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AN588
6. Energy Harvesting System
The energy harvesting system used in this reference design, shown in Figure 8, consists of three components:
energy management circuitry to harvest energy, energy storage, and energy management circuitry to convert the
stored energy into a form usable by the wireless sensor.
Solar
Thermal
Vibration
RF Energy
Energy
Management
(input)
Energy
Management
(output)
Regulated
System
Voltage
Energy Storage
Figure 8. Energy Harvesting System
The energy management circuitry used for harvesting energy consists of a solar cell which provides dc energy, a
rectifier which can be used to convert ac vibration energy into dc energy, and the LTC 4071 which takes in dc
energy and regulates it to a constant 4.1 V. The LTC4071 also protects the battery from overdischarge (by
disconnecting it from the circuit if its voltage gets too low) and provides a “ship mode” that disconnects the battery
during shipping and allows it to hold its energy until the end user starts up the system.
The energy storage used in this reference design is a 4.1 V, 700 μA-H thin film battery from IPS. This battery
provides enough energy storage to keep the system running for many days without any “harvested” power. In any
energy harvesting system, it is important to keep the energy used by the system lower than the amount of
“harvested” energy in order to prevent a steady depletion of the stored energy. The larger the energy storage
reservoir, the longer the system can go without “harvesting” new energy from the environment.
The energy management circuitry at the output of the energy storage converts the 4.1 V thin film battery voltage to
a regulated 2.7 V for use by the Si1012 Wireless MCU. The main components of this circuit are an ultra low power
LDO (ADP162), a brownout detector (NCP302), and a 100 μF tantalum capacitor to supply the peak currents
required for RF transmission. The LDO’s shutdown pin is tied to the output of the brownout detector, so that the
system is not powered until the 100 μF capacitor is charged up to at least 3.0 V. This ensures that the system will
not attempt to power up unless it has enough stored energy to get it through the power up sequence.
The energy harvesting system requires approximately 3 μA to operate. This is easily cancelled out by as little as
50 lux light shining on the solar cell. The energy harvesting system can remain in a dark closet for 1 week before all
the stored energy is depleted. When the system is going to be placed in a dark area for a prolonged period of time,
it is best to place the S2 switch in “OFF” mode, which activates the “ship mode” and disconnects the battery from
the system. This allows the system to hold its current state until the S2 switch is placed in the “SOLAR” position.
6
Rev. 0.2
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