Volture datasheet 001

1REVISION N0. 002 REVISION DATE: 01-23-2013

PIEZOELECTRIC ENERGY HARVESTERS

TYPICAL APPLICATION

FEATURES

� Enables Vibration Energy Harvesting� Robust Piezo Packaging� Pre-Attached Electrical Lead Wires and Connector� Hermetically Sealed for Use in Harsh Environments� Low Profile� Available in Different Sizes to Match to Application� Directly Integrate with COTS Products Such As The

Linear LTC3588 and Thin Film Batteries

APPLICATIONS

� Industrial Health Monitoring Network Sensors� Condition Based Maintenance Sensors� Wireless HVAC Sensors� Mobile Asset Tracking� Tire Pressure Sensors� Oil and Gas Sensors� All Air, Land and Sea Vehicle Sensors� Battery and Hard Wired Power Replacement

VIN

GND

OUT

VSTORE

VCC

MICROCONTROLLER

TX/RX

CHARGEMANAGEMENT

SENSORSENSOR SENSOR

DESCRIPTION

Volture™ vibration energy harvesters convert otherwisewasted energy from mechanical vibrations into useableelectrical energy. The Volture™ accomplishes this byutilizing normally brittle piezoelectric materials. TheMidé Volture™ energy harvester is unique amongstother piezo based energy harvesters because itincorporates Midé's patented piezoelectric transducerpackaging technology.

Through a proprietary manufacturing process, theVolture™ packages piezoelectric materials in aprotective skin with pre-attached electrical leads,producing a robust component with no soldered wires.The Volture’s™ protective skin also provides electricalinsulation and defense against humidity and harshcontaminants.

The Volture™ is available in six standard sizes. Customsizes are available and a cost effective alternative.

If a custom size is required please contact MidéTechnology Corporation by emailing:[email protected].

.100

.600 .660

.300.910

3.3103.560

2 X .110

.570

1.400

.175

1.085CLAMP LINE

.883

2.2832.533

.600.240

.100.150 1.000

1.290CLAMP LINE

2.720

.660

.300

2 X .110

.910

2.470

.600.570

1.400

.175

.100

1.085CLAMP LINE

3.190

1.2501.500

2 X .129

.690.865

2.940

.6001.31

1.81

.250

.100

1.115CLAMP LINE

.883

3.3833.633

.600.240

.1001.000.150

1.290

CLAMP LINE

V21BL

V22B

V21B

V20WV25W

V22BL

PRODUCT DIMENSIONS


NOTE:1. All dimensions are in inches

2. Connector thickness = 0.100”

Product TypicalThickness (in)

V20W 0.034

V25W 0.024

V21B 0.031

V21BL 0.031

V22B 0.031

V22BL 0.031


DEFLECTION LIMITS

ABSOLUTE MAXIMUM RATINGS

Operating Temperature Range -40 to 90 C

Operating Temperature Range (Without Connector) -40 to 150 C

Storage Temperature Range -60 to 90 C

Storage Temperature Range (Without Connector) -60 to 150 C

Lead Temperatures (Soldering, 10 sec) 300 C

Piezo Strain, max 800 micro-strain*

Maximum Voltage Output Product and Vibration Dependent**

Maximum Current Output Product and Vibration Dependent**

**See Performance Curves For Typical Values

*Related to max. tip deflection, see Deflection Limits

ProductSingle Wafer SeriesCapacitance (nF),

measured at 100 Hz

Single Wafer SeriesResistance (Ohm),

measured at 100 Hz

Single Wafer SeriesCapacitance (nF),

measured at 120 Hz

Single Wafer SeriesResistance (Ohm),

measured at 120 HzV20W 69 390 69 340

V25W 130 210 130 175

V21B 26 950 26 770

V21BL 26 950 26 770

V22B 9 2400 9 2000

V22BL 9 2400 9 2000

Energy HarvesterProduct Number

Max. Tip-to-TipDisplacement (in)

V20W 0.10

V25W 0.15

V21B 0.06

V21BL 0.18

V22B 0.03

V22BL 0.12

ELECTRICAL CHARACTERISTICS

TIP-TO-TIPDISPLACEMENT

FIXED TO VIBRATING SURFACE

PIN FUNCTIONS


P1, P2: Piezo wafer 1 output P3, P4: Piezo wafer 2 output

Each Volture contains two electrically isolated piezowafers, which may be used independently or bridgedfor increased voltage (series configuration) or currentoutput (parallel configuration). Series connection willdouble the open-circuit voltage compared to a singlewafer, and the effective capacitance will be 1/2 thesingle-wafer capacitance listed in the “ElectricalCharacteristic Table (Pg. 3)”. Parallel connection willdouble the current compared to a single wafer, and theeffective capacitance will be double the single-wafervalue. For most applications, parallel connection is

recommended. Please refer to the connection diagramabove. Regardless of series or parallel connection, thepower generated by the Volture™ Energy Harvester willbe the same.

In typical energy harvesting usage, the raw output is anAC waveform as the Volture deflects in both directions.For sensing or dual-use applications where it is desiredto know the direction of deflection at any given time,please refer to the relationship between deflection andoutput polarity for each wafer diagram below.

