Immunity analysis of an LDO using identification of operating region transitions

DC operating point shifts due to Electromagnetic Interference (EMI) are a major cause of EM immunity failures. Accurate analysis of such nonlinear effects demands the use of time-consuming simulations that are not practical for large-scale circuits. A method is presented that points to the causes of immunity failures by identifying devices that show strongly nonlinear behavior as a result of operating region transitions. It does this without using time-consuming simulations. To further relax the requirements on computational power of the simulator, the method splits the circuit into smaller sub-circuits that are each characterized by small-signal linear models. Using these models, interference propagation through the chip can be estimated. A tool based on the proposed method is implemented and applied to a large-scale test circuit of a Low-Dropout (LDO) regulator.