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LM34936-Q1: How to calculate the EMI Filter before designing the board or the system

Expert 1140 points
Part Number: LM34936-Q1
Other Parts Discussed in Thread: LM34936


We are designing an EMI Filter with the AN 2162 Document. As per the equation |Att|dB=20 log (Vin-ripple-p2p/ 1µV) - Vmax

How can i determine the Vin-ripple-p2p before designing the board. I found an equation in one of the TI reference designs for calculating the Vin-ripple-p2p by using the capacitance ESR value but i lost the Document.

Can you please share that way how to calculate the Vin-ripple-p2p value using the ESR value of the input capacitance so that i can calculate the required attenuation for my DC DC Converter.



  • Hi Amal,

    Thank you for posting. You need to first calculate the input ripple current by assuming no input capacitor, and this includes calculating all three operating mode: Buck, boost, and buck-boost, of the LM34936.  Then, you need to use these ripple current to calculate your ripple voltage across your selected input capacitors, and this is your initial Vin-ripple to be attenuated, based on that, you select your additional EMI filter.

    Note that your EMI filter's parasitic can affect the effectiveness.  Like the inductor's parasitic capacitors actually passes the high frequency EMI signal without much filtering, and the capacitor's ESR and ESL reduces the filtering performance.  

    Which this clarifies.


    Youhao Xi, Applications Engineering

  • Hi Youhao,

    Thankyou so much.

    I have one more request.. could you please share a relevant document which has the equation to calculate the ripple current without the input caps in all modes of operation as you specified

    I couldnt find any documents related to same.

    it's a request.



  • Hi Amal,

    What I said in my previous reply was a generic approach.  To calculate these values, you can follow any power electronics textbook. TI engineers also put together a handbook which you can refer to, which include all basic circuit topologies.