Design of a current-ripple-based controller for integrated DC-DC converters

• Main Author: Xie, Yanshan
• Other Authors: Chang Joseph
• Format: Thesis-Master by Coursework
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/160747
• Institution: Nanyang Technological University

Power management ICs (PMICs) are essential in portable electronic devices to manage the voltage conversion and regulation, and often employ DC-DC converters due to the high power-efficiency of these converters. In small form-factor portable applications where size is paramount, there is an increasing trend of operating the DC-DC converter at high frequencies (>10 MHz) to shrink the size of the required inductor so that the inductor can be integrated monolithically or within the semiconductor package of the PMIC. Nonetheless, compared to the external (discrete) inductors, the integrated inductors can have higher variations that degrade the steady-state stability and the transient response of the converters. To compensate for the variation, algorithms for auto-tuning the control loop are employed in digitally-controlled DC-DC converters, but these algorithms are often computationally complex and require significant hardware circuities that are power-inefficient for the high-frequency operation. Hence, there is a need for a digital controller that can yield stable steady-state and satisfactory transient response performance despite the variation in the integrated inductor, yet simple for realization in high-frequency DC-DC converters. In this dissertation, a novel current-ripple-based digital-controller is proposed for high-frequency DC-DC converters. The inductor-current sensor — the critical part of the controller — is proposed and designed using digital circuits to take advantage of the higher immunity to process variation and flexible programmability. Unlike conventional analog sensors where tuning is generally difficult, the proposed digital sensor provides a means to easily adapt to the inductance and the series resistance of the inductor. Further, its computation is simple for low hardware complexity, hence low power dissipation at high-speed operation. The proposed digital controller incorporating the digital sensor is designed for operation at high switching-frequencies while ensuring that the output of the DC-DC converter, wherein the controller is employed, is stable at steady-state and exhibits fast transient response performance. At the same time, to achieve lower power consumption while maintaining its functionality, this digital inductor current sensor is mathematically simplified by implementing less computation. To verify the characteristics of the proposed current-ripple-based digital controller, a digitally-controlled DC-DC buck converter embodying the controller is designed and thereafter, simulated using MATLAB Simulink. Simulation results at ∼50 MHz switching frequency, 3.3 V input, and 1.2 V output show that the converter remains stable under both the CCM and the DCM, with the load current up to 200 mA. The output ripple magnitude is low at ∼5.7% of the output voltage under the CCM load currents from 59 to 200 mA. The settling time for a load step from 75 to 150 mA is fast at 160 ns (8 cycles of the ∼20 ns switching period) The digital part of the proposed controller is coded in the Verilog hardware description language, and thereafter synthesized using the logic circuits from a standard cell library of a CMOS 180 nm process. The synthesized circuit embodying the controller is finally placed and routed to form a compact IC layout of 0.21 mm × 0.21 mm in size.