Analog IC Design with Low-Dropout Regulators (LDOs).pdf

CHAPTER 1 System Considerations Regulators in Power Management Supplying and conditioning power are the most fundamental func- tions of an electrical system. A loading application, be it a cellular phone, pager, or wireless sensor node, cannot sustain itself without energy, and cannot fully perform its functions without a stable sup- ply. The fact is transformers, generators, batteries, and other off-line supplies incur substantial voltage and current variations across time and over a wide range of operating conditions. They are normally noisy and jittery not only because of their inherent nature but also because high-power switching circuits like central-processing units (CPUs) and digital signal-processing (DSP) circuits usually load it. These rapidly changing loads cause transient excursions in the sup- posedly noise-free supply, the end results of which are undesired volt- age droops and frequency spurs where only a ponent should exist. The role of the voltage regulator is to convert these unpredictable and noisy supplies to stable, constant, accurate, and load-indep endent voltages, attenuating these ill-fated fluctuations to lower and more acceptable levels. The regulation function is especially important in high-performance applications where systems are increasingly more integrated and complex. A system-on-chip (SoC) solution, for instance, incorporates numerous functions, many of which switch simultaneously with the clock, demanding both high-power and fast-response times in short consecutive bursts. Not responding quickly to one of these load-curr ent transitions (., load dumps) forces storage capacitors to supply the full load and subsequently suffer considerable transient fluctuations in the supply. The bandwidth performance of the regulator, that is, its ability to respond quickly, determines the magnitude and extent of these transient variations. Regulators also protect and filter integrated circuits (ICs) from expo- s