Time-Based Control Techniques for Highly Integrated Power Management Circuits
Dr. Qadeer Ahmad Khan
Indian Institute of Technology Madras
Abstract
This tutorial discusses design of a highly integrated, low quiescent current continuous time controller using time-based signal processing. By virtue of the continuous-time digital nature of the time-based controller, it is capable of achieving very high resolution and speed without using any error amplifier and large compensation capacitor or any high resolution A/D converter and digital PWM while preserving all the benefits of both analog and digital PWM controllers. Using time as the processing variable, the controller operates with CMOS-level digital-like signals but without adding any quantization error. A voltage or current controlled ring oscillator is used as an integrator in place of conventional opamp-RC or Gm-C integrator while a voltage or current controlled delay line is used to perform voltage-to-time conversion. Starting with trade-offs with conventional analog and digital PWM controllers, the concept of time-based proportional-integral-derivative (PID) controller with complete architecture of a time-based buck converter is presented. The technique was successfully demonstrated on silicon with implementation of 10-25MHz single phase and 30-70MHz 4-phase buck converters in 180nm and 65nm CMOS processes, respectively. The time-based control technique can also be used to implement a low drop-out regulator (LDO) to achieve high bandwidth, low quiescent current and smaller area compared to state-of-the art LDOs.
This tutorial discusses design of a highly integrated, low quiescent current continuous time controller using time-based signal processing. By virtue of the continuous-time digital nature of the time-based controller, it is capable of achieving very high resolution and speed without using any error amplifier and large compensation capacitor or any high resolution A/D converter and digital PWM while preserving all the benefits of both analog and digital PWM controllers. Using time as the processing variable, the controller operates with CMOS-level digital-like signals but without adding any quantization error. A voltage or current controlled ring oscillator is used as an integrator in place of conventional opamp-RC or Gm-C integrator while a voltage or current controlled delay line is used to perform voltage-to-time conversion. Starting with trade-offs with conventional analog and digital PWM controllers, the concept of time-based proportional-integral-derivative (PID) controller with complete architecture of a time-based buck converter is presented. The technique was successfully demonstrated on silicon with implementation of 10-25MHz single phase and 30-70MHz 4-phase buck converters in 180nm and 65nm CMOS processes, respectively. The time-based control technique can also be used to implement a low drop-out regulator (LDO) to achieve high bandwidth, low quiescent current and smaller area compared to state-of-the art LDOs.
History, Motivation and Focus
Power management is an essential module of battery-powered devices such as smartphones and tablets. With an effort to integrate more hardware features, there is a tremendous increase in the number of power supplies required on these devices. The increasing power demand while maintaining the small form factor has made power management ICs as one of the most challenging and complex modules in a portable/handheld device. For instance, battery capacity of 100mAh-200mAh used in a feature phone now increased to 3000mA-4000mAh in a smartphone. Similarly, number of power supplies used in a smartphone are much larger compared to power supplies used in a feature phone. This power supply demand is catered by voltage regulators which could be either linear low-drop out (LDO) regulator or switching dc-dc converter. These regulators often require large capacitors and/or inductors that are either impossible to integrate on-chip or incur prohibitively large area penalty. Using external components takes away premium board space and increases system cost. Therefore, size of a power module is mainly driven by the size of on and off-chip passive components (mainly capacitors and inductors). The most viable approach to achieve smaller form factor and fast transient response is increasing the bandwidth and switching frequency. However, conventional analog and digitally controlled regulators suffer from trade-offs between quiescent power and bandwidth/speed.
