Harvesting tiny amounts of scavenged energy seems like a “something for almost nothing” technique, but it has a major “vicious circle” impediment: the interface circuitry between the source transducer (temperature, vibration, moisture/galvanic, coil/magnet motion, photoelectric) and the application needs some power to operate and thus capture and transform the micropower available; at the same time, that interface can't begin to operate until it has captured some power. In other words, it needs some priming to get going, but unless it gets that priming, it can't get going. [Note that these applications typically have very lower average power needs, with occasional higher-power load pulses, such as when transmitting periodic data.]
Linear Technology Corp. claims that their LTC3108 is the first IC solution to the problem, a step-up DC/DC converter designed to store, regulate, and manage power from extremely low voltage sources such as Peltier-effect thermopiles, thermoelectric generators (TEGs), and even solar cells (for thermal sources, a small temperature difference is the source of the scavenged power). The vendor says that the alternative is a discrete design, requiring tens of components, a tricky circuit design, and more board space–all while yielding inferior performance than the IC approach.
The self-resonant design operates from inputs as low as 20 mV and provides a pin-selectable output voltage of 2.35, 3.3, 4.1, or 5V, which matches many common circuit and battery design requirements. It also provides a switched output which can power devices which do not have their own micropower shutdown capability; this output is enabled by the host.
Critical to the functioning of the device is a tiny 1:100 step-up transformer (a standard off-the-shelf catalog item) placed between the source and the IC input. In addition, by adding an external storage (reservoir) capacitor, the LTC3108 can provide operating power to its host and system even when input source is not available. It also contains a 2.2V LDO to power a host microcontroller.
The self-resonant boost topology requires only a few standard external components for operation. Quiescent current, of course, must be low for this device to be effective; in this case, it is less than 6 microamps, and internal efficiency is also high to ensure fastest charge time for the reservoir capacitor.–Bill Schweber
Packaging, pricing, and availability : The LTC3108 is available immediately, in 12-lead 3mm × 4mm DFN and SSOP-16 packaging. Prices begin at $2.95 (1000 pieces) for the standard-temperature grade; an industrial-temperature grade is available for $3.45. While the LTC3108 is optimized for thermal-difference sources, future members of the family will be designed for other energy-harvest sources.
For more information : Linear Technology Corp., http://www.linear.com.