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Original question:
TI app note #SNVA538 has an equation on page 13 used to calculate the input impedance of a switching regulator circuit using the LM2675-5.0.
Ro = 5 ohm, Rl = 0.088 ohm, ESR = 0.09 ohm, D = 0.458, Lo = 68 uH, Co = 68 uF
I generated a graph using the equation and page 13 assuming Si = 2 x 3.14 x Fi where Fi = frequency 1 Hz to 100,000 Hz
Here is the resulting graph using an Excel spreadsheet:
My graph obviously doesn't match the graph on page 13.
The value at 100 Hz in my graph is 20.3 ohms, in the TI graph it is roughly the same.
The value at 900 Hz in my graph is 10.6 ohms, in the TI graph it is roughly the same.
The value at 2500 Hz in my graph is 9.6 ohms, in the TI graph it is roughly 1.8 ohms which is a big discrepancy.
The value at 6000 Hz in my graph is 13.0 ohms, in the TI graph it is roughly 10 ohms, another big discrepancy.
I verified the Zi values at 100, 900, 2500 and 6000 Hz manually using a hand calculator to eliminate possible issues with the Excel spreadsheet calculations.
On page 5 of the application note it says that the curves for the output impedance of the input filter and the input impedance of the switching power supply should be well separated.
If I can precisely replicate the graph on page 13 it will give me the confidence to use that equation for the Zi impedance calculations.
Any help to resolve the discrepancies would be much appreciated.