In an effort to overcome the above issues with conventional analog and digitally controlled voltage regulators, a highly integrated, continuous time controller based on time-based signal processing (TSP) is presented in this tutorial. The concept of time-based signal processing was originally derived from phase locked loop where VCO acts as a phase integrator. So far, the application of TSP has been mostly in A/D converters where op-amp based integrators are replaced with VCOs [1-2]. Previously published work have shown application of VCOs in dc-dc converter but only in the PWM generation while still requiring a separate compensator for stabilizing the loop therefore posing the similar challenges as conventional analog or digital controllers [3-4]. By implementing the complete compensator using time-based control, it eliminates the need for wide bandwidth error amplifiers, large on-chip compensation capacitor, PWM modulator, high resolution ADC and digital pulse width modulator (DPWM), while preserving the benefits of both analog (high accuracy, low quiescent current) and digital (low voltage operation, smaller area and process scalability) controllers. A voltage/current controlled ring oscillator is used as an integrator in place of conventional opamp-RC or Gm-C integrator while a voltage/current controlled delay line is used to perform voltage-to-time conversion. Starting with basics of power management ICs, trade-offs with conventional analog and digital controllers, the tutorial provides insight of time-based controller for switching dc-dc converters and LDO [5-8]. Performance comparison with state-of-the-art voltage regulators and limitations of time-based control techniques are also discussed.
Power management is an essential module of battery-powered devices such as smartphones and tablets. With an effort to integrate more hardware features, there is a tremendous increase in the number of power supplies required on these devices. The increasing power demand while maintaining the small form factor has made power management ICs as one of the most challenging and complex modules in a portable/handheld device. For instance, battery capacity of 100mAh-200mAh used in a feature phone now increased to 3000mA-4000mAh in a smartphone. Similarly, number of power supplies used in a smartphone are much larger compared to power supplies used in a feature phone. This power supply demand is catered by voltage regulators which could be either linear low-drop out (LDO) regulator or switching dc-dc converter. These regulators often require large capacitors and/or inductors that are either impossible to integrate on-chip or incur prohibitively large area penalty. Using external components takes away premium board space and increases system cost. Therefore, size of a power module is mainly driven by the size of on and off-chip passive components (mainly capacitors and inductors). The most viable approach to achieve smaller form factor and fast transient response is increasing the bandwidth and switching frequency. However, conventional analog and digitally controlled regulators suffer from trade-offs between quiescent power and bandwidth/speed.
In an effort to overcome the above issues with conventional analog and digitally controlled voltage regulators, a highly integrated, continuous time controller based on time-based signal processing (TSP) is presented in this tutorial. The concept of time-based signal processing was originally derived from phase locked loop where VCO acts as a phase integrator. So far, the application of TSP has been mostly in A/D converters where op-amp based integrators are replaced with VCOs [1-2]. Previously published work have shown application of VCOs in dc-dc converter but only in the PWM generation while still requiring a separate compensator for stabilizing the loop therefore posing the similar challenges as conventional analog or digital controllers [3-4]. By implementing the complete compensator using time-based control, it eliminates the need for wide bandwidth error amplifiers, large on-chip compensation capacitor, PWM modulator, high resolution ADC and digital pulse width modulator (DPWM), while preserving the benefits of both analog (high accuracy, low quiescent current) and digital (low voltage operation, smaller area and process scalability) controllers. A voltage/current controlled ring oscillator is used as an integrator in place of conventional opamp-RC or Gm-C integrator while a voltage/current controlled delay line is used to perform voltage-to-time conversion. Starting with basics of power management ICs, trade-offs with conventional analog and digital controllers, the tutorial provides insight of time-based controller for switching dc-dc converters and LDO [5-8]. Performance comparison with state-of-the-art voltage regulators and limitations of time-based control techniques are also discussed.
Speakers
• Dr. Qadeer Ahmad Khan, Department of Electrical Engineering, Indian Institute of Technology Madras, Chennai, India
Basic Structure of the Tutorial
• Dr. Qadeer Ahmad Khan, Department of Electrical Engineering, Indian Institute of Technology Madras, Chennai, India
Basic Structure of the Tutorial
- Part-I Overview of Power Management ICs: Introduction to power management IC, market overview, need of power management, power management system of a smartphone, voltage regulators, discrete vs. integrated voltage regulators, types of voltage regulators (switching vs linear) and applications, performance parameters.
