3.2 Minimum Energy Tracking Loop with Embedded DC-DC Converter Delivering Voltages down to 250mV in 65nm CMOS

Minimizing the energy consumption of battery powered systems is a key focus in integrated circuit design. Switching energy of digital circuits reduces quadratically as VDD is decreased below VT (i.e. sub-threshold operation), while the leakage energy increases exponentially. These opposing trends result in a minimum energy point (MEP), defined as the operating voltage at which the total energy consumed per operation (Eop) is minimized [1]. Operating circuits at their MEP [1, 2] has been proposed as a solution for energy critical applications and the analytical solution of the MEP has been derived in [3]. The MEP can vary widely for a given circuit depending on its workload and environmental conditions (e.g., temperature). By tracking the MEP as it varies, energy savings of 50 100% are demonstrated and even greater savings can be achieved in circuits dominated by leakage. In this paper, a 65nm CMOS circuit that can dynamically track the MEP of a digital circuit with varying operating conditions is presented. Embedded within the tracking loop is an ultra-lowpower switching DC-DC converter that can efficiently deliver supply voltages down to 250mV, enabling minimum energy operation.