This week’s EYE ON MPI (video) is celebrating Earth Day, with the eco-friendly MAX20361 Tiny Single-/Multi-Cell Solar Harvester (Link). This little chip is designed for making solar-harvesting projects and products that can sip power from a small inexpensive solar cell, to charge up a battery or super-capacitor.
Maxim Integrated’s MAX20361 is a fully-integrated solution for harvesting energy from single-/multi-cell solar sources. It has an ultra-low quiescent current (360 nA) boost converter that can start from input voltages as low as 225 mV (typical). To maximize the power extracted from the source, the MAX20361 implements a proprietary maximum power point tracking (MPPT) technique that allows efficient harvesting from 15 μW to over 300 mW of available input power. The MAX20361 also features an integrated charging and protection circuit that is optimized for Li-ion batteries but can also be used to charge supercapacitors, thin-film batteries, or traditional capacitors. The charger features a programmable charging cut-off voltage with thresholds programmable through I²C interface as well as temperature shutoff. The MAX20361 is available in a 12-bump, 0.4 mm pitch, 1.63 mm x 1.23 mm wafer-level package (WLP).
This chip is perfect for miniature sensor nodes, maybe with a LoRa radio or Bluetooth Low Energy. We could imagine this on an asset tracker, environmental or agriculture sensor network. While we have a 5-6V cell charging board, (Adafruit Universal USB DC Solar charger) this one is designed specifically for small panels with one to three which are low cost and easier to get in a particular size/configuration. (Usually, the smaller the physical cell, the lower the voltage because there are not as many cells spot-welded together). Since it’s really designed for approximately 3V boost output, you’ll want to stay under 3V Open-Cell voltage (so, say, 3 cells in series, max).
The boost converter has to be designed very specifically for use with solar cells because cells collapse under high current draw. When drawing current, at the very beginning the voltage is very high, then is slowly drops down as more is drawn until the input voltage collapses completely. So you have to be very careful when drawing current – too little and you lose out on efficiency, too much and your power goes away completely. There’s a ‘sweet spot’ right in the middle, where you can get the most power output, which is called the Max Power Point. (Wiki Link) And this point varies with how much sunlight you get – so it isn’t something you can pre-program in with a comparator or anything. Instead, the boost converter has to wiggle the power draw forward and back to find the maximum point. For large-scale solar installations, you’d have a microcontroller do the math for you, and adjust throughout the day – it isn’t hard to do MPPT it just takes measurement and computation. But in low-low-low-current situations, you can’t spend all your power budget on a microcontroller to manage your boost converter!
That’s what the MAX20361 does – and it even has I2C for configuration, you can set up the desired boost output, as well as some heuristics it can follow: thermal measurements, open-circuit voltage checks and max current output. You can use the MAX20361EVKIT eval board (Link) which has a very nice user interface to set up desired values as well as a panel for querying the register values. It’s just fully-featured enough to easily implement into a design without turning into a large integration project. It’s also really really small, only 1.6x 1.2mm in size the MAX20361 comes in a chip-wafer-scale package with 0.4mm pitch ball grid array.
So, if you want to power your product from solar cells, especially if its a tiny product and a small amount of sun, the MAX20361 is tiny, efficient and easy to add for instant max-efficiency solar! You can charge up a small battery or capacitor, then have it wake up when you get enough energy to do a data transmission, then go back to sleep. Digi-Key has the MAX20361 in stock right now for instant shipping: (Short Link) order today and you can harness THE SUN and help keep our Earth nice and clean.