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LMR36520: How to use PSpice to achieve open loop bode plot?

Part Number: LMR36520

Hi,

I want to use PSpice's AC sweep function to achieve open loop bode plot of LMR36520, while the results don't match analysis well.

The process of my simulation is followed, hope you can help check it and tell me what is the wrong. Thanks.

Step 1: Add a AC source to inject a small signal, shown in the following picture.

Step 2: AC sweep setup like this.

Step 3: Run simulation and add traces.

However, the result is obviously wrong, since the amplitude at low frequency is less than zero.

Furthermore, since the circuit has not achieved expected output because of soft start at the beginning, so I am confused what is the steady state point is used to calculate the bode plot in the PSpice? The first or the last small signal cycle? However, nowhere to set the simulation time in the as sweep setup dashboard.