Tutorial: Fundamentals of Digital and Digitally Assisted Linear Voltage Regulators

Linear voltage regulators, including low-dropout (LDO) regulators, are a popular choice for on-die voltage regulation in SoC processors. Although analog LDOs have been used in supply-sensitive applications over the years, there has been a recent resurgence in research and development of all-digital and digitally assisted LDOs targeted for digital applications. In some of these topologies, traditional design metrics such as power-supply rejection or load regulation are often traded-off for higher energy efficiency and performance during large current and voltage transients, as well as a wide dynamic operating range. This tutorial introduces the fundamentals of all-digital and digitally assisted LDOs and then presents recent advances in circuit topologies. While highlighting the trade- offs between digital and analog LDOs, this presentation covers key metrics such as the transient response, current efficiency, dropout-voltage requirements, and stability via analysis of theoretical control models. Finally, this tutorial explores circuit topologies to unify voltage and clock frequency regulation, to allow designers to reduce voltage guardbands in digital circuits.