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LM5177: Output capacitor for LM5177 design

Michael Emery
Prodigy 30 points
Part Number: LM5177

Hello,

I used the web bench power designer to create a design for a wide input buck boost converter.

Actually I created two identical designs. 1 with an electrolytic polymer cap (PCR1J390MCL1GS) as the main output capacitor.  Another with multiple MLCCs.

The switching frequency in this design is 1.31 MHz.

The capacitance of the MLCCs starts ramping up at 1 MHz.

Is it possible to get a TI power expert to look at my power bench designs and share thoughts?

cga6p1x7r1n106m250ac_200131.pdfe-pcr.pdf

  • 0
    TI__Mastermind 19665 points

    Hello Michael,

    Thanks for reaching out to us via e2e.
    It is expected behavior of ceramic capacitors that the capacitance goes down at higher frequencies.
    At the point in the diagram that you are looking at it does not increase again, instead the capacitor even behaves like an inductor.

    You will need special high frequency capacitors for your converter.
    Then determine the derating and increase the capacitance you use accordingly (above the nominal value).

    Best regards,
    Harry

  • 0
    Prodigy 30 points

    Hello Harry,

    Thank you for that.  It sounds like rolling with the Nichicon polymer aluminum electrolytic might be the better route assuming I’m not going the wrong direction to begin with,

    I may have mistook what was happening with workbench.  Are you able to access my design?

    Workbench originally spit the design out with three 4.7uF ceramics at Coutx and one custom cap at Cout specified as 1uF with either 0 or 1mOhm ESR.

    I’m used to seeing led driver docs recommending a 1uF ceramic with an electrolytic 220uF or greater for Cout and the more the better.

    I took the three 4.7 ceramics to be for smoothing and the custom part to be a bulk storage capacitor that it wanted me to pick on my own and the 1uF just to be a place holder.

    From their I determined 40uF was about as big as I could go without creating an error for phase margin falling below 35 degrees.

  • 0 in reply to Michael Emery
    TI__Mastermind 19665 points

    Hello Michael,

    You should always use ceramic capacitors for your converter which can handle these high frequencies without too much derating.

    For big input/output capacitors you can use the ceramics in combination with aluminium electrolytic or aluminium polymer capacitors.

    But it is very important that those are high quality with Low ESR and Low ESL. 

    Best regards,
    Harry

  • 0 in reply to Harry Hofner
    TI__Mastermind 19665 points

    Hello Michael,

    I have not heard from you for a while, so I am closing this thread now.
    You can re-open it or simply start a new one if this one got locked.

    Best regards,
    Harry