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.
Highlights
How to prolong battery life is one of the main goals for researchers and engineers around the world
We propose an improved delay element for cold start-up ring oscillators, which can be employed in battery-assistance DC energy harvesters: for example, a battery-assistance thermoelectric energy harvesting system in wearable devices; as the temperature difference between the skin and the ambient environment is only 1–2 ◦ C, the generated input voltage for the energy harvester is only
By inserting a voltage level shifter (VLS) to achieve dynamic control of the transistors’ threshold voltage with a higher voltage swing, a cold start-up ring oscillator with this proposed delay element can oscillate under a lower supply voltage as ion is aggressively enhanced
Summary
How to prolong battery life is one of the main goals for researchers and engineers around the world. Battery replacement for those nodes will have an overwhelming cost and be an obstacle in the deployment of the IoT application To solve this issue, the energy harvesting technique as a potential candidate has attracted massive research [2,3,4,5,6,7,8,9,10,11,12,13,14]. 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
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