Design of monolithic digital current-mode controllers for DC-DC converters

Abstract

Monolithic digital controllers for dc-dc converters are the future trend of voltage regulators for space-constrained portable devices because they can be highly integrated on-chip. This advantage is more prominent in an advanced CMOS process (e.g. 45nm/22nm process) because designing analog circuits in such a process becomes very challenging with reduced supply voltage and lower transistor output impedance. Digital controllers are also known to have a number of advantages over analog controllers. For example, they are more robust and reprogrammable. In particular, by using both the output voltage and the inductor current of dc-dc converters as feedback signals, monolithic digital current-mode controllers (DCMCs) have additional advantages of over-current protection and faster transient response. Despite all the advantages of DCMCs, they have not yet been widely adopted in the portable devices. This is mainly due to concerns over high power consumption of the DCMCs and design challenges in inductor current-sensing and quantization. Conventionally, inductor current is sensed and quantized before it can be used by the DCMCs. These are intuitively done by using two separated functional blocks - an analog current sensor and an ADC. Few research works have investigated other possible ways of obtaining the inductor current information in the digital domain. Therefore, this thesis proposes different approaches for achieving it. Firstly, an on-chip digital inductor current sensor is proposed for obtaining the averaged inductor current in the digital domain. It combines both the inductor current-sensing and quantization into a single functional block. In this way, the redundancies found in the conventional approach can be reduced and optimizations can be made to save chip area and power consumption. An 8-bit digital inductor current sensor has been designed and fabricated with UMC 0.13μm digital CMOS process. The measurement results show that the digital sensor can provide digital inductor current information with a conversion time of 225ns. This can be used by a buck converter with a switching frequency up to 4MHz. The digital sensor has linear and monotonic input-output transfer curve properties, with an LSB of 6.79mA. It consumes current of 700μA at 1.2V supply voltage. Secondly, when DCMCs require inductor current ripples as feedback signals, analog RC inductor current sensors can be used for sensing the ripples. However, the passive RC components are too bulky to integrate on-chip and the DCMCs cannot use the analog ripples for the control purpose unless extra ADCs are available to quantize them. Therefore, another digital inductor current sensor is designed in this thesis for obtaining the ripples in the digital domain. As compared to the existing designs, it does not require extra ADCs or knowledge of the inductor value. A ripple-based digital controller is also designed to demonstrate how the digital sensor can be utilized. Both the digital sensor and controller are fully synthesizable with UMC 0.13μm digital CMOS process. They occupy a small chip area of 220μm×220μm. Measurements results show that a 2MHz buck converter achieves load-transient responses of 10μs by using the digital controller. The peak efficiency is 91% at a nominal load current of 100mA.

Date of Award	2013
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology