Minimum convertible voltage analysis for ratioless and robust subthreshold level conversion

Subthreshold logics and memories are crucial elements in ubiquitous-computing applications. The subthreshold circuits need level converters to communicate with the other circuits that use high supply voltages. However, level conversion from sub-to-super-threshold voltage suffers from weak driving strength and severe process variations. The level conversion may depend on the size or threshold voltage of the transistors. This paper therefore analyzes level converters that can robustly convert subthreshold signals without using ratio-sized or multiple-threshold-voltage transistors. The limit of the subthreshold level conversion is thoroughly analyzed. This study suggests that level converters using current-mirror structures are superior candidates for converting subthreshold signals, according to the analytical and simulation results.