Prototyping of an All-Pmos-Based Cross-Coupled Voltage Multiplier in Single-Well CMOS Technology for Energy Harvesting Utilizing a Gastric Acid Battery

electronics

Article Prototyping of an All-pMOS-Based Cross-Coupled Voltage Multiplier in Single-Well CMOS Technology for Energy Harvesting Utilizing a Gastric Acid Battery

Shinya Yoshida 1,* , Hiroshi Miyaguchi 2 and Tsutomu Nakamura 3

1 Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan 2 Micro System Integration Center, Tohoku University, Sendai 980-8579, Japan 3 Promotion Oﬃce of Strategic Innovation, Tohoku University, Sendai 980-8579, Japan * Correspondence: [email protected]; Tel.: +81-22-752-2192

Received: 1 June 2019; Accepted: 13 July 2019; Published: 18 July 2019

Abstract: A gastric acid battery and its charge storage in a capacitor are a simple and safe method to provide a power source to an ingestible device. For that method, the electromotive force of the battery should be boosted for storing a large amount of energy. In this study, we have proposed an all-p-channel metal-oxide semiconductor (pMOS)-based cross-coupled voltage multiplier (CCVM) utilizing single-well CMOS technology to achieve a voltage boosting higher than from a conventional complementary MOS (CMOS) CCVM. We prototyped a custom integrated circuit (IC) implemented with the above CCVMs and a ring oscillator as a clock source. The characterization experiment demonstrated that our proposed pMOS-based CCVM can boost the input voltage higher because it avoids the body eﬀect problem resulting from an n-channel MOS transistor. This circuit was also demonstrated to signiﬁcantly reduce the circuit area on the IC, which is advantageous as it reduces the chip size or provides an area for other functional circuits. This simple circuit structure based on mature and low-cost technologies matches well with disposal applications such as an ingestible device. We believe that this pMOS-based CCVM has the potential to become a useful energy harvesting circuit for ingestible devices.

Keywords: ingestible device; gastric acid battery; cross-coupled voltage multiplier; charge pump; body eﬀect

1. Introduction An ingestible device has been developed as an ideal device for diagnosis and healthcare for use in the near future [1–3]. Various ingestible devices implemented with temperature [4,5], pH [6], and gas [7] sensors have been released or developed thus far. However, most devices generally use a button battery as the power source. This has a significant risk of accidentally injuring the inside of the body, and thus, has been one of the barriers for such ingestible devices to be widely supplied for consumer applications. Its lifetime and expiry are also inconvenient factors. Recently, a wireless power supply based on induction from outside of the body was proposed [8]. However, the ingestible device remains at a deep position in the body and moves along the gastrointestinal tract. Therefore, it is difficult to efficiently supply power to the device. The exposure dose amount of the body might be increased if the fed power is increased for forcibly providing a rapid energy charge. Recently, a gastric acid battery using galvanic couples has received considerable attention as a safe power source for ingestible devices. However, a drawback is that the power is basically generated only in the stomach while the battery electrode survives. The generated power decreases significantly in the intestinal fluids even if galvanic electrodes operable in the intestine are employed [9]. Moreover, the

