Analysis and design of monolithic, high PSR, linear regulators for SoC applications

Linear regulators are critical analog blocks that shield a system from fluctuations in supply rails and the importance of determining their power supply rejection (PSR) performance is magnified in SoC systems, given their inherently noisy environments. In this work, a simple, intuitive, voltage divider model is introduced to analyze the PSR of linear regulators, from which design guidelines for obtaining high PSR performance are derived. The PSR of regulators that use PMOS output stages for low drop-out (LDO), crucial for modern low-voltage systems, is enhanced by error amplifiers which present a supply-correlated ripple at the gate of the PMOS pass device. On the other hand, amplifiers that suppress the supply ripple at their output are optimal for NMOS output stages since the source is now free from output ripple. A better PSR bandwidth, at the cost of dc PSR, can be obtained by interchanging the amplifiers in the two cases. It has also been proved that the dc PSR, its dominant frequency breakpoint (where performance starts to degrade), and three subsequent breakpoints are determined by the dc open-loop gain, error amplifier bandwidth, unity-gain frequency (UGF) of the system, output pole, and ESR zero, respectively. These results were verified with SPICE simulations using BSIM3 models for the TSMC 0.35 μm CMOS process from MOSIS.