Linear Technology has entered the energy-harvesting business with two chips, the LTC3108 step-up converter/power manager and the LTC3588-1 power supply for piezoelectric vibrating-beam energy harvesters. The transducer-specific LTC3588 incorporates design features of the general-purpose LC3108, adapting circuitry expressly for piezoelectric harvesters.
More harvester-specific devices are planned, as the power supply/power manager targets thermo-electric, photovoltaic, galvanic, and coil/magnet harvest rings as potential applications. As with most micro-power energy harvesting, typical applications involve sensors of some sort and wireless mesh networks that pass on data from a host of sensors to a central processor.
Practical applications could range from environmental control in buildings to monitoring the structural integrity of highway structures. In building monitoring, energy could be harvested from ambient light or temperature differences. In bridges, piezoelectric harvesters could collect energy from the vibrations caused by vehicles driving on the structure.
The LTC3108 neatly meets the fundamental challenge of startup. It uses an external step-up transformer and a small coupling capacitor to form a resonant step-up oscillator driving a MOSFET switch. An external charge pump capacitor boosts and rectifies the ac voltage produced on the secondary winding of the transformer. That voltage in turn charges another external capacitor (on the VAUX pin) that runs the chip and its external outputs. Once the VAUX capacitor is charged to 2.5 V, the chip can charge a standard capacitor, supercapacitor, or rechargeable battery.
Outputs include a low-dropout regulator (LDO), VSTORE, and two outputs designated VOUT and VOUT2, plus a “power-good” pin (figure a). VSTORE is a trickle-charge output intended to charge a large storage capacitor or rechargeable battery up to the VAUX voltage. VOUT provides the main output voltage, which can be pin-strap programmed by the user to provide a regulated 2.35-, 3.3-, 4.1-, or 5-V output. The host can turn the VOUT2 output on and off.
When enabled, VOUT2 is connected to VOUT through a 1.3-Ω P-channel MOSFET switch. This output can be used to power external circuits such as sensors and amplifiers that do not have a low-power sleep or shutdown capability, so they only draw power when needed. Quiescent power for the LTC3108 itself is only 1.6 µA.
The LDO output is for running the system controller. It provides a regulated 2.2-V output at 3 mA. The higher of VAUX or VOUT powers the LDO, enabling it to become active as soon as VAUX has charged to 2.3 V, while the VOUT storage capacitor is still charging. In the event of a step load on the LDO output, current can come from the main VOUT capacitor if VAUX drops below VOUT.
While the LTC3108 boosts the transducer input voltage via a transformer-based circuit, the LTC3588-1, intended for piezo energy harvesters, implements a buck converter. Its internal full-wave bridge rectifier handles the ac input from a piezoelectric transducer directly. The rectified output, stored on a capacitor at the VIN pin, is used as an energy reservoir for the buck converter (figure, b). The low-loss bridge rectifier has a total drop of about 400 mV with typical piezo generated currents (around 10 µA). The bridge can carry up to 50 mA.
The buck regulator uses a hysteretic voltage algorithm to control the output through internal feedback from the VOUT sense pin. It charges an output capacitor through an inductor to a value slightly higher than the regulation point by ramping the inductor current up to 260 mA through an internal PMOS switch and then ramping it down to zero through an internal NMOS switch.
The LTC3108 comes in a 3- by 4-mm dual flat no-lead (DFN) package or small-outline SSOP-16. Pricing starts at $2.95 each for 1000-piece quantities. Industrial temperature-grade versions are available for slightly more. The DFN version of the LTC3588 is slightly smaller (3 by 3 mm) but costs the same.