Analysis and Optimization of Sleep Modes in Subthreshold Circuit Design

Subthreshold operation is a promising method for reducing power consumption in ultra-low power applications, such as active RFIDs and sensor networks. It was shown in previous works that operating at the V
_min
supply voltage results in optimal energy operation, where V
_min
typically falls below the threshold voltage. However, all previous subthreshold analyses ignore the leakage current in standby mode. Hence, for applications where operation at V
_min
results in completion of the task well ahead of the required deadline, the energy consumption can be significantly under-estimated In this paper, we investigate the effect of the non-zero standby energy on the optimal energy consumption in subthreshold operation. We first analyze energy consumption both with and without a cutoff technique in standby mode. Two parameters are proposed to capture the cutoff structure's effect on the energy consumption. Second, a methodology to minimize the total energy consumption is addressed The selection of the cutoff structure is examined by comparing three different structures. Then, a co-optimization method to optimize the size of the cutoff structure concurrently with the supply voltage, is proposed. This approach reduces energy by 99.2% compared to standby-energy-unaware optimization.