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TPS568215: Input capacitance

Elder Costa17
Expert 2825 points
Part Number: TPS568215


Hello,

I would like to know how critical is the input capacitance. The data sheet recommends 40uF and Webench does add two 22uF. However, the derating in the tool is too optimistic. For example, one design I tried (Vin 11-13V, Vout 1Vx4A) it suggested 2x TDK C3216X5R1V226M160AC. It turns out the effective capacitance @12V is around 5uF whereas the tool derates it to approximately 14uF (28uF total).

Thanks in advance.

  • 0
    TI__Mastermind 27470 points
    Hi Costa:
    The input capacitance major affecting: 1 input ripple 2 the temperature raising of itself.
    Our datasheet gives the formula calculate the Cin and Irms of input current on 8.2.2.15 . The larger and multi Cin will have lower ripple and smaller RMS current go through input capacitors. for 1V@4A, with 10uF effective capacitance, the Vin ripple will be around 83mV(not include voltage on ESR) and Irms on each cap will be 2A based on 400Khz Fsw. And temp raise around 5 deg C based on C3216X5R1V226M160AC datasheet. Too small input cap, you will see you circuit have a large input ripple on you input capacitor which make the output unstable. and input capacitor will be hot.


    hope this explanation helpful

    Thanks
  • 0 in reply to Daniel Li14
    Expert 2825 points
    Hi, Daniel,

    Thank you very much for your quick response.

    I thought I had mentioned the capacitor P/N was suggested by Webench, which wrongly derates it to 14uF (28uF total) when the derating graph on this capacitor's data sheet shows ~5uF@12V.

    I will take a look at the information you provided.

    Thank you again.

    Elder.
  • 0 in reply to Elder Costa17
    TI__Mastermind 27470 points

    Hi Costa:

    the modeling team will be hand all the component model including capacitance dc bias in the wenbench. we will feedback this information to wenbench team. see any improvement they can make.

    Thanks

    Daniel Li

  • 0 in reply to Daniel Li14
    Expert 2825 points

    Hello, Daniel.

    Thank you for the update.

    I followed your first advice and performed the calculations to determine the recommended capacitances for the several supplies based on TPS568215 in my design. I added the capacitors required to reach those capacitances, derating per capacitors data sheet.

    Thank you very much for your support.

    Best Regards,

    Elder.