LDO Question about a 100dB loop gain and -30dB PSR in low frequency

[bhl777]

Hi All, I have designed a LDO, with low loop gain~100dB. In light load, there is only one single pole within UGF, and PM is around 90 degree.

However, when I used an AC signal in VCC and use AC simulation in Vout to plot PSR, its low frequency gain is only -30dB. I have read some of the treads in this forum in discussing the relationship between them, but still have some questions:

(1) If 100dB DC loop gain and -30dB low frequency PSRR makes no sense, does that mean my sim setup have some problem? I am confident with my loop gain analysis, because it was performed by STB analysis in cadence. The setup for PSR is like this


(2) If that makes sense, would anyone advise what are the other main parameters that can impact PSR significantly?

Thank you!

 

[FvM]

You can check if your line regulation ("PSR") setup is correct if you inspect the other node DC and AC voltages.

 

[bhl777]

Hi FvM, the DC voltages are checked when running DC operating point. Would you tell me how can I check AC voltage at differengt node, and how can I use the AC voltage to judge if the setup is correct or not?

You can check if your line regulation ("PSR") setup is correct if you inspect the other node DC and AC voltages.