LTC3331_15 Datasheet, PDF (21/34 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Charger

LTC3331
APPLICATIONS INFORMATION
The LTC3331 allows for energy harvesting from a variety
of alternative energy sources in order to power a wireless
sensor system and charge a battery. The extremely low
quiescent current of the LTC3331 facilitates harvesting
from sources generating only microamps of current. The
onboard bridge rectifier is suitable for AC piezoelectric
or electromagnetic sources as well as providing reverse
protection for DC sources such as solar and thermoelec-
tric generators. The LTC3331 powers the V_OUT output
continuously by seamlessly switching between the energy
harvesting and battery inputs.
When harvestable energy is available, it is transferred
through the bridge rectifier where it accumulates on the V_IN
capacitor. A low quiescent current UVLO mode allows the
voltage on the capacitor to increase towards a programmed
UVLO rising threshold. When the voltage rises to this level,
the buck converter turns on and transfers energy to V_OUT.
As energy is transferred the voltage at V_IN may decrease
to the UVLO falling threshold. If this happens, the buck
converter turns off and the buck-boost then turns on to
service the load from the battery input while more energy
is harvested. When the buck is running the quiescent
current on the BB_IN pin drops to the 200nA required by
the shunt battery charger.
The LTC3331 is well suited to wireless systems which
consume low average power but occasionally need a
higher concentrated burst of power to accomplish a task. If
these bursts occur with a low duty cycle such that the total
energy needed for a burst can be accumulated between
bursts then the output can be maintained entirely by the
harvester. If the bursts need to happen more frequently or
if harvestable energy goes away the battery will be used. If
enough energy is available the energy harvester will bring
the output up and enter the low quiescent current sleep
state and excess energy can be used to charge the battery.
Piezo Energy Harvesting
Ambient vibrational energy can be harvested with a
piezoelectric transducer which produces a voltage and
current in response to strain. Common piezoelectric
elements are PZT (lead zirconate titanate) ceramics, PVDF
(polyvinylidene fluoride) polymers, or other composites.
Ceramic piezoelectric elements exhibit a piezoelectric effect
when the crystal structure of the ceramic is compressed
and internal dipole movement produces a voltage. Polymer
elements comprised of long-chain molecules produce
a voltage when flexed as molecules repel each other.
Ceramics are often used under direct pressure while a
polymer is commonly used as a cantilevered beam.
A wide range of piezoelectric elements are available and
produce a variety of open-circuit voltages and short-circuit
currents. Typically the open-circuit voltage and short-circuit
currents increase with available vibrational energy as shown
in Figure 3. Piezoelectric elements can be placed in series
or in parallel to achieve desired open-circuit voltages.
12
9
INCREASING
VIBRATION ENERGY
6
3
0
0
10
20
30
PIEZO CURRENT (µA)
3331 F03
Figure 3. Typical Piezoelectric Load Lines for Piezo Systems
T220-A4-503X
Piezos produce the most power when they operate at
approximately half the open circuit voltage for a given
vibration level. The UVLO window can be programmed to
straddle this voltage so that the piezo operates near the
peak power point. In addition to the normal configuration
of connecting the piezo across the AC1 and AC2 inputs, a
piezo can be connected from either AC1 or AC2 to ground.
The resulting configuration is a voltage doubler as seen
in Figure 4 where the intrinsic capacitance of the piezo is
used as the doubling capacitor.
For more information www.linear.com/LTC3331
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