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TMDS570LS31HDK: Load Capacitance Calculation

Mitchell
Expert 4915 points
Part Number: TMDS570LS31HDK

Hi Team,

I have a question on the calculation of load capacitance provided in the Hercules EVM (http://www.ti.com/tool/TMDS570LS31HDK ).

The load capacitance was spec'd at 10pF (C27/28), however, the 7A-16.000MAAJ crystal (https://www.mouser.com/datasheet/2/417/7a-13881.pdf ) specifies the load cap ~ 18pF and strap cap of ~ 5pF.

Can you share some insights as how 10pF was obtained? Shouldn't C27/28 be ~36-26pF, according to this post: https://e2e.ti.com/support/microcontrollers/hercules/f/312/t/667495?TMS570LS0432-Resonator-Selection

Thanks,
Mitchell

  • 0
    TI__Guru**** 186196 points

    He Mitchell,

    You are right, the capacitor for this crystal should be 26pF. The crystal external load capacitance can be calculated:

    cLOAD = (C27 * C28)/(C27 + C28) + Cstray = 0.5*C27 or 0.5*C28 + Cstray

    C27 or C28 should be (Cload-Cstray)*2 =(18-5)*2=26pF, normally Cstray is 2~5pF

    Please don't use the value used on HDK and Launchpad.