Series

OUT

OUT1

23

4

Parallel

OUTOUT1

23

4

Compared to Single Wafer ValueDouble VoltageSame CurrentCapacitance: Half the Single-Wafer Value

Compared to Single Wafer ValueSame VoltageDoubled CurrentCapacitance: 2x the single-wafer value

V21B / BLV22B / BL

V20WV25W

+-

+-

-+-

+

OPERATION


The Volture™ vibration energy harvester is designed toextract useable electrical energy from wastemechanical vibrations. The best means to accomplishthis is to mount the Volture™ product in a cantileveredconfiguration on the vibration source and tune thenatural frequency of the Volture™ harvester to matchthat of the vibration source.

Vibration Source CharacterizationThe first step in successful energy harvesting is to fullyunderstand the vibration environment in which theVolture™ will be operating. The most effective meansto accomplish this is to measure the vibration using anaccelerometer, capture the data, and perform an FFT(Fast Fourier Transform) on the data to extract therelevant frequency information.

Some applications will not require this step since theirdominant frequencies are inherently known. Anexample of this would be a 120 Hz AC motor or a 60Hz appliance. However, most applications will requiresome form of vibration characterization to besuccessful.

Midé offers a vibration characterization product andservice, the VR001. The VR001 is a small device thatcan be easily installed into many different vibrationenvironments. The device is completely stand aloneand can be applied to hard to reach areas. Built in timerdelays allow for capture of many different types ofvibration environments. A simple USB interface with

provided software allows the user to easily characterizeany vibration.

Attaching And Clamping The Volture™For optimized energy harvesting from vibrations it isbest to mount the Volture™ products in a cantileveredconfiguration. This takes advantage of resonant beamharvesting. If the natural frequency of the Volture™ issuccessfully tuned to that of the vibration source, themost energy will be harvested.

The first step in successful clamping is to ensure thatboth the base and clamp are constructed of rigidmaterials completely free of burrs and defects. Using arigid material will minimize dissipation of energythrough the clamp structure and avoiding burrs anddefects will minimize the potential for stressconcentrations on the Volture™ which could lead topremature failure.

The clamp should completely extend beyond the piezoelement within the Volture product. The suggestedclamp line shown in the product dimensions section ofthis document ensures that the clamp is clamping onthe piezo element.

For long term installation, the fasteners used to securethe clamp should be properly torqued and should bereinforced either using lock washers or some kind oflocking adhesive. This will ensure a proper long termclamp that will not loosen over time.

TOP VIEW

SIDE VIEW

FIXED TO VIBRATING SURFACEBEAM SECTION FREE TO DISPLACE

CLAMP LINE CLAMP CONNECTORTIP MASS

VIBRATION

OPERATION

Straight clamps are the simplest and often most costeffective clamps. However, curved clamps, as shownin the diagram below, have shown the capability toslightly increase performance of the Volture. Straightclamps are sufficient for the majority of applications.

Tuning The Volture™To ensure the most efficient harvesting, it is essential totune the Volture’s™ natural frequency to match that ofthe vibrating source. Tuning is performed by adding atuning mass to the end of the cantilevered Volture™

until the natural frequency of the piezo beam is thesame as the vibration source. The larger the tuningmass the lower the natural frequency of the Volture™.

For non permanent installations or for active tuning it isbest to use bee’s wax or some other form of non-permanent attachment for the tuning mass to theVolture. Bee’s wax allows the mass to be moved alongthe beam, toward and away from the clamp, for tuning.

There are multiple means of tuning the Volture™depending on the equipment available to the user. Ifonly the vibration source that will ultimately beharvested from is available to the user, it isrecommended that the Volture™ be properly mountedand clamped to the vibration source. The output of theVolture should then be attached to an oscilloscope formonitoring. The output can be either the raw output ofthe Volture™ (directly on two of the output pins) orthrough whatever electronics the user is using so longas the electronics allow for some measure of optimal

power output. The tuning mass can then be adjusteduntil the maximum power is achieved.

If the user has a shaker available, the tuning can beperformed by driving the Volture™ at the desirednatural frequency and adjusting the mass until optimalpower output is achieved. If connecting directly to theVolture™ pins, optimal power output will be where thevoltage output is maximized.

Another simple way to tune your Volture™ product is tomeasure the frequency at which the device "rings out"when excited by an impulse mechanical load. Theeasiest way to perform this type of tuning is to properlymount and clamp the Volture to a rigid structure. Next,attach at least one of the piezo's within the Volturedirectly to an oscilloscope for monitoring (ex: connectto pins P1 and P2). Add the appropriate tip mass (SeeRELATION BETWEEN TIP MASS & NATURALFREQUENCY section) to the end of the cantileveredVolture, do not permanently adhere the tip mass yet.Bee's wax or tape is often the best material to use fornon permanent tip mass installations. Apply an impulsemechanical load by very lightly "flicking" the end of theVolture. This will cause the beam to "ring out". Thefrequency of the the decaying wave is the naturalfrequency that the Volture is currently tuned to. Todecrease this frequency move the mass farther awayfrom the clamp point, to increase the frequency movethe mass closer to the clamp point. If the naturalfrequency is not close to the desired frequency either adifferent tip mass or a different product may berequired.

Once the tip mass is in the proper location for optimalenergy harvesting it should be permanently adhered tothe Volture™. This ensures that the tip mass remains inplace for the life of the Volture™. It is recommendedthat a robust adhesive such as Loctite™ 404 be usedfor this permanent installation. Keep in mind that anyadded mass will impact the tuning of the system.


