Nanowatt-Scale Power Management for On-Chip Photovoltaic Energy Harvesting Beacons

A power-management architecture is presented for bulk-CMOS indoor beacons powered by on-chip photovoltaic energy conversion and storage. The extremely restrictive power and form-factor constraints of indoor operation are met by adopting an asynchronous transmit-only functionality, a distributed power-supply network of dual-supply converters, and a minimalist approach at the topological level of design. Architectural functionality is demonstrated on a radio-frequency identification tag application integrated in a 0.18- μm CMOS technology with deep n-well option. Load voltage is boosted to 1.9 V, stored across a 240-pF on-chip capacitor, and let drop by 0.6 V to release 230-pJ energy for transmitting a 16-bit codeword in each beacon interval. The measured value of beacon interval is 2.7 s at the lower limit 20 μW/cm² of indoor irradiance, and 25.6 ms at the higher limit 2 mW/cm². The electrical power consumed in the entire system varies from less than 2 nW to 250 nW between these limits. Duty cycle of transmission is relatively insensitive to irradiance, and remains below 0.2% over the entire range. Storage-capacitor and converter footprints are 0.07 mm² and 0.35 mm², respectively. Upgrading to 64-b codeword and 920-nJ transmission energy quadruples the footprint of the capacitor and triples that of the converters.