Figure 1. Typical wireless sensor system configuration
An ideal power management solution for energy harvesting should be small, easy to apply and perform well while operating from the exceptionally high or low voltages produced by common energy harvesting sources, ideally providing a good load match to the source impedance for optimal power transfer. The power manager itself must require very little current to manage the accumulated energy and produce regulated output voltages with a minimal number of discrete components.
Some applications, such as wireless HVAC sensors or geothermal powered sensors present another unique challenge to an energy harvesting power converter. These applications require that the energy harvesting power manager be able to operate not only from a very low input voltage, but one of either polarity as the polarity of the ∆T across the thermoelectric generator (TEG) changes. This is a particularly challenging problem, and at voltages in the tens or hundreds of millivolts, diode bridge rectifiers are not an option.
The LTC3109, available in either a 4mm × 4mm × 0.75mm 20-pin QFN or 20-pin SSOP package, solves the energy harvesting problem for ultralow input voltage sources of either polarity. It provides a compact, simple, highly integrated monolithic power management solution for operation from input voltages as low as ±30mV. This unique capability enables it to power wireless sensors from a thermoelectric generator (TEG), harvesting energy from temperature differentials (∆T) as small as 2°C. Using two small (6mm × 6mm), off-the-shelf step-up transformers and a handful of low cost capacitors, it provides the regulated output voltages necessary for powering today’s wireless sensor electronics.
The LTC3109 uses these step-up transformers and internal MOSFETs to form a resonant oscillator capable of operating from very low input voltages. With a transformer ratio of 1:100, the converter can start up with inputs as low as 30mV, regardless of polarity. The transformer secondary winding feeds a charge pump and rectifier circuit, which is used to power the IC (via the VAUX
pin) and charge the output capacitors. The 2.2V LDO output is designed to be in regulation first, to power a low power microprocessor as soon as possible. After that, the main output capacitor is charged to the voltage programmed by the VS1
pins (2.35V, 3.3V, 4.1V or 5.0V) for powering sensors, analog circuitry, RF transceivers or even charging a supercapacitor or battery. The VOUT
reservoir capacitor supplies the burst energy required during the low duty cycle load pulse when the wireless sensor is active and transmitting. A switched output (VOUT2
), easily controlled by the host, is also provided for powering circuits that don’t have a shutdown or low power sleep mode. A power good output is included to alert the host that the main output voltage is close to its regulated value. Figure 2
shows the circuit schematic for the LTC3109.
Figure 2. LTC3109 schematic for unipolar input operation