Optimized Charge Pumps for Subthreshold Operation

Over the last decade, the use of Field Programmable Gate Arrays (FPGAs) has grown from the design verification of digital systems to a wide variety of commercial and space applications for which low power consumption and re-programmability are paramount. To address the issue of low power, researchers have reported FPGAs incorporating a variety of design techniques to improve energy efficiency. However, to date no work has focused on the combination of reconfigurable circuits with ultra-low voltage subthreshold logic. Powered at a voltage that is less than the threshold of the transistor – far below conventional levels subthreshold circuits provide significant energy savings. Although subthreshold operation results in performance degradation of several orders of magnitude, some applications in which energy efficiency is prioritized can tolerate the reduced performance. The marriage of subthreshold logic and FPGAs is inevitable and therefore deserves investigation. Given the importance of boosted internal voltages in FPGAs for improved performance in the switching fabric as well as programmability of non-volatile configuration memory, this report describes a comprehensive comparison of existing charge pumps in the context of subthreshold operation and proposes a novel charge pump optimized for ultra-low voltage operation.