CURVE CLAMP

POWER MEASUREMENTS

Piezoelectric material produces mechanical strainunder the influence of an externally applied electricalfield, and conversely produces electrical potential inresponse to applied mechanical strain. Products suchas the Volture™ piezo energy harvester are typicallyused in a cantilevered-beam configuration, in which thepiezoelectric beam is clamped at one end and the otherend allowed to oscillate freely in response to vibrationnormal to the flat surface of the beam, convertingthese vibrations to in-plane material strain. The beamdimensions and tip mass determine the resonantfrequency of the beam, which is tuned to match thedominant vibrational frequency of its environment,mechanically amplifying this typically small vibration.

Power MeasurementsThe power output capability of the Volture™ productswas measured in the following manner. In thecantilevered beam configuration above, the Volture wasmounted to a shaker capable of generating vibrations ofvarying frequency and amplitude. Tip masses (four foreach product) were added to alter the natural frequencyof the Volture™ products. The vibration frequency beinggenerated by the shaker was then matched to thefrequency of the Volture™ product to provide resonantand therefore optimized energy harvesting. Fourdifferent amplitudes were tested (0.25, 0.375, 0.5, and1.00g) at each of these frequencies. The piezo’s outputwas rectified and then placed across a purely capacitiveload. The capacitor value was chosen using thefollowing equation for average power, where C is the

capacitance in Farads, V is the piezo’s open circuitvoltage, and Δt is a reasonable time interval (~ 10seconds), and solving for C:

Yielding:

The figure below shows the voltage (operating voltage)on the capacitor and instantaneous power into capacitorvs. time for a representative vibration level andfrequency. The V25W product was used, demonstratingthat the power increases until it peaks when theoperating voltage is at about half its open circuit value.After that, it decreases.

See Application Note: Load Isolation Example.


00

Typical Power & Voltage vs Time

Time (Seconds)

Inst

anta

neos

Pow

er (m

W)

PPEAK VOPEN CIRCUIT

VOPEN CIRCUIT

PAVG t

2V1

2 C

P

V

AVGt

2C

2


-2 0 2 4 6 8 10 12-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Tuned to 180 Hz | 0 gram Tip Mass

Operating Voltage

Pow

er (m

W)

Tuned to 130 Hz | 2.4 gram Tip Mass

Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)

Tip Mass (grams)

Natu

ral F

requ

ency

(Her

tz)

-2 0 2 4 6 8 10 12 14 16 18-0.5

0

0.5

1

1.5

2

2.5

3

3.5

-5 0 5 10 15 20 25 30-1

0

1

2

3

4

5

6

0 2 4 6 8 10 12 14 1660

70

80

90

100

110

120

130

140

150

160

-2 0 2 4 6 8 10 12 14 16-0.5

0

0.5

1

1.5

2

2.5

3

0.250 g, Pmax=0.532 mW0.375 g, Pmax=0.969 mW0.500 g, Pmax=1.428 mW1.000 g, Pmax=3.005 mW




0 180 0.25 4.70 180 0.375 6.50 180 0.5 7.70 180 1 12.82.4 130 0.25 6.72.4 130 0.375 92.4 130 0.5 112.4 130 1 187.8 95 0.25 8.37.8 95 0.375 11.87.8 95 0.5 16.47.8 95 1 23.115.6 75 0.25 13.315.6 75 0.375 1915.6 75 0.5 22.615.6 75 1 34.7

Tip Mass(gram)

Fn(Hz)

Amplitude(g)

Open CircuitVoltage*

* piezo wafers connected in series

V20W TYPICAL PERFORMANCE POWER CHARACTERISTICS

V20W RELATION BETWEEN TIP MASS & NATURAL FREQUENCY



Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)

Tip Mass (grams)

Natu

ral F

requ

ency

(Her

tz)

-2 0 2 4 6 8 10 12-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

-2 0 2 4 6 8 10 12 14 16 18-0.5

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25-1

0

1

2

3

4

5

6

7

8

-5 0 5 10 15 20 25 30-2

0

2

4

6

8

10

0 1 2 3 4 5 6 7 850

60

70

80

90

100

110

120

130





0 120 0.25 3.20 120 0.375 4.40 120 0.5 5.50 120 1 10.12.4 75 0.25 4.72.4 75 0.375 6.52.4 75 0.5 7.52.4 75 1 11.57.8 50 0.25 10.37.8 50 0.375 15.47.8 50 0.5 18.67.8 50 1 29.515.6 40 0.25 14.515.6 40 0.375 21.215.6 40 0.5 2715.6 40 1 36.6

Tip Mass(gram)

Fn(Hz)

Amplitude(g)



V25W TYPICAL PERFORMANCE POWER CHARACTERISTICS

V25W RELATION BETWEEN TIP MASS & NATURAL FREQUENCY


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)

Tip Mass (grams)

Natu

ral F

requ

ency

(Her

tz)

-5 0 5 10 15 20 25-0.5

0

0.5

1

1.5

2

2.5

-1 0 1 2 3 4 5 6 7-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

-2 0 2 4 6 8 10 12-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-2 0 2 4 6 8 10 12 14 16 18-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 580

