to suggest if it's ok to replace input 68uF POSCAP to the 7*10uF MLCC?

Thank you.

  • 0
    TI__Mastermind 31315 points

    Hi,

    I would recommend keeping the footprint for the POSCAP on the design, but you can populate it as needed. If you were to add in more MLCCs, please keep in mind with MLCC's there will be a capacitance de-rating based on the voltage bias. 

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    TI__Intellectual 2800 points

    Hi Richard,

    Because there is layout space consideration, customer can't reserve POSCAP location.

    Could we suggest them to put the MLCC capacitance but need to consider DC bias? 

    Thanks.

  • 0 in reply to user4979989
    TI__Mastermind 31315 points

    Hi user,

    If that is the case, I would recommend putting the MLCC capacitance on the board, but have one of the capacitors be in series with a damping resistor. Yes, also tell them to consider DC bias.

    The resistor can be populated as 0-ohm if not needed, but it will be good to have. If there is insignificant damping, you may find that during a load step, oscillations may occur due to the resonance between the parasitic inductance and capacitance; and may destabilize the dc/dc converter. 

    Thanks,

    Richard