Electronics 2019, 8, 804; doi:10.3390/electronics8070804 www.mdpi.com/journal/electronics Electronics 2018, 7, x FOR PEER REVIEW 2 of 11 Electronics 2019, 8, 804 2 of 11 generated only in the stomach while the battery electrode survives. The generated power decreases significantly in the intestinal fluids even if galvanic electrodes operable in the intestine are employed [9].generated Moreover, power the might generated become power unstable might in the become intestine owingunstable to thein contaminationthe intestine ofowing theelectrode to the contaminationsurface by stool. of Thisthe electrode feature of surface the gastric by stool. acid battery This feature significantly of the gastric limits its acid applications, battery significantly although it limitsis apparently its applications, suitable although for medication it is apparently management suitable devices for whichmedication are intended management to operate devices only which in the arestomach intended [10 ,to11 ].operate only in the stomach [10,11]. Therefore,Therefore, we we have have proposed proposed to store to store the thegenerat generateded energy energy in a storage in a storage medium medium such as such a multi- as a layermulti-layer ceramic ceramic capacitor capacitor (MLCC) (MLCC) for device for operation device operation even in the even intestines, in the intestines, as shown asin shownFigure in1 [12,13].Figure 1First,[ 12, 13in]. the First, energy in the harvesting energy harvesting domain, the domain, power the generated power generated by the gastric by the acid gastric battery acid operatesbattery operatesan oscillator an oscillator and a voltage-boosting and a voltage-boosting circuit. circuit.Then, the Then, generated the generated energy energy is stored is stored in the in capacitorthe capacitor at a atboosted a boosted voltage voltage permitted permitted by bythe the hardware hardware specifications specifications while while the the device device remains remains in in thethe stomach. stomach. In In the the sensing, sensing, processing, processing, and and commu communicationnication domain, domain, the the stored stored energy energy is is distributed distributed toto other other components components such such as as the the sensor, sensor, processing processing circuit, circuit, and and driver driver circuit circuit for for telecommunication. telecommunication. ThisThis method allowsallows devicedevice operation operation until until the the voltage voltage of the of storage the storage capacitor capacitor falls below falls thebelow operable the operablevoltage ofvoltage the electrical of the electrical components. components. Following Foll thisowing principle, this principle, and unlike and a unlike typical a battery, typical abattery, voltage aregulator, voltage regulator, which is associatedwhich is associated with power with loss, powe is notr loss, needed. is not Aneeded. short timeA short charge time is charge also expected. is also expected.Using this Using method, this we method, tune the we number tune the of number voltage-boosting of voltage-boosting stages to store stages greater to store energy greater in the energy MLCC, inbut the not MLCC, to exceed but not the to hardware exceed the specifications. hardware specifications.

Figure 1. Schematic of an ingestible sensor system based on a stored charge as a power source utilizing Figure 1. Schematic of an ingestible sensor system based on a stored charge as a power source utilizing a gastric acid battery via a voltage-boosting circuit. a gastric acid battery via a voltage-boosting circuit. In our previous study, a conventional complementary metal-oxide semiconductor (CMOS)-based In our previous study, a conventional complementary metal-oxide semiconductor (CMOS)- cross-coupled voltage multiplier (CCVM) was used for the voltage boosting of a gastric acid based cross-coupled voltage multiplier (CCVM) was used for the voltage boosting of a gastric acid battery [12,13]. Figure2a shows one stage of the CCVM. Vgen corresponds to the output voltage battery [12,13]. Figure 2a shows one stage of the CCVM. Vgen corresponds to the output voltage generated by the gastric acid battery. CLK_1 and CLK_2 are two-phase clocks and have amplitudes of generated by the gastric acid battery. CLK_1 and CLK_2 are two-phase clocks and have amplitudes Vgen. These clocks are symmetrically connected to pumping capacitors and pump Vgen up alternatively. of Vgen. These clocks are symmetrically connected to pumping capacitors and pump Vgen up We prototyped a four-stage cascaded CCVM and successfully demonstrated that it boosted the output alternatively. We prototyped a four-stage cascaded CCVM and successfully demonstrated that it of the gastric acid battery. boosted the output of the gastric acid battery. However, for the above-mentioned CCVM, the boosting performance gradually decreases with However, for the above-mentioned CCVM, the boosting performance gradually decreases with the number of stages [14,15]. This is caused by the body effect of the n-channel MOS (nMOS) transistor. the number of stages [14,15]. This is caused by the body effect of the n-channel MOS (nMOS) One approach to compensate the degradation in the boosting performance is to increase the transistor transistor. One approach to compensate the degradation in the boosting performance is to increase size or parallel number in the latter stages. However, this approach increases the occupied area of the the transistor size or parallel number in the latter stages. However, this approach increases the CCVM in an integrated circuit (IC), which will limit the usable areas for other circuits or enlarge the occupied area of the CCVM in an integrated circuit (IC), which will limit the usable areas for other chip size. Another method is to use a special process such as a double-well structure or a silicon on circuits or enlarge the chip size. Another method is to use a special process such as a double-well insulator (SOI) process for separating the p-well in each stage. However, the fabrication process cost structure or a silicon on insulator (SOI) process for separating the p-well in each stage. However, the increases significantly. An IC cost should be inexpensive for such a disposal device. fabrication process cost increases significantly. An IC cost should be inexpensive for such a disposal Consequently, p-channel MOS (pMOS)-based charge pumps were proposed for eliminating this device. body effect problem of an nMOS transistor. For example, a charge pump with four-phase clocks is Consequently, p-channel MOS (pMOS)-based charge pumps were proposed for eliminating this reported to exhibit a highly efficient voltage boosting [16–18]. However, such a multiple-clock-based body effect problem of an nMOS transistor. For example, a charge pump with four-phase clocks is charge pump requires a circuit with a complex design. By contrast, charge pumps utilizing twin- or reported to exhibit a highly efficient voltage boosting [16–18]. However, such a multiple-clock-based triple-well CMOS technologies have been reported [19–21]. However, these multiple-well technologies charge pump requires a circuit with a complex design. By contrast, charge pumps utilizing twin- or

