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62 • 2007 IEEE International Solid-State Circuits Conference
ISSCC 2007 / SESSION 3 / TD: EMERGING DEVICES AND CIRCUITS / 3.1
3.1 Efficient Power Management Circuit: Thermal Energy Harvesting to Above-IC Microbattery Energy Storage
H. Lhermet1, C. Condemine1, M. Plissonnier2, R. Salot2, P. Audebert1, M. Rosset1
1CEA/LETI, Grenoble, France2CEA/LITEN, Grenoble, France
Autonomous devices that are self-powered over a full lifetime, byextracting their energy from the environment, are crucial forapplications such as ambient intelligence, active security insmart cards or monitoring. As the energy availability and powerdissipation are not constant over time, energy managementbecomes a key function and determines the potential for informa-tion processing. All these challenging constraints have beentaken into account to develop an autonomous system enablingthermal energy harvesting and power storage in the microwattrange.
The microsystem architecture, illustrated in Fig. 3.1.1, is com-prised of two power sources, RF and thermoelectric, a microbat-tery used as a storage unit and integrated circuits to transformand manage the harvested energy and interface the microbattery.Both sources are managed by the ICs: the microbattery beingcharged either using thermal energy harvested by the thermo-generator associated with the DC/DC converter or using externalRF power converted by the RF converter. The state of charge ofthe storage device is monitored periodically.
Thermoelectric power generators have three main advantages: nohuman intervention is required throughout their lifetime, theyare highly reliable and quiet since there are no moving mechani-cal parts and the materials used are environment friendly. Microand nanotechnologies enable production of the small power gen-erators required to match the decreasing dimensions of standardwireless sensor modules. The thermal micro-generator illustrat-ed in Fig. 3.1.2 has an output power of 4µW/cm2 per degree C, a90Ω series resistance and generates 1V for a temperature differ-ence of 60°C.
A micropower up-converter switching power supply is used toconvert the available power from the thermogenerator into a reg-ulated power supply (Fig. 3.1.3). The difference between one partof the boost filter output voltage (α*Vout) and a voltage setpoint(SP) is amplified, then modulated into pulse density informationfor control of the MOS power switch. A sub-1V bandgap voltagereference [1] is used as the voltage reference (570mV). The erroramplifier is comprised of a 50dB gain OTA and a buffer. Theinnovative pulse density modulation (PDM) is based on an asyn-chronous passive ∆Σ modulator instead of the traditional PWMcontroller for simplicity of implementation (2 RC filters and 3inverters), very low power consumption (1µW) and spectralspread of the switch noise. A low-voltage, high-performancecharge pump, composed of one clock booster and two stages ofvoltage doublers [2], is used to increase the PDM signal voltagefour-fold. This allows a large decrease in the equivalent Ron ofthe MOS switch.
The RF converter is composed of a limiter, a rectifier and a con-trol loop to provide a stabilised DC output. In standard 13.56MHzRFID applications, RF power and conversion efficiency depend onthe distance between the RF source and the IC; the input RFpower is much greater than the needed power and the superflu-ous current is diverted through ballast MOS.
The microbattery can be charged by the thermogenerator’sDC/DC output or by the RF converter. Therefore, the power sup-ply manager, comprising a specific unit along with an asynchro-nous finite state machine, manages priority between the twosources when they are present simultaneously and activates self-
powered microbattery protection in the case of external powersource interruption. The charge controller provides constant cur-rent for a one hour microbattery charge.
The lithium, solid-state microbatteries [3] are processed on Siwafers using thin film technology. The low process temperature(< 350°C) required and surface compatibility with ASIC materi-als allow the use of the ASIC as a substrate for the microbattery.
In order to ensure permanent monitoring, the microbattery dis-charge monitor is supplied by the above-IC deposited microbat-tery itself. Therefore, since the available capacity provided bythese on-chip batteries is in the range of 100µAh/cm2, all levels ofmonitor development (from architecture to design) are made toachieve an ultra-low-power monitor [4, 5], the aim being longerbattery operation and preservation of energy for the application.The discharge monitor must also be resistant to supply voltagevariations (1.6V to 2.8V) to be compatible with the microbattery.The microbattery is considered discharged if its voltage is lowerthan 1.6V±0.1V. The discharge monitor compares this voltage toa reference voltage for one second every hour. This sampling rateis sufficient since the microbattery lasts for about one year for theenvisaged low duty-cycle applications.
