Low Voltage Delay Element with Dynamic Biasing Technique for Fully Integrated Cold-Start in Battery-Assistance DC Energy Harvesting Systems

Abstract

This paper proposes an ultra-low voltage delay element for battery-assistance DC energy harvesting systems. By inserting a low voltage level shifter (VLS), a wider voltage range is obtained to bias the body of the delay element. Thus, both the voltage transfer curve (VTC) and the DC gain of the delay element are enhanced. Due to the introduction of the VLS, the cold start-up ring oscillator constituted by the proposed delay element can achieve oscillation under an extremely low input voltage. The fully integrated cold start-up ring oscillator with 21 stages of the proposed element is implemented in a standard 180 nm complementary metal oxide semiconductor (CMOS) process. The post-layout experimental results indicate that the cold start-up ring oscillator can retain oscillation when the power supply voltage (VDD) is 24 mV under a typical corner at room temperature. The output voltage swing of the cold start-up ring oscillator based on the proposed delay element is improved by more than 55% under VDD = 40 mV compared with a stacked inverter-based cold start-up ring oscillator. Monte Carlo (MC) simulation from 100 samples shows the enhanced output swing with the proposed delay element under process variation.

Recent Start-Up Techniques Intended for TEG Energy Harvesting: A Review

“…Thus, the reliability and cost of such an approach constitute real obstacles. Some researchers attempted to address this problem by proposing battery-assisted energy harvesting systems to reduce the rate of battery replacement and to realize self-charging for the rechargeable battery [59]. Consequently, battery-assisted energy harvesting systems can significantly prolong the battery life compared with conventional battery-powered IoT nodes.…”

“…Thus, supply voltage reduction is advantageous for supply voltage-constrained applications. However, it is limited by on-to-off current ratios degradation observed with decreasing supply [30], [59], [75]. Many scientists have attempted to provide valid solutions for this issue by either adopting a new delay cell architecture or by applying new biasing mechanisms to existing structures.…”

“…An improved delay element based on work from [77] for cold start-up RO was reported in [59]. As shown in Fig.…”

“…Furthermore, postlayout simulation results of this 21-stage RO show that it maintains oscillation under V DD = 36 mV power supply, in addition to a wider swing compared with other cold start-up ROs. The authors extended their work to cover battery-assisted DC energy harvesting systems in [59] to reduce battery replacement frequency by realizing selfcharging for rechargeable batteries. Thus, the batteryassisted energy harvesting system can significantly prolong battery life compared with conventional systems.…”

A Novel Cross-Latch Shift Register Scheme for Low Power Applications

“…This circuit block can be applied in conversion between serial and parallel interfaces during data transmission, and it can be used as a delay circuit too. The shift register is a key component in digital circuits and is generally used in active-matrix displays, sensors, memories, communication receivers, and real-time image processing chips [1][2][3][4][5][6][7][8]. The shrinking feature size of process technology has enhanced the potential performance of electronic devices, and the capacity of shift registers has increased significantly.…”