Low voltage delay element with dynamic biasing technique for fully integrated cold-start in DC energy harvesting systems

Abstract

As the ratio of charging current ion of the linear-mode PMOS and leakage current ioff of the corresponding NMOS in the general inverter is aggressively dropped down along with the decrease of power supply VDD, low voltage cold-start has greatly challenged energy harvesting systems. In order to enhance the ratio of ion/ioff under low power supply, a novel delay element is proposed with both the body dynamic biasing technique and the stack technique. The body dynamic biasing technique is introduced to enhance ion by dynamically controlling the threshold voltage of transistors. The stack technique is employed to reduce ioff. The ring oscillator with 21 stages of the proposed element was implemented in a standard 180 nm CMOS process for fully integrated cold-start. The post-layout simulation results show both the DC gain of the proposed delay element and the output voltage swing of the ring oscillator based on the proposed delay element is improved under VDD = 40 mV. The ring oscillator based on the proposed delay element can maintain oscillation under VDD = 36 mV. Monte-Carlo (MC) simulation shows it presents wider swing under the process variation and the device mismatch compared with the ring oscillator with the stacked inverter.