THESIS
2013
xiv, 80 pages : illustrations ; 30 cm
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 t...[
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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.
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