A novel retargeting methodology in computer aided design of nano-watt CMOS reference circuit based on advanced compact MOSFET model

Focusing on a nano-watt CMOS reference circuit, we propose a novel retargeting methodology by adopting an ACM (advanced compact MOSFET) model to describe the drain current consistently in strong and weak inversion levels. Taking the effect of very long channels into account, we formulate the threshold voltage as a function of the drain-source voltage. While supply voltage is changing, the variable of the drain-source voltage causes reference current and reference voltage outputs at various tilts. Thus redesigning geometric size of transistors is required for different process. Based on the ACM model, we approximate all base conditions in terms of ratios of the channel widths and lengths. From the derivation, we define a tuning parameter with an empirical range and fix all transistor sizes by sweeping this parameter value as well as applying a computer aided design. In our case studies, we retarget a circuit from 1.8V/180nm process to 1.2V/90nm, 3.3V/90nm and 1.8V/65nm processes respectively. Besides, we fabricate the reference circuit in the 1.2V/90nm process, and confirm that good measurement results are obtained with less than 12.8%/V supply voltage variation and only 1.1nW power consumption.