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SNVA538 input Impedance calculation of a buck converter

Hi everyone,

I would like to calculate the input impedance of a Buck converter in general.

In fact I want to design a proper input filter, thats what I need the impedance for.

Therefore I found the application note SNVA538 from TI. Which is a really helpful document in this case.

www.ti.com/lit/an/snva538/snva538.pdf

In that document on page 13, there is a big formula for calculating the input impedance.

The formula looks very handy compared to other ones i have found but I am not sure if it fits my case.

Does anybody know on which assumptions this formula is based on? Or does anybody know how to derive it?

Thank you very much,

Markus