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Phase in Bode plots starts from "0" in LDO loop?

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Nurahmad

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Hi,
I'm simulating LDO in Spectre (Cadence). I used 2 methods in simulating the loop stability.
1. Break the loop and add an AC voltage source.
2. Use iprobe instance.

The gain plot is correct, but the phase plot is different in 2 methods. What's more, using method 1, the phase plot starts from "0", regardsless of the load. While using 2nd method, the phase plot starts from "180" at light load, but starts from "0" at heavy load.

Can u explain this phenomenon, where did I got wrong. thank you!
 

Loop gain starting at phase 0 means you didn't manage to implement negative DC feedback, thus the LDO won't work as voltage regulator.

As for method1, you most likely broke the loop incorrectly. Show your circuit and test setup.
 

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