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LTC3331

3331fc

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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

protectionforDCsourcessuchassolarandthermoelectric

generators. The LTC3331 powers the VOUT output con-

tinuously by seamlessly switching between the energy

harvesting and battery inputs.

When harvestable energy is available, it is transferred

throughthebridgerectifierwhereitaccumulatesontheVIN

capacitor. A low quiescent current UVLO mode allows the

voltageonthecapacitortoincreasetowardsaprogrammed

UVLOrisingthreshold.Whenthevoltagerisestothislevel,

the buck converter turns on and transfers energy to VOUT.

As energy is transferred the voltage at VIN 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 cur-

rent 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

higherconcentratedburstofpowertoaccomplishatask.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.

Ceramicpiezoelectricelementsexhibitapiezoelectriceffect

when the crystal structure of the ceramic is compressed

andinternaldipolemovementproducesavoltage.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

produceavarietyofopen-circuitvoltagesandshort-circuit

currents.Typicallytheopen-circuitvoltageandshort-circuit

currentsincreasewithavailablevibrationalenergyasshown

in Figure 3. Piezoelectric elements can be placed in series

or in parallel to achieve desired open-circuit voltages.

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.

Figure 3. Typical Piezoelectric Load Lines for Piezo Systems

T220-A4-503X

PIEZO CURRENT (µA)

3331 F03

INCREASING

VIBRATION ENERGY

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