Hi Team,
Good day! I need your help to check our customer's concern below.
TI has an application note #SNVA538 which explains how to properly design an input filter for a switching power supply using the LM2675N-5.0 as an example. https://www.ti.com/lit/an/snva538/snva538.pdf
On page 13 there is an equation provided for the input impedance calculation which can be used to generate a graph of impedance versus frequency to help in the design of the input filter for the power supply.
I presumed that Si = 2 x PI x frequency and input the appropriate equations into an Excel spread sheet in an attempt to reproduce the graph shown on page 13
My graph doesn't show a peak of 2 ohms at 2500 Hz as shown on page 13.
My graph shows a minimum impedance of 9.2 ohms at 2000 Hz.
At 1000Hz my spread sheet result was 10.2 ohms for Zi which matched the graph on page 13.
I have checked the result for 1000Hz and 2000 Hz using a hand calculator and they match what the spread sheet results were. I am trying to figure out where my error is and was hoping TI could provide some assistance. I am assuming the equation provided on page 13 is correct and that the value to substitute for Si = 2 x PI x frequency.
Thank you
Best regards,
Jonathan