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Design Techniques for Fully Integrated Switched-Capacitor Voltage Regulators

  • Author(s): LE, HANH-PHUC
  • Advisor(s): Alon, Elad
  • Sanders, Seth R.
  • et al.
Abstract

As parallelism increases the number of cores integrated onto a chip, there is a clear need for fully integrated DC-DC converters to enable efficient on-die power management. Due to the availability of high density and low series resistance capacitors in existing CMOS processes, switched-capacitor DC-DC converters have recently gained significant interest as a cost-effective means of enabling such power management functionality.

In this thesis, described are design techniques to implement fully integrated switched-capacitor DC-DC converters with high power density and efficiency. The area required by a fully integrated switched-capacitor DC-DC converter in order to deliver a certain level of power to the load has direct implications on both cost and efficiency, and hence in Chapter 2 a methodology is presented to predict and minimize the losses of such a converter operating at a given power density. Chapter 3 further introduces gate driver and level shifter circuit design strategies to enable topology reconfiguration and hence efficient generation of a wider range of output voltages. In order to demonstrate the possibility of replacing all off-chip PMICs, Chapter 4 presents a battery-connected switched-capacitor DC-DC converter that is able to convert the wide input voltage range from Li-ion battery to an output regulated at ~1V using cascode switches and intermediate voltage rails. The SC converter in Chapter 4 also employs a fast control loop to regulate the output with sub-ns response times.

Measured results from the converters presented in Chapters 3 and 4 match with the analytical prediction and, thus, confirm the design methodology presented in Chapter 2. The 32nm SOI prototype presented in Chapter 3 achieves ~80% efficiency at a power density of ~0.5-1W/mm2 for a 2:1 step-down converter operating from a 2V input and utilizing only standard MOS capacitors. Reconfiguration of the converter's topology enables it to maintain greater than 70% efficiency for most of the output voltage range from 0.7V to ~1.15V. The 65nm Bulk CMOS prototype discussed in Chapter 4 also utilizes only standard MOS capacitors to regulate the output voltage at ~1V from a ~2.9V-4V input. It achieves ~73% efficiency at 0.19 W/mm2 output power density and maintain efficiency above 72% over the whole range of target power density. The sub-ns response control loop maintains <76 mV voltage droop out of a 1V regulated output under a full load step of 0 → 0.253 A/mm2 in 50ps.

Given that these results were achieved in a standard CMOS process with no modifications or additions, they illustrate that fully integrated switched-capacitor converters are indeed a promising candidate for low-cost but efficient power management on a per-core or per-functional unit basis. They can possibly replace all the off-chip PMICs and passive components and free up significant PCB area to be used to implement new functions on next-generation mobile devices.

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