- Part-II Time-Based Switching DC-DC Converters: Analog vs digitally controlled DC-DC converters, high resolution digital PWM, trade-offs with high speed controllers, concept of time based control, time based Integral (type-I) PWM controller, time based PID (type-III) controlled, voltage-to-time transformation, time-based multi-phase converter, design examples and experimental results.
- Part-III Time-Based Low Drop-Out Regulator (LDO): Linear regulators and LDOs, error amplifier and compensation techniques, power supply rejection ratio (PSRR), capacitor less LDO, digitally controlled LDO, using type-II PLL as an error amplifier, time-based LDO, design example and simulation results
References
1. M. Park and M.H. Perrott, “A 78 dB SNDR 87 mW 20 MHz Bandwidth Continuous-Time Delta-Sigma ADC With VCO-Based Integrator and Quantizer Implemented in 0.13 um CMOS,” IEEE JSSCC, vol. 44, pp. 3344-3358, Dec 2009.
2. M. Park and M.H. Perrott, “A VCO-based Analog-to-Digital Converter with Second-Order Sigma-Delta Noise Shaping”, ISCAS 2009, pp. 3130-3133, May 2009.
3. J. Xiao, A. Peterchev, J. Zhang, and S. Sanders, “A 4A-quiescent-current dual-mode buck converter IC for cellular phone applications ISSCC Dig. Tech. Papers, Issue , 2004, pp. 280 - 528 Vol.1.
4. J. Zhang, and S. Sanders, “An Analog CMOS Double-Edge Multi-Phase Low-Latency Pulse Width Modulator”, APEC'07, pp.355-360, March 2007.
5. Q. Khan, S. J. Kim; M. Talegaonkar, A. Elshazly, A. Rao, N. Griesert, G. Winter, W. McIntyre and P. K. Hanumolu, “A 10-25MHz, 600mA buck converter using time-based PID compensator with 2uA/MHz quiescent current, 94% peak efficiency, and 1MHz BW,” Symposium on VLSI Circuits, June 2014, Honolulu.
6. S. J. Kim, Q. Khan, M. Talegaonkar, A. Elshazly, A. Rao, N. Griesert, G. Winter, W. McIntyre, and P. K. Hanumolu, “High frequency buck converter design using time-based control techniques,” IEEE Journal of Solid-State Circuits, Apr. 2015.
7. S. J. Kim, R. K. Nandwana, Q. Khan, R. Pilawa-Podgurski, and P. K. Hanumolu, “A 4-phase 30-70 MHz switching frequency buck converter using a time-based compensator,” IEEE Journal of Solid-State Circuits, Dec. 2015.
8. Q. A. Khan, S. Saxena and A. Santra, "Area and Current Efficient Capacitor-Less Low Drop-Out Regulator Using Time-Based Error Amplifier," 2018 IEEE International Symposium on Circuits and Systems (ISCAS), Florence, Italy, 2018, pp. 1-5.
1. M. Park and M.H. Perrott, “A 78 dB SNDR 87 mW 20 MHz Bandwidth Continuous-Time Delta-Sigma ADC With VCO-Based Integrator and Quantizer Implemented in 0.13 um CMOS,” IEEE JSSCC, vol. 44, pp. 3344-3358, Dec 2009.
2. M. Park and M.H. Perrott, “A VCO-based Analog-to-Digital Converter with Second-Order Sigma-Delta Noise Shaping”, ISCAS 2009, pp. 3130-3133, May 2009.
3. J. Xiao, A. Peterchev, J. Zhang, and S. Sanders, “A 4A-quiescent-current dual-mode buck converter IC for cellular phone applications ISSCC Dig. Tech. Papers, Issue , 2004, pp. 280 - 528 Vol.1.
