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TIDM-02002: What is the formula to calculate the value of the low-pass filter (C245, R254)?

Pawel Z
Prodigy 70 points
Part Number: TIDM-02002
Other Parts Discussed in Thread: TMDSCNCD280049C

There is a low-pass RC filter based on R254 and C245 with 56.2Ohm and 10pF accordingly. This gives roughly a cut-off frequency at 283MHz.

What is the calculation formula or a reason of choosing 283MHz specifically? How should I calculate this filtering, if I'm going to choose different switching frequency in own design?

Should I go lower with a cut-off if the switching frequency is lower too?

  • 0
    TI__Mastermind 41415 points

    I've reached out to the designer, will respond on Monday.

    Regards,
    Cody

  • +1
    TI__Expert 3650 points

    Hi,

    C245 is actually not populated. We are using TMDSCNCD280049C F280049 controlCARD for this design. And the "ISNS_PRIM_TANK" node has tie to 2 ADC pins. There is 330pF total capacitance at those 2 ADC pins on the controlCARD. Therefore, the cut-off frequency will be ~8.6MHz.

    But it will work even without the filter capacitor as for the tank current sensing, we are actually care the zero current crossing point (around turn off transient), which is pretty quite compare to the noisy turn on transient.

    Regards,

    Sheng-Yang Yu