hi,

i am simulating the open loop response of an LDO regulator using 2 methods and i expected identical result.

1st, i place a iprobe in series with the feedback signal that went back to the error amplifer reference. this gave me a gain and phase plot. from it i could roughly tell where the pole and zeros are.
all is good. for example, dominate pole is at 1KHz.

2nd, with the same circuit, i manually break the loop (at the same place where i placed the iprobe in 1st case) using an inductor and inject an AC source into the left of it. then run pz analysis.
i expect pz analysis to return the numerical location of the pole and zero. HOWEVER, the result i got had pole/zero location that was way small. about 1E-6 Hz.
all dc bias is preserved.

questions:

1. am I simulating the open loop response in 2nd case correctly?

2. if so, why does the 2nd case result is so odd. i use simple RC circuit to test this setup and the result is correct.

thanks please help.

Frank

thanks Ken for your reply

sorry my question wasn't clear before
attached is the schematic of the pz analysis

is this the correct way to find the open loop pole/zero of an LDO?

I used an AC source to inject ac=1 through a large cap. while the dc bias is preserved using a big inductor (bottom of picture).

thank you again.

Frank