A Scalable High-Current High-Accuracy Dual-Loop Four-Phase Switching LDO for Microprocessors

Abstract

High-performance microprocessors need high cur- rent (ampere-level), high accuracy, and fast-response power supplies. Comparing to analog and digital low-dropout (LDO) regulators, the switching LDO can be a better candidate for such requirements, as it can drive large power transistor(s) fast and accurately. However, conventional switching LDOs need large load capacitance to reduce the output ripple, which restricts their applications. This article presents a 1.5-A fully-integrated switching LDO for microprocessors, with an easily scalable load capability. Here, we introduce three techniques together to relief the output capacitance requirement: 1) four-phase 500-MHz pulsewidth modulation (PWM) with inherent current balancing; 2) current-limited power cells resisting processor voltage and temperature (PVT) variations; and 3) hybrid fast-slow power transistors. Therefore, we reduce significantly the load capaci- tance to maximum load current ratio C L/IMAX when compared with the prior switching LDOs. Also, the proposed dual-loop architecture not only achieves a fast transient response, but also provides high-accuracy regulation. In addition, we design the tunable active voltage positioning (A VP). Fabricated in 28-nm CMOS, the proposed switching LDO measures a maximum 70- mV undershoot with a 1-A load current step with 10-ns edge time. The measured load regulation is 1 mV/A and the line regulation is 1.5 mV/V . Also, we obtain a good power s upply rejection (PSR) of −63 dB at 1