Electronics 2018, 7, x FOR PEER REVIEW 3 of 11 Electronics 2019, 8, 804 3 of 11 triple-well CMOS technologies have been reported [19–21]. However, these multiple-well aretechnologies generally moreare generally expensive more than expensive a single-well than CMOS a single-well technology. CMOS A lowtechnology. cost is more A low important cost is more than eimportantﬃciency for than a disposal efficiency ingestible for a disposal device. ingestible device.

FigureFigure 2.2. SchematicSchematic ofof ((aa)) aa conventionalconventional CMOS-basedCMOS-based CCVMCCVM andand ((bb)) anan all-p-channelall-p-channel metal-oxidemetal-oxide semiconductorsemiconductor (pMOS)-based(pMOS)-based CCVM.CCVM.

Therefore,Therefore, wewe have have proposed proposed a pMOS-baseda pMOS-based CCVM CCVM operated operated by two-phaseby two-phase clocks clocks in a in single-well a single- CMOSwell CMOS technology, technology, which iswhich assumed is assumed in this study in this to bestudy utilized to be for utilized an ingestible for an device ingestible application. device Weapplication. prototyped We this prototyped pMOS-based this CCVMpMOS-based and a conventional CCVM and a one conventional in the same one IC diein the and same compared IC die their and voltage-boostingcompared their voltage-boosting performances. A performances. two-phase-clock A two-phas generatore-clock based generator on an oscillating based on circuit an oscillating was also fabricatedcircuit was and also characterized. fabricated and Finally, characterized. we demonstrated Finally, we the demonstrated operation of these the oper circuitsation and of these the boosting circuits ofand the the electromotive boosting of forcethe electromotive of a pair of Mg force and of Pt a electrodespair of Mg acting and Pt as electrodes a gastric acid acting battery as a gastric by dipping acid thembattery in by an dipping artiﬁcial them gastric in juice.an artificial gastric juice.

