Transfer Function Analysis of the Power Supply Rejection Ratio of Low-Dropout Regulators and the Feed-Forward Ripple Cancellation Scheme

This paper presents a rigorous transfer function analysis of the power supply rejection ratio (PSRR) of low-dropout (LDO) regulators and the LDO with the feed-forward ripple cancellation (FFRC) scheme. Four main noise sources of conventional LDOs are first clarified. An efficient and accurate model is proposed to facilitate the analysis, and the transfer function, along with the poles and zeros, is derived based on the simplified model. Software simulation results and numerical plots are both demonstrated and compared to verify the analytical results, with respect to two specific design examples. The widely-used FFRC scheme for PSRR enhancement is also reinvestigated based on the proposed model. With the inclusion of the FFRC circuit, the new transfer function with new poles and zeros is presented. The working principles of the FFRC scheme are illustrated and the optimal FFRC case is clarified. The proposed model and the derived transfer function match well with the simulation results and the measurement results in prior works. The proposed model and the analytical results serve as the theoretical foundation of the design of high-PSRR LDOs. The recommended simulation methodology could be helpful during the circuit design phase.