Design of an ultra-low powered DC-DC buck converter for wireless sensor networks

Wireless sensor networks (WSNs) are normally installed in remote, inaccessible locations. Energy harvesting technique is used in such systems which periodically charges the battery automatically. The life-cycle of such a rechargeable battery depends on the number of charge/discharge cycles. To prolong the battery life, an ultra-low powered DC-DC converter, which interfaces between the battery and the load, is required. This paper describes an ultra-low powered, switched capacitor based DC-DC buck converter, in 180 nm standard CMOS technology, targeted for WSN applications. The input voltage can vary from 2.1-4.5 V and the output voltage to be regulated is 1.8 V. The load requires a maximum of 4 mA of load current for 15 ms of on-time, and thereafter a mere 2 μA for 3 minutes of sleep time. The duty cycle of the clock is typically of the order of 1:12000 or even more, and the typical average output power is around 4.2 μW. In this design, an ultra low quiescent current of 500 nA for the whole converter is achieved over a desired load range of 0.002-4 mA. Consequently, the peak power efficiency has been improved by 20% compared to existing designs. Also, a unique low power consuming two-phase non-overlapping clock generation circuit with an adjustable skew margin is designed. A basic block diagram of this circuit is described.