Gain margin and phase margin expressions from nyquist plots

Are you producing the nyquist plots by plotting the loop gain from a Spectre stb analysis? If not, how are you doing this?

Andrew

Are you producing the nyquist plots by plotting the loop gain from a Spectre stb analysis? If not, how are you doing this?

Andrew

Correct,

However, when using the pm/gm calculations from the stb analysis it says the circuit is unstable.

WARNING (SPECTRE-16922): Cannot obtain the phase margin and gain margin because the circuit is a positive feedback system and is unstable. This is because the magnitude of loopGain is greater than one at 112.202 Hz when the phase of loopGain crosses zero degree. To make the circuit stable, ensure that the magnitude of loopGain is less than one when the phase of loopGain crosses zero degree.

The circuit is an LDO regulator, so I sweep the load current to check for the stability at all load conditions. For some values of the load current, the phase starts at 180 and rolls down whereas for others, the phase starts at -180 and goes up (and then back down eventually)

These are the simulation results (it's an RC extracted view, so I used spectre X set in LX, as the GUI indicates this is a good setting for power management circuits). The load is steped down by halving it every 50us and running an stb at selected times.