Low Voltage Delay Element with Dynamic Biasing Technique for Fully Integrated Cold-Start in Battery-Assistance DC Energy Harvesting Systems  
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be able to activate the whole system during the initial phase. However, the battery-assisted energy harvesting system employs a rechargeable battery as an energy reservoir, which stores available energy in advance, as shown in Figure 1b; and the surplus energy can also be stored on this rechargeable battery. Although the battery-assisted energy harvesting system cannot totally solve the issue of battery replacement, this solution can reduce the frequency of battery replacement and realize self-charging for the rechargeable battery. Thus, the battery-assisted energy harvesting system can aggressively prolong battery life compared with conventional battery-powered IoT nodes.  
One of the main issues for both battery-less energy harvesting systems and battery-assistance energy harvesting systems is how to activate the whole system when the input voltage is not enough. This application scene is the most common mode, as the voltage generated by the surrounding sources is low and unstable. Thus, how to achieve a low voltage cold start-up has attracted extensive research over the past few years [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. Several techniques have been reported to solve this issue, which can be divided into four categories: i.e., the mechanical method [2], the transformer-based method [3], the inductor-based method [4], and the fully integrated method [5; 6; 7; 8; 9; 10; 11; 12]. Although the first three methods can realize an extremely low cold start voltage, usually below 50 mV, it is difficult to integrate the core devices with other circuits in one die. Thus, it not only makes the cost increase but also enlarges the form factor. To achieve a fully integrated solution, a cold start-up scheme based on a ring oscillator is a kind of fully integrated solution and has attracted massive research [5; 6; 7; 8; 9; 10; 11; 12].  
To ensure that the cold start-up ring oscillator can achieve oscillation under a low voltage, several techniques have been proposed from circuit level [5; 6; 7; 8; 9; 10; 11; 12; 15]. However, the minimal cold start-up voltage in these works is still not enough to achieve a start-up below 30 mV. In this paper, a low voltage start-up ring oscillator based on the proposed low voltage delay element is presented for battery-assistance DC energy harvesting systems. A high voltage (VDDH) generated by the battery is employed to assist the oscillation of the cold start-up ring oscillator under an extremely low input voltage (V\({}_{\rm in}\)).  
Figure 1: General categories for energy harvesting systems: (**a**) battery-less energy harvesting system [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]; (**b**) battery-assistance energy harvesting system [13; 14].  
The rest of the paper is organized as follows. Section 2 is a literature review of the published low voltage delay element, which can be employed in the fully integrated cold start-up ring oscillator. Section 3 illustrates the proposed low voltage delay element. Section 4 presents the post-layout experimental results and a comparison with prior arts. Finally, a conclusion is drawn in Section 5.