55pW/pixel Peak Power Imager with Near-Sensor Novelty/Edge Detection and DC-DC Converter-Less MPPT for Purely Harvested Sensor Nodes

Cost and form factor reductions in CMOS imagers are enabling the deployment of increasingly distributed vision sensor systems. Due to battery cost and size, aggressive power reductions are crucial to make vision sensor nodes more inexpensive, smaller, and longer-lived (Fig. 5.7.1). Purely harvested imagers further reduce cost and size at unbounded lifespan, although imposing a tight peak power budget (e.g.,
$\mu\text{Ws}$
for mm-scale solar cells). Accordingly, such imagers target ultra-low power/pixel as a key efficiency metric, as opposed to the energy figure-of-merit (FOM) commonly adopted for battery-powered systems. Aggressive power reductions in low-cost CMOS imagers were recently achieved through fully digital time-domain readout schemes with low bit depth (e.g., 4 to 8b) [1], [2], although at a relatively high power/pixel (~1nW) [1], and without incorporating any higher-level function for routine scene/event monitoring to filter out unimportant data and transmit/process only when/where strictly necessary [1], [2]. In- or near-sensor coupling of higher-level tasks was widely explored to reduce system power, as in computational imagers that efficiently compute features and few neural network layers for reduced subsequent transmission/processing [3], [4]. Unfortunately, their higher power/pixel pushes the overall imager power to tens or hundreds of
$\mu\text{Ws}$
regardless of the importance of the scene content. To reduce system power, event detection/recognition [5] (motion, simple object) and log-gradient extraction [6] have been embedded at 300 to 3,800pW/pixel. Pure harvesting with
$\mu\mathrm{W}$
-range peak power (rather than average) would instead require power/pixel down to tens of pWs [1] (e.g., to support near-QVGA resolutions), including higher-level tasks, harvesting circuitry for adaptation to uncertain supply (usually not included), and the ability to detect regions of interest (ROls) for further reduction in subsequent transmission/processing [3].