2.2. Fabrication ofof thethe CCVMCCVM CircuitsCircuits inin aa CustomCustom ICIC Figure2b shows the schematic of a single stage of the pMOS-based CCVM. Here, M and M as Figure 2b shows the schematic of a single stage of the pMOS-based CCVM. Here, M11 and M22 as thethe pMOSpMOS transistorstransistors areare focusedfocused onon toto easilyeasily explainexplain thethe boostingboosting procedure.procedure. DuringDuring thethe ﬁrstfirst halfhalf cycle, CLK_1 changes from 0 to Vgen and CLK_2 changes from Vgen to 0, and then, M and M turn cycle, CLK_1 changes from 0 to Vgen and CLK_2 changes from Vgen to 0, and then, M1 and1 M2 turn2 OFF OFF and ON, respectively. Thus, the coupling capacitor, C , is charged by Vgen via the M and N and ON, respectively. Thus, the coupling capacitor, C2, is charged2 by Vgen via the M2 and N2 2 nodes.2 nodes. Next, in the second half cycle, when CLK_1 and CLK_2 respectively change to 0 and Vgen, M Next, in the second half cycle, when CLK_1 and CLK_2 respectively change to 0 and Vgen, M1 and M12 and M turn ON and OFF, respectively. At this instant, node N is ideally boosted to 2 Vgen by the turn ON2 and OFF, respectively. At this instant, node N2 is ideally2 boosted to 2 × Vgen by× the coupling coupling of C . Therefore, the storage capacitor is charged at the boosted voltage. A similar circuit of C2. Therefore,2 the storage capacitor is charged at the boosted voltage. A similar circuit composed composed of M , M , and N nodes is fabricated and driven by the same CLK_1 and CLK_2 but in a of M3, M4, and 3N1 nodes4 is fabricated1 and driven by the same CLK_1 and CLK_2 but in a reversed reversedphase for phase dual-phase for dual-phase charge pumping. charge pumping. The voltage The can voltage be boosted can be up boosted further up by furthercascading by the cascading stages. theThe stages. ideal output The ideal voltage output of a voltage circuit ofwith a circuit n stages with is nshownstages as is follows: shownas follows:

Vout =VoutV gen= V+genn + n V× genVgen.. (1) (1) × In practice, Vout is decreased by certain factors such as the parasitic stray capacitance at each In practice, Vout is decreased by certain factors such as the parasitic stray capacitance at each pumping node, undesired leakage current, time constant, and threshold voltage of the transistors. pumping node, undesired leakage current, time constant, and threshold voltage of the transistors. Figure 3a shows the circuit schematic of the pMOS-based CCVM as designed and prototyped in Figure3a shows the circuit schematic of the pMOS-based CCVM as designed and prototyped this study. In this figure, the expression “P: W/L = 20/0.6 × 10” implies that 10 pMOS transistors with in this study. In this ﬁgure, the expression “P: W/L = 20/0.6 10” implies that 10 pMOS transistors a gate width and length of 20 and 0.6 μm, respectively, are connected× in parallel. The N-wells of the with a gate width and length of 20 and 0.6 µm, respectively, are connected in parallel. The N-wells of transistors, which are surrounded by the dashed blue lines, are separated for applying a different the transistors, which are surrounded by the dashed blue lines, are separated for applying a diﬀerent potential to each transistor. MLCCs of 0.1 μF (C11, C12, C13, C14, C21, C22, C23, C24) are used as the coupling potential to each transistor. MLCCs of 0.1 µF(C11, C12, C13, C14, C21, C22, C23, C24) are used as the capacitors. The charge is stored in the intermediate storage capacitors (Cs1, Cs2, Cs3) of 0.1 μF MLCC coupling capacitors. The charge is stored in the intermediate storage capacitors (Cs1, Cs2, Cs3) of 0.1 µF at the intermediate boosted voltage at each stage and eventually in the final storage capacitor (Cs4).

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Electronics 2018, 7, x FOR PEER REVIEW 4 of 11 MLCC at the intermediate boosted voltage at each stage and eventually in the final storage capacitor A 22 μF electrolytic capacitor is used as the final storage capacitor (Cs4). The transition of the boosting (Cs ). A 22 µF electrolytic capacitor is used as the final storage capacitor (Cs ). The transition of the at 4each stage was observed by monitoring the voltage at the intermediate4 and the final storage boosting at each stage was observed by monitoring the voltage at the intermediate and the final storage capacitors (Cs1, Cs2, Cs3, Cs4), which is described using a symbol of voltage meter V1–V4 in this figure. capacitors (Cs1, Cs2, Cs3, Cs4), which is described using a symbol of voltage meter V1–V4 in this figure.

