Is there an issue with Virtuoso stb analysis and coupled C? I have a regulator giving a reasonable RCdecoupled phase margin but when RCcoupled is chosen then the PM drops below zero. If i use the Parasitic Aware Design tool to print reports on all the tracks in the FB loop for coupled C, most coupling is to supply/ground due to the tracks being shielded. Only 10fF coupling to a non supply track is found.
IC6.1.8-64b.500.19