100

120

140

160

180

200

220

240

260





0 275 0.25 4.10 275 0.375 5.90 275 0.5 7.60 275 1 12.31 175 0.25 7.61 175 0.375 10.91 175 0.5 13.61 175 1 23.52.4 140 0.25 10.92.4 140 0.375 15.22.4 140 0.5 18.82.4 140 1 324.8 105 0.25 15.94.8 105 0.375 21.64.8 105 0.5 28.14.8 105 1 46.5

Tip Mass(gram)

Fn(Hz)

Amplitude(g)



V21B TYPICAL PERFORMANCE POWER CHARACTERISTICS

V21B RELATION BETWEEN TIP MASS & NATURAL FREQUENCY




Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)

Tip Mass (grams)

Natu

ral F

requ

ency

(Her

tz)

-1 0 1 2 3 4 5 6 7 8 9-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

-2 0 2 4 6 8 10 12 14-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

-5 0 5 10 15 20 25-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

-5 0 5 10 15 20 25 30-0.5

0

0.5

1

1.5

2

2.5

3

0 1 2 3 4 540

50

60

70

80

90

100

110

120

130





0 110 0.25 3.950 110 0.375 5.350 110 0.5 6.60 110 1 12.11 65 0.25 81 65 0.375 9.91 65 0.5 12.41 65 1 22.12.4 50 0.25 9.82.4 50 0.375 13.72.4 50 0.5 19.12.4 50 1 27.54.8 40 0.25 13.24.8 40 0.375 19.24.8 40 0.5 25.94.8 40 1 44.4

Tip Mass(gram)

Fn(Hz)

Amplitude(g)



V21BL TYPICAL PERFORMANCE POWER CHARACTERISTICS

V21BL RELATION BETWEEN TIP MASS & NATURAL FREQUENCY

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 1 2 3 4 5 6 7 8-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

-2 0 2 4 6 8 10-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

-2 0 2 4 6 8 10 12 14 16-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.5 1 1.5 280

100

120

140

160

180

200

220

240

260


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)

Tip Mass (grams)

Natu

ral F

requ

ency

(Her

tz)

0.25 g, Pmax= 0.00 mW0.375 g, Pmax= 0.01 mW0.5 g, Pmax= 0.01 mW1.0 g, Pmax= 0.05 mW




0 240 0.25 2.20 240 0.375 3.40 240 0.5 4.60 240 1 7.20.5 125 0.25 6.70.5 125 0.375 9.30.5 125 0.5 11.60.5 125 1 18.51 105 0.25 91 105 0.375 12.11 105 0.5 14.71 105 1 25.12 80 0.25 132 80 0.375 17.42 80 0.5 22.32 80 1 33.4

Tip Mass(gram)

Fn(Hz)

Amplitude(g)



V22B TYPICAL PERFORMANCE POWER CHARACTERISTICS

V22B RELATION BETWEEN TIP MASS & NATURAL FREQUENCY



-5 0 5 10 15 20 25 30 35 40-0.5

0

0.5

1

1.5

2

0 0.5 1 1.5 220

30

40

50

60

70

80

90

100

110

-2 0 2 4 6 8 10 12-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

-5 0 5 10 15 20 25 30-0.2

0

0.2

0.4

0.6

0.8

1

1.2

-5 0 5 10 15 20 25-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)


Operating Voltage

Pow

er (m

W)

Tip Mass (grams)

Natu

ral F

requ

ency

(Her

tz)





0 110 0.25 20 110 0.375 3.10 110 0.5 4.20 110 1 6.90.5 45 0.25 60.5 45 0.375 8.50.5 45 0.5 110.5 45 1 17.51 35 0.25 81 35 0.375 11.51 35 0.5 141 35 1 23.42 30 0.25 122 30 0.375 16.12 30 0.5 20.82 30 1 31.8

Tip Mass(gram)

Fn(Hz)

Amplitude(g)



V22BL TYPICAL PERFORMANCE POWER CHARACTERISTICS

V22BL RELATION BETWEEN TIP MASS & NATURAL FREQUENCY

APPLICATIONS INFORMATION - SWITCHED CAPACITOR BOOST CIRCUIT


Switched Capacitor Boost Circuit for One-Shot Sensors and Low Vibration LevelsIt is often difficult to make use of low-voltage energyscavenging sources, such as piezo energy harvestersat low vibration amplitudes, solar cells in overcast orindoor environments, or Seebeck devices, etc., asmost microcontrollers and sensors require minimumvoltages of 1.8V or greater to operate. Using thesesources requires a voltage boost converter with aminimum star t-up voltage, low star t-up currentrequirement, and graceful handling of undervoltage andslow input voltage rise times.

The circuit shown, based on the Seiko Epson S-882Zseries charge pump IC, provides bursts of powerstarting at approximately 2.4VDC (ending at 1.6-2VDC) from input voltages as low as approximately380mV. This IC provides boosting using small on-chipswitched (“flying”) capacitors, rather than inductiveboost conversion. Thus the initial input currentrequirements are reduced compared to a typical boostconver ter, requiring only a small power-supply

bypass capacitance to reliably start up. The circuitcan begin operating almost immediately when powerbecomes available. This circuit is ideal for directlypowering small sensors that can perform theirfunction (e.g. record or transmit a measurement) in aknown amount of time and power. It could also beused to extend battery life, or as a trigger source fordevices with their own battery/supercap powersource.

