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LM25143-Q1: Input Impedance

Part Number: LM25143-Q1

Hi team,

How to calculate or derive input impedance of a LM25143-Q1 circuit? It is used to design the input filter before the power supply. 

In the below app note: The Middlebrook’s extra element theorem (paper [2]), explains that the input filter does not significantly modify the converter loop gain if the output impedance curve of the input filter is far below the input impedance curve of the converter. In other words to avoid oscillations it is important to keep the peak output impedance of the filter below the input impedance of the converter. (See figure 3)