Get a copy of Larry Smith's paper : "Power Distribution System Design Methodology and 5 `" w' T+ f' C0 a+ V OCapacitor Selection for Modern CMOS Technology" - i/ L2 k2 K( J4 d* C$ [$ y% xThis is the original paper that talked about VRM modeling and its parameter calculation. Most of PI tools use the formula and parameters he gave in the paper. You can find the paper through Google search.