When the voltage at VIN exceeds the minimum start-upvoltage, the IC begins pumping charge to a storagecapacitor connected across the CPOUT pin. When thecapacitor voltage reaches 2.4V, the output is enabledand this voltage is applied to the load. The load isautomatically disconnected when the storage capacitorvoltage drops below approx. 1.8V (see Figure 6). Thisbehavior allows the circuit to directly power most 3.3Vmicrocontroller circuits, with a typical operating voltagerange of 1.8V ~ 3.6V, without exposing the circuit tosignificant undervoltage or unfavorable voltage rise-time conditions.

QPXX_RA

J3D1

BAT54STA

GND

D2

VH

C1

10uF

100uF

C3

4

5

3

2

1

U1

S-882Z

VIN

CPOUT

VM

GND

VOUTVCC

GND

GND

1

Piezo Bimorph

X1

X2


Goals� Allow intermittently operating circuits to be

powered from extremely weak vibration sources,or moderate vibration sources at frequencies sig-nificantly different from the energy harvester's res-onant frequency (Figure 3)

� Physically disconnect the load during periods ofinsufficient voltage to avoid "over-the-hump" prob-lems of cold circuit start-up from harvested power

� Maximize chances of success where vibrationsource's characteristics (amplitude and frequencycontent) cannot be known in advance.

Simple “One-Shot” ApplicationA simple usage scenario is an embedded sensor withdata storage/transmission capability, which takes oneset of measurements each time it is powered up(relying on the loss and subsequent re-application ofpower to start the next measurement). In this case, themeasurement frequency is variable and depends on thevibration amplitude. To operate the sensor directly fromthe boost circuit requires:� Estimation (or measurement of) the run-time and

power consumption of your application within itsvoltage limits

� Sizing the CPout according to worst-case usage,allowing some headroom

For such one-shot sensors, it is recommended tocreate a large load (e.g. drive an LED or GPIO pin tiedto ground) after completing the task in order to ensurethe power output cycles in high-vibration conditions.

A typical microcontroller sensor application’s loadprofile will be “bursty”, complicating the task ofestimating the required value of CPout. However, if theload can be approximated in terms of a resistive load,the following equations can be used to estimate therequired capacitance, available runtime, energy perdischarge or power stored.

In the equations above, Td is the runtime or dischargetime in seconds, Tc is the charge time in seconds, R isthe equivalent load resistance in ohms, V0 is thestarting output voltage (2.4), V is the final outputvoltage (1.8V or the minimum operating voltage of thesensor, whichever is greater), and C is the capacitancein Farads. Likewise, the output voltage can be modeledas a simple RC time constant, V = V0e-T/RC.

Continuously-Powered Application withInput-Dependent TriggeringSometimes it may be advantageous to incorporatevibration-powered battery maintenance and/ortriggering into a continuously-running circuit. Forexample, a sensor may require low-level continuouspower to maintain a realtime clock, but measurementtasks are only needed when a piece of machinery isknown to be operating. Alternately, it may be desired todynamically adjust the measurement rate based on theincoming power to maintain a given power budget. In


R lnC

11 V

VTo

d

T R C lnVVo

d

W12

C V V2 20

PW

T Tc d

Equation 1: Capacitance needed fora given runtime (F)

Equation 2: Runtime for a givencapacitance (sec.)

Equation 3: Energy per discharge(Joules or watt-seconds)

Equation 4: Average Power (Watts)


these cases, the application can use Vout as aninterrupt source in addition to power source, with arising or falling edge triggering the desired action.

Typical ResultsThe following are measured results using the circuitshown in a typical configuration.


0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

10

20

30

40

50

60

70

80

90

Averaged Power Output vs. AccelerationVolture V20W @ 60Hz

Acceleration (g)

Aver

age

pow

er o

utpu

t (uW

)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.710

0

101

102

103

Discharge Interval vs. AccelerationVolture V20W @ 60Hz, 100uF

Acceleration (g)

Tim

e be

twee

n di

scha

rge

cycl

es (s

)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.20

10

20

30

40

50

60

Usable Bandwidth vs. AccelerationVolture V20W

Acceleration (g)

Usab

le V

20W

ban

dwid

th a

t Fc=

60Hz

(Hz)

Figure 3: Usable Bandwidth vs. Acceleration at 60Hz centerfrequency. Usable bandwidth refers to the frequency range

over which the setup produced voltages exceeding thecircuit’s turn-on threshold (2.4V), producing output pulses.

Figure 2: Discharge Interval vs. Acceleration, measuredresults using configuration shown

Figure 4: Averaged Power Output vs. Acceleration



Tek Stop

Ch1 500mV Ch2 500mV Ch3 500mV M 2.00 s A Ch1 0.00 V

3

: 570mV@: 2.42 V

Figure 5: Rectified input (aqua), VCAP (navy) and switchedoutput (violet) from initial start-up to stable output, 10K

resistive load.

Figure 6: Rectified input (aqua), VCAP (navy) and switchedoutput (violet) output pulse detail, 10K resistive load.

Tek Stop

Ch1 500mV Ch2 500mV Ch3 500mV M 200ms A Ch1 0.00 V

3

: 570mV@: 2.42 V

APPLICATIONS INFORMATION - LOAD ISOLATION EXAMPLE

Load Isolation Example for MaximumEfficiency with Low-Impedance LoadsA cantilevered-beam piezoelectric energy harvester is acomplex electromechanical system in which theelectrical and mechanical loading of the beam are inter-related. Understanding of this relationship is critical togetting the most out of the system.