The DC/DC converter for the thermogenerator illustrated in Fig.3.1.4 is implemented in a traditional 0.35µm CMOS process andoccupies a core area of 0.5mm2. Measurement results indicate atotal power consumption of 70µW with a 1V supply voltage,including clock generation, charge pump, ∆Σ modulator, erroramplifier, bandgap reference and I/O pads (Fig. 3.1.5). Thebandgap reference presents a voltage variation of 104ppm/˚C in a[-40, +160]°C temperature range. The output voltage can be reg-ulated within a 2.5V range from 1.75V to 4.3V. Figure 3.1.6 showsDC/DC converter block efficiency at maximum output current fordifferent voltage levels. Higher efficiency is obtained for a 1.75Voutput and maximum output power is obtained at an output volt-age of 2.25V. The efficiency factor can be strongly enhanced byreducing losses in the power filter (and especially in the diode).
Figure 3.1.7 illustrates an IC, containing an RF converter, thepower supply manager and the microbattery charger and dis-charge monitor, fabricated in 0.18µm CMOS with a 30mm2 micro-battery deposited on the surface. A 78% efficient power supplymanagement and microbattery charge unit is obtained with a27µA charge current and a 12pA standby current consumptionfrom the microbattery (for permanently supplied protection). Theoverall consumption of the battery-powered discharge monitor isless than 5nW (3.3nW for sequencing, 1.3nW for sampled com-parison, as detailed in Fig. 3.1.5). These results are confirmed bysystem level tests with above-IC microbattery powered ASICs,showing furthermore that the microbattery process does notaffect ASIC performance.
In order to achieve thermal energy harvesting and storage, amicropower DC/DC up-converter and a controller chipset wasdesigned and tested. The next step of this project is to design anautonomous microsystem including the two proposed functionstogether with a traceability application in the same die and tech-nology.
References:[1] K. Lasanen, V. Korkala, E. Raisanen-Ruotsalainen, J. Kostamovaara,“Design of a 1-V Low Power CMOS Bandgap Reference Based on ResistiveSubdivision”, IEEE MWSCAS , vol. 3, pp. 564-567, Aug., 2002.[2] Y. Moisiadis, I. Bouras, A. Arapoyanni, “Charge Pump Circuits for Low-Voltage Applications,” VLSI Design, vol.15, pp. 477-483, Jan., 2002.[3] C. Navone, R. Baddour-Hadjean, J. P. Pereira-Ramos, and R. Salot“High-Performance Oriented V2O5 Thin Films Prepared by DC Sputteringfor Rechargeable Lithium Microbatteries,” J. Electrochem. Soc., vol.152(9), p. A1790-1796, 2005.[4] H.J. Oguey, and D. Aebischer, “CMOS Current Reference WithoutResistance,” IEEE J. Solid-State Circuits, vol. 32, pp. 1132-1135, July1997.[5] S.F. Al-Sarawi, “Low Power Schmitt Trigger Circuit,” ElectronicsLetters, vol. 38, no.18, pp. 1009-1010, Aug. 29, 2002.
1-4244-0852-0/07/$25.00 ©2007 IEEE.
63DIGEST OF TECHNICAL PAPERS •
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ISSCC 2007 / February 12, 2007 / 1:30 PM
Figure 3.1.1: System architecture. Figure 3.1.2: Thermo-elements.
Figure 3.1.3: DC/DC converter architecture.
Figure 3.1.5: Current consumption. Figure 3.1.6: DC/DC converter efficiency.
Figure 3.1.4: Thermal DC/DC converter micrograph.
0/3V
0/3V
Power supply
manager
Charger
Micro -battery
External supplies ASIC
Thermogenerator
RF power
DC/DC converter
Discharge monitor
RFconverter
Passive asynchronous sigma-delta
Charge Pump(0/4V)
Ring oscillator
Bandgap
Erroramplifier
Boost LCPower filter
Thermo -generator
Load
15 MHz
570mV
α * Vout
Vout
0/1V
1V
0/4V
1.3m
m
2.4mm (pad limited)
<<<<10pA60pAHold140pA68pAStandby current consumption1.87µA2.2µAActive current consumption (act time = act period/212)596pA605pASampled comparison (bandgap, adaptation and comparator)50pA20pASequencing signal shaping10pA40pAFrequency 212 divider<10pA40pASchmitt trigger400pA450pAOscillator
1.05nACurrent reference1.52nA1.60nASequencing signal generation
2.17nA2.2nAMicrobattery discharge monitor30µA23µACharge pump and MOS power switch
26.9µA10.8µAClock generator and IO pads1.1µA1.1µAAsynchronous passive ∆Σ5µA5µAError amplifier7µA6.1µABandgap reference
70µA46µADC/DC converter
TestSimulation
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Vout (V)
Iout
max
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