(a)

(b)

FigureFigure 3. 3.Schematic Schematic ofof thethe ((aa)) all-pMOS-basedall-pMOS-based CCVM and ( (bb)) conventional conventional CMOS-based CMOS-based CCVM CCVM with with fourfour stages. stages. “P: “P: W W/L/L = =20 20/0.6/0.6 ×10” 10” implies implies that that 10 10 pMOS pMOS transistors transistors with with a a gate gate width width and and length length of of 20 × and20 0.6andµ m,0.6 respectively,μm, respectively, are connected are connected in parallel. in parallel. The N-wells The ofN-wells the transistor, of the transistor, which are surroundedwhich are bysurrounded dashed blue by lines,dashed are blue separated. lines, are separated.

AA conventional conventional CCVMCCVM circuitcircuit waswas alsoalso fabricated for comparison of of the the boosting boosting performance, performance, asas shown shown in in Figure Figure3 b.3b. The The number number of of nMOS nMOS transistorstransistors waswas graduallygradually increasedincreased with the increase inin the the stages stages to to suppress suppress thethe degradationdegradation ofof thethe boostingboosting performance due due to to the the body body effect. effect. In In this this study,study, the the number number of of transistors transistors in in the the pMOS-based pMOS-based CCVM CCVM in in each each stage stage was was kept kept the the same same because because of noof bodyno body effect effect problem problem inherently inherently found found in thisin this circuit. circuit. AA ring ring oscillator oscillator circuit circuit was was also also implemented implemented in the in ICthe for IC generating for generating two-phase two-phase clocks toclocks operate to theseoperate CCVMs, these asCCVMs, shown as in shown Figure 4in. TheFigure output 4. The port output of the port ring of oscillator, the ring oscillator,OSC out, andOSC the out input, and portthe ofinput the buportffer of circuit, the bufferBuf incircuit,, are separatedBuf in, are forseparated characterizing for characterizing these circuits these independently. circuits independently. These are connectedThese are connected in the practical in the usepractical for generating use for generating the two-phase the two-phase clocks, clocks,CLK_1 CLK_1and CLK_2 and CLK_2. The. The ring oscillatorring oscillator is operated is operated by the by power the power generated generat fromed afrom gastric a gastric acid battery acid battery and then and drives then drives the charge the charge pumping process. Its duty ratio and period are tuned by tuning the R and C values. Figure 5

Electronics 2018, 7, x FOR PEER REVIEW 5 of 11 Electronics 2019, 8, 804 5 of 11 Electronicsshows the 2018 photograph, 7, x FOR PEER of REVIEW the prototyped IC chip implemented with these circuits. It is clearly5 ofseen 11 thatElectronics the occupied 2018, 7, x FOR area PEER of the REVIEW pMOS-based CCVM is half of the conventional CMOS-based CCVM.5 of 11 pumpingshows the process. photograph Its duty of the ratio prototyped and period IC are chip tuned impl byemented tuning with the R theseand C circuits.values. It Figure is clearly5 shows seen shows the photograph of the prototyped IC chip implemented with these circuits. It is clearly seen thethat photograph the occupied of area the prototyped of the pMOS-based IC chip implemented CCVM is half with of the these conventional circuits. It CMOS-based is clearly seen CCVM. that the occupiedthat the areaoccupied of the area pMOS-based of the pMOS-based CCVM is halfCCVM of the is half conventional of the conventional CMOS-based CMOS-based CCVM. CCVM.

Figure 4. Schematic of the generation circuit of two-phase clocks using a ring oscillator circuit.

Figure 4. Schematic of the generation circuit of two-phase clocks using a ring oscillator circuit. FigureFigure 4. 4.Schematic Schematic of of the the generation generation circuit circuit of of two-phase two-phase clocks clocks using using a ringa ring oscillator oscillator circuit. circuit.