A properly tuned switched boost-buck circuit willalways outperform any circuit in which you do nottune to the characteristics of its application. Thisapplication note demonstrates a high-efficiencyswitched step-down (buck) conver ter andoptimizations for maximum performance in a real-world application. This circuit isolates the end-application’s electrical load from the piezo beam,providing proper impedance-matching of the circuit’s“virtual load” to the beam, as well as minimizingmechanical loading effects. Such a circuit is ideal forlow-impedance loads such as rechargeable batteries,“bursty” loads such as intermittently-operatingsensors/transmitters, and applications where theelectrical load cannot be known in advance.

Basic Piezoelectric Beam ModelBefore mechanical loading effects are taken intoaccount, each piezo beam can be thought of as a smallcurrent source in parallel with a capacitor and parasiticresistance, as shown in Figure 1. Typical values for thisparallel capacitance and resistance are on the order of10nF and >40M, respectively. This parallel resistanceis insignificant for our purposes and may be ignored.The current flow is equal to the derivative of the strain-induced charge, or dQ/dt. The voltage transfer functionof each beam therefore is V(s)/q(s) = sR/(1+sRC),where R is the parallel resistance and C is the parallelcapacitance of the beam.

In its simplest form, power is drawn from a smallcapacitor which is constantly recharged by its

environment. Care must be taken to choose when thispower is drawn. To maximize transfer efficiency, theload must be matched to the piezo’s equivalentimpedance. In practice, the piezo impedance at a givenamplitude and frequency, as well as the loadimpedance, can be thought of as a pair of simple (butunknown) resistances which make up a resistordivider. The power transfer between the two isoptimized when their values match. This correspondsto the point at which the piezo’s loaded voltage is equalto half its open-circuit voltage. Thus the impedancematch can be optimized without formally measuring orknowing the impedance of the piezo source or load in-situ.

ImplementationThe circuit of Figure 2 provides a simple but effectiveapproach to meeting these goals.

The main components are a bridge rectifier formed byD1-D4, low-power comparator (U1) and buckconver ter (U2). During vibration, main storagecapacitor C1 slowly charges until its voltage reachesthe operating point (Voc/2) set by R1 and R2. Buckconverter U2 is enabled once the stored voltageexceeds this value plus a small hysteresis. At typicalloads, the buck converter operation into the load will


R leak

R leak

Figure 1: Piezo parallel and series equivalent circuits


deplete C1 until its voltage drops below the hysteresisband, at which point the converter is disabled and thecycle repeats. Thus C1 is maintained at approximatelyVoc/2. The typical output is a train of voltage-regulatedpulses ideally suited to charging a battery or capacitor,or directly powering an intermittently-operating sensor.

Since the voltage at C1 is held approximately constant,the effective load seen by the piezo is not significantlyaffected by activation of U2 or changes to the actualload. Additionally, the voltage maintained on C1ensures charge is drawn from the piezo beam onlywhen its voltage exceeds C1’s voltage by one diodedrop, which occurs only when the beam approaches itsmaximum deflection. Thus a limited amount of chargeis drawn on each cycle of the piezo beam, timed tocoincide with the maximum deflection of the beam.This combination of factors helps prevent excessivemechanical damping of the beam, allowing usableoutput voltages to be output at lower vibrationamplitudes while the piezo beam is driven near itsmechanical resonance.

U2 provides a regulated output voltage determined bythe ratio of R7 and R8. The power output per G ofvibration will remain relatively constant across G-levelsand loads, provided the load is capable of drawing at

least as much power as is being supplied (Figure 5).The converter duty cycle is approximated by the ratioof input power (G-level) vs. output power (voltage *current * time), less any small conversion losses andleakage. When the input power exceeds the output andlosses, duty cycle will be 100%, and excess voltage atC1 is safely disposed of through D5.

R6 sets the peak current through inductor L1, and canbe specified according to the equation:

where IMAX is the maximum desired output current tothe load. Generally, the peak inductor current should beset to maximum to improve efficiency with smaller(<300uH) inductors in space-limited applications;refer to the LT1474 datasheet for more thoughtfultrade-offs between peak current and inductor size.Battery-charging applications requiring a (pulsed)constant-current charge phase, or where both thecharge circuit and the load are directly connectedacross the battery pack, may require a lower IPEAKsetting to reduce voltage ripple caused by the chargepulses.


Piezo Bimorph

X1

X2

VCC

+ C1

V REF 18

R3

BAT43W

D6

R6

U2

VIN

SENSE

LBI

RUN

VFB

LBO

SW

GND

-INOUTV+

V-

+IN

HYSREF

15M (Optional)

72

3

5

6

7VCC

VFB

D8

VOUT

GND

V4

12pF

BAT43W

C3+

1uF

VOUT

SW1

RUN

63

8

125

4

R52.2M

R4

C2

10nF

100K LTC1474CMS8

100uF

GND

D7 BAT4

3W

Zene

r 11V

D5

1M R2

5.7M R1

100u

H

2.4M R8

4.7M R7GN

DA

C

LTC154 0CMS8

D1

BAV199BAV199

D2

Figure 2: Simple high-efficiency voltage conversion and load-isolation circuit for piezo energy harvesting applications. The C1voltage setpoint in this example is fixed, but could be made adaptive using additional circuitry.

RSENSEMAX

0.067I

0.25


Considerations for Conversion EfficiencyConverter efficiency is defined as the output powerdivided by the input power times 100%.

