LTC3330_15 Datasheet, PDF (19/32 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Life Extender

LTC3330
APPLICATIONS
The LTC3330 allows for energy harvesting from a variety
of alternative energy sources in order to extend the life of
a battery powered wireless sensor system. The extremely
low quiescent current of the LTC3330 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 LTC3330 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 pro-
grammed 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 BAT pin is essentially zero.
The LTC3330 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.
Piezo Energy Harvesting
Ambient vibrational energy can be harvested with a piezo-
electric transducer which produces a voltage and current
in response to strain. Common piezoelectric elements are
PZT (lead zirconate titanate) ceramics, PVDF (polyvinyli-
dene 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. Ceram-
ics 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)
3330 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/LTC3330
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