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In order to minimize total active power consumption in digital circuits, one must take into account subthreshold leakage currents that grow exponentially as technology scales. This research develops a theoretical model to predict how dynamic power and subthreshold power must be balanced to give an optimal VDD/Vt operating point that minimizes total active power consumption for different workload and operating conditions. A 175-mV multiply-accumulate test chip using a triple-well technology with tunable supply and body bias values is measured to experimentally verify the tradeoffs between the various sources of power. The test chip shows that there is an optimum VDD/Vt operating point, although it differs from the theoretical limit because of excessive forward bias currents. Finally, we propose a preliminary automatic supply and body biasing architecture (ASB) that automatically configures a circuit to operate with the lowest possible active power consumption.
Date of Publication: Nov 2002