The losses from the circuit’s front-end components(leakage across the storage capacitor, comparator, andR1-R3) will be small (uA) and largely proportional tothe C1 voltage setpoint. Minimizing losses here isstraightforward by appropriate component selection ofC1 and suitably large values of R1 – R3. These smalllosses will occur for the entire time that voltage ispresent on C1.

The remaining controllable losses will occur in thestep-down converter section and, apart from theLT1474’s shutdown current, occur only during the ON-time of the output. The main losses will come fromthree sources: VIN current, I2R losses, and catch diodelosses.

VINThe VIN current of the LT1474 is due to twocomponents: the DC bias current and the gate chargecurrent of its internal P-channel switch. Both areproportional to VIN; however, at load currents > 1mA,the DC bias current (from 9uA at no load to 100uA incontinuous mode) is negligible compared to the gatecharge losses. Each time the gate is switched on andoff, a packet of charge dQ moves from VIN to ground.In continuous mode, IGATE = fQp where Qp is the gatecharge of the internal switch and f is the switchingfrequency. Since VIN is pre-set according to piezoefficiency considerations, the best way to minimize thisloss is to keep the output duty cycle low.

Resistive (I2R) Losses

The resistances in the current path (The ON-resistance

of the internal switch, current sense resistor, andinductor) contribute resistive losses. At low values ofVIN, switch losses will dominate, and can be minimizedby using a suitably large inductor and low IPEAK. Athigher supply voltages, these losses are proportional toload.

Catch Diode Losses

The catch diode, D7, introduces a loss (V*I) as itconducts during the switch off-time, proportional to itsforward voltage, and more pronounced at high supplyvoltage where the switch on-time is shorter. Again, VINis fixed for piezo efficiency reasons, so this loss is bestminimized by minimizing the diode Vf and converterduty cycle. By lowering the duty cycle, a reduction in Vfof the catch diode will improve efficiency even thoughlow-Vf diodes tend to have much higher reverseleakage currents, which will produce loss during theswitch ON-time. In any event, the catch diode must besized to safely handle IPEAK at nearly 100% duty cycle(worst-case condition; output shorted).

The selection of 100uH inductor should be considereda minimum, for use in conjunction with high IPEAKvalues. Particularly in applications where space is notconstrained or IPEAK must be reduced, inductors aslarge as 1000uH may be required for maximumefficiency. The ideal range of inductor size at a givenIMAX is a trade-off between the increased resistance ofhigher-value inductors and the increased switchingrates required for lower-value inductors.

At light loads, the output duty cycle will rise and lossesin the conversion stage (leakage, switching and catchdiode losses) will accrue. This may or may not be aproblem; it could simply mean that more power isavailable than the load can use.



Application ExampleThe steps to configure this circuit optimally for a givenload are presented below. This example assumes thefollowing are known:

� Required output voltage� Required output current� Nominal piezo open-circuit voltage

1) Set output voltageThe output voltage is determined by

Where Vf is the forward voltage drop of D8 under load.In direct sensor-power applications where reverseleakage into the circuit during its off-time is not aconcern, D8 may be omitted (but see importantwarnings below). To minimize no-load supply current,resistor values in the megaohm range should be used.

2) Choose inductor size and maximum inductorcurrent, following the guidelines of the LT1474datasheet. For greatest efficiency where space is not aconcern, first choose the inductor current senseresistor R6 according to desired output current(RSENSE Equation above), then size the inductoraccordingly. For space-limited applications, size L1 asdictated by the available space and set IPEAK as highas the inductor will tolerate without saturating.

3) Choose C1 bias voltage to be approximately ½ theopen-circuit piezo voltage in its natural vibrationenvironment. In applications with highly variableamplitudes, a trade-off must be made betweenefficiency at higher amplitudes and the ability toharvest power from lower amplitudes: no charge willbe drawn by this circuit if the piezo open-circuit voltageafter rectification never reaches the set turn-on voltage.

(Circuit adaptations for self-adjusting bias voltage maybe explored if the power gains exceed the consumptionof the added circuitry.)

The LTC1540 features a programmable hysteresisband of up to 100mV. However, a larger hysteresis isgenerally preferable to allow for smaller values of C1and/or longer output ON-times, particularly for directly-powered sensors. This can be accomplished bybypassing the onboard hysteresis adjustment (R4=0ohms; R5=open) and adding a small amount ofpositive feedback via R3.

First select the desired center voltage (Vc) for C1 andthe desired hysteresis band (Vhb). Choose a value forR1 such that the current across it at the trip point issubstantially greater than the comparator’s inputleakage (1nA typ.), for example 100nA. The 5.7M valueshown is perfectly reasonable, but the following willmore formally dictate its upper limit. The LTC1540’sinternal reference (Vref) is 1.182V. The current throughR1 at the lower trip point is (Vc-(Vhb/2)-Vref) / R1. Thus the upper limit for R1 is

Choose R2 to set the center voltage, Vc. Neglecting thesmall influence of R3,

Choose R3 to set the hysteresis. At the lowercomparator trip point, the threshold voltage will beequal to


Vout = 1.23 1+ VR7R8

R1Vc

V

I R1

21.182hb

R21.182 R1Vc 1.182

VR2

R2 R1 R3L


as the comparator will be ON when approaching thispoint. Solving for R3,

The addition of R3 will slightly affect the actual centervoltage; however, this change will typically not besignificant in comparison with the size of the hysteresisband. For a typical Vc (>3.3V), the final value of Vc willbe lowered. As indicated by Figure 5, however, drivingthe piezo to moderately higher open-circuit voltagesthan optimal does not have an excessive impact onefficiency.