4. J. Zhang, and S. Sanders, “An Analog CMOS Double-Edge Multi-Phase Low-Latency Pulse Width Modulator”, APEC'07, pp.355-360, March 2007.
5. Q. Khan, S. J. Kim; M. Talegaonkar, A. Elshazly, A. Rao, N. Griesert, G. Winter, W. McIntyre and P. K. Hanumolu, “A 10-25MHz, 600mA buck converter using time-based PID compensator with 2uA/MHz quiescent current, 94% peak efficiency, and 1MHz BW,” Symposium on VLSI Circuits, June 2014, Honolulu.
6. S. J. Kim, Q. Khan, M. Talegaonkar, A. Elshazly, A. Rao, N. Griesert, G. Winter, W. McIntyre, and P. K. Hanumolu, “High frequency buck converter design using time-based control techniques,” IEEE Journal of Solid-State Circuits, Apr. 2015.
7. S. J. Kim, R. K. Nandwana, Q. Khan, R. Pilawa-Podgurski, and P. K. Hanumolu, “A 4-phase 30-70 MHz switching frequency buck converter using a time-based compensator,” IEEE Journal of Solid-State Circuits, Dec. 2015.
8. Q. A. Khan, S. Saxena and A. Santra, "Area and Current Efficient Capacitor-Less Low Drop-Out Regulator Using Time-Based Error Amplifier," 2018 IEEE International Symposium on Circuits and Systems (ISCAS), Florence, Italy, 2018, pp. 1-5.
Dr. Qadeer Khan is an assistant professor in the Integrated Circuits and Systems group of the department of Electrical Engineering, Indian Institute of Technology Madras, India. He received the Bachelor's degree in electronics and communication engineering from Jamia Millia Islamia University, New Delhi, India, in 1999 and the Ph.D. degree in electrical and computer engineering from Oregon State University, USA in 2012. His Ph.D. work was focused on developing novel control techniques for high performance switching dc-dc converters.
From 2012 to 2015, he worked as a Staff Engineer, Power Management Systems with Qualcomm, San Diego and from 2015 to 2016 with Qualcomm, Bangalore where he was involved in defining system and architecture of various power management modules for snapdragon chipsets catering to different smartphone markets. From 1999 to 2005, he worked with Motorola and Freescale Semiconductor, India, where his main responsibilities were designing mixed-signal circuits for baseband and network processors and full-chip integrated solutions for high-voltage motor drives.
Dr. Qadeer Khan holds 18 U.S. patents and authored/co-authored over 20 IEEE publications in the area of analog, mixed-signal and power management ICs. He serves as reviewer of the IEEE Journal of Solid-State Circuits, IEEE Transactions on Very Large Scale Integration Systems, IEEE Transaction on Power Electronics and IEEE Power Electronics Letters.
His research interests involve high-performance linear regulators, LDOs, switching dc-dc converters and power management ICs for portable electronics and energy harvesting.
From 2012 to 2015, he worked as a Staff Engineer, Power Management Systems with Qualcomm, San Diego and from 2015 to 2016 with Qualcomm, Bangalore where he was involved in defining system and architecture of various power management modules for snapdragon chipsets catering to different smartphone markets. From 1999 to 2005, he worked with Motorola and Freescale Semiconductor, India, where his main responsibilities were designing mixed-signal circuits for baseband and network processors and full-chip integrated solutions for high-voltage motor drives.
Dr. Qadeer Khan holds 18 U.S. patents and authored/co-authored over 20 IEEE publications in the area of analog, mixed-signal and power management ICs. He serves as reviewer of the IEEE Journal of Solid-State Circuits, IEEE Transactions on Very Large Scale Integration Systems, IEEE Transaction on Power Electronics and IEEE Power Electronics Letters.
His research interests involve high-performance linear regulators, LDOs, switching dc-dc converters and power management ICs for portable electronics and energy harvesting.