Check: Actual Vh and VlOnce all resistor values are chosen, the actual upperand lower trip points will be defined by:

Latch-Up ConsiderationIf diode D8 is removed, the designer must pay attentionto D6, which is necessary to prevent latch-up of theLT1474’s output switch if the voltage at its SW pin isheld up while VIN drops. This situation may easilyoccur in an application where the load is powered bymultiple sources or includes a storage element, suchas supercapacitor or battery, whose voltage dropsmore slowly than that of C1 when no harvested poweris available. The user must then choose D6 so that thevoltage at SW cannot exceed the voltage at VIN by the0.6V required for latch-up. Particularly at high C1voltages, the designer must also take steps to ensurethat reverse leakage over D6 will not allow the voltagesat the VFB and SW pins to float above their maximumratings at no load.


R3R1 R2 VL Vth

R1 Vth R2 Vth R2 VL

Vh 1.182

Vl 1.182

R2 R3 R1R2 R3

R1 R3 R2R2

APPLICATIONS INFORMATION - POWER MANAGEMENT IC INTEGRATION

Integration with LTC3588 ChargeManagement ICThe LTC3588-1 is an ultra low quiescent current powersupply designed specifically for energy harvestingapplications. The part is designed to interface directly toa piezoelectric or alternative A/C power source such asthe Volture™. The part rectifies a voltage waveform andstores harvested energy on an external capacitor,bleeds off any excess power via an internal shuntregulator, and maintains a regulated output voltage bymeans of a nanopower high efficiency synchronousbuck regulator. When combined with a Volture™ energyharvester the LTC3588-1 forms the base of a full energyharvesting solution.

The LTC3588-1 will gather energy and convert it to auseable output voltage to power any type of electronicsystem. Some applications may require much morepeak power than a typical piezo can produce. TheLTC3588-1 accumulates energy over a long period oftime to enable efficient use for short power bursts. Thefrequency of bursts allowed is directly proportional tothe power coming in from the piezo, and the total energyper burst. The LTC3588-1 allows for four different pinselectable output voltages (1.8, 2.5, 3.3 and 3.6).

PGOOD SignalThe PGOOD Signal is a logical high when VOUT reaches92% of the target value. This behavior is ideal for anenable or interrupt pin on a microprocessor. That way,the microprocessor can be idle until there is enoughenergy to supply the burst.

Capacitor SelectionThe input and output capacitors should be selectedbased on the energy needs and load requirements ofthe application. The VIN capacitor is placed betweenVIN and GND. It should be rated to withstand thehighest voltage that the piezo can put out. For 100mAor smaller loads, storing energy at the input takes

advantage of the high voltage input since the buckconverter on the chip can deliver 100mA averagecurrent efficiently to the load. The input capacitorshould then be sized to store enough energy to provideoutput power for the length of time required, while alsonot dropping to the undervoltage lockout fallingthreshold (UVLO falling). This threshold isapproximately 300mV above the selected regulatedoutput. The following simple equation shows energyout on the left-hand side, and effective energy in on theright-hand side.

and

Where η is the efficiency of the buck converter, andVSHUNT is the voltage at which the input shuntregulator bleeds power from the input (typically 20V).See LTC3588 datasheet for more detail on η, as itdepends on the selected regulated output, the value ofthe inductor, the load current, and temperature.Keeping CIN at this value or greater ensures that thereis enough power stored at the input to perform eachburst.

To size the output capacitor, the following equation isused:

For capacitor values on the order of 10uF, some non-ideal delays are introduced, resulting in VOUT ripple.For smoother operation, it is recommended that COUTbe kept higher than 10uF.


P t12 C V VLOAD LOAD IN IN UVLOFALLING

2 2

t CSLEEP OUTLOAD

24mVI

V V VUVLOFALLING IN SHUNT

APPLICATIONS INFORMATION - POWER MANAGEMENT IC INTEGRATION

InductorThe buck converter is optimized to work with aninductor in the range of 10uH to 100uH. 10uH isadequate for space-limited applications, but 100uHmay provide greater efficiency, particularly as the ratiobetween input and output voltage increases (seeLTC3588 datasheet). Choose an inductor with a DCcurrent rating greater than 350mA. Lower DCRinductors can impact the efficiency of the buckconverter.

VIN2 and CAP CapacitorsA 1uF capacitor should be connected between VIN andCAP and a 4.7uF capacitor should be connectedbetween VIN2 and GND. These capacitors hold up theinternal rails during buck switching and compensatethe internal rail generation circuits. In applicationswhere the input source is limited, to less than 6V, theCAP pin can be tied to GND and the VIN2 pin can be tiedto VIN. In this scenario, a 5.6V Zener diode can beused to clamp VIN.


10uF25V

1uF6V

47uF6V

100uH 3.3VLTC3588-1

PZ1

VIN

CAP

VIN2

D1

D0

EN

MICROPROCESSOR

CORE

PZ2

PGOOD

SW

VOUT

4.7uF6V

TX

GND

GND

Education

Volture datasheet 001