Figure 5. Layout schematic of the prototyped custom IC (0.6 μm/5 V CMOS process) for the energy harvesting of the gastric acid power generation. Figure 5. Layout schematic of the prototyped custom IC (0.6 µm/5 V CMOS process) for the energy Figure 5. Layout schematic of the prototyped custom IC (0.6 μm/5 V CMOS process) for the energy harvestingFigure 5. of Layout the gastric schematic acid power of the generation. prototyped custom IC (0.6 μm/5 V CMOS process) for the energy 3. Resultsharvesting and of Discussion the gastric acid power generation. harvesting of the gastric acid power generation. 3. Results and Discussion 3.3.1. Results Evaluation and Discussionof the Voltage-B oosting Performance of the CCVMs 3.1.3. Results Evaluation and of Discussion the Voltage-Boosting Performance of the CCVMs Figure 6 shows the evaluation setup of the voltage-boosting performances of these CCVMs. 3.1. Evaluation of the Voltage-Boosting Performance of the CCVMs Here,3.1.Figure Evaluation to characterize6 shows of the the Voltage-Bthe evaluation CCVMsoosting setup independently Performance of the voltage-boosting fromof the th CCVMse characteristics performances of of the these ring CCVMs. oscillator Here, circuit, to characterizesquareFigure wave the6 showspatterns CCVMs the independentlywith evaluation 50% duty setup from ratio theof signals the characteristics vo ltage-boostingwere supplied of the ring performafrom oscillator a patternnces circuit, of generator these square CCVMs. waveto the Figure 6 shows the evaluation setup of the voltage-boosting performances of these CCVMs. patternsHere,buffer to circuit with characterize 50% on dutythe theIC. ratio CCVMsIts signals high-level independently were voltage supplied was fromfrom set ath to patterne thecharacteristics same generator with toVofgen thethe as buffer ring an input oscillator circuit voltage. on circuit, the IC.We Here, to characterize the CCVMs independently from the characteristics of the ring oscillator circuit, Itssquarealso high-level simulated wave voltage patterns the operation was with set to50% thebeha duty sameviors ratio with of the Vsignals CCVMsas anwere input using supplied voltage. LTspice from We® (version also a pattern simulated IV, generatorAnalog the operation Devices, to the square wave patterns with 50% duty ratio gensignals were supplied from a pattern generator to the behaviorsbufferInc., Norwood, circuit of the on CCVMs MA,the IC. USA). usingIts high-level LTspice® voltage(version was IV, set Analog to the Devices, same with Inc., V Norwood,gen as an input MA, USA).voltage. We buffer circuit on the IC. Its high-level voltage was set to the same with Vgen as an input voltage. We also simulated the operation behaviors of the CCVMs using LTspice® (version IV, Analog Devices, also simulated the operation behaviors of the CCVMs using LTspice® (version IV, Analog Devices, Inc., Norwood, MA, USA). Inc., Norwood, MA, USA).

Figure 6. Setup schematic of the characterization of the prototyped CCVMs. Figure 6. Setup schematic of the characterization of the prototyped CCVMs.

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FigureFigure7 plots7 plots the the typical typical simulation simulation result result of of the the voltage-boosting voltage-boosting behavior behavior for for each each CCVMCCVM andand dependency on the clock frequency when V was set as 1.3 V. V of the conventional CMOS-based dependency on the clock frequency when Vgengen was set as 1.3 V. V11 of the conventional CMOS-based CCVM is always higher than those of the pMOS-based CCVMs. The boosting performance from V CCVM is always higher than those of the pMOS-based CCVMs. The boosting performance from V1 to V is also relatively high. However, the boosting performance worsens gradually as the stages to 2V2 is also relatively high. However, the boosting performance worsens gradually as the stages increaseincrease owing owing to to the the body body e ffeffectect of of the the nMOS nMOS transistors transistorseven even whenwhen thethe numbernumber of nMOS transistors isis intentionally intentionally increased increased in in the the latter latter stages. stages. The The final final boosted boosted voltagevoltage i.e.,i.e., VV44 inin thethe conventionalconventional CCVMCCVM is is less less than than 4 4 V. V. By By contrast, contrast, the the ratio ratio of of the the boosting boosting voltagevoltage atat eacheach stagestage isis nearlynearly constant andand approximately approximately 1.0 1.0 V V in in the the pMOS-based pMOS-based CCVMs. CCVMs. Thus, Thus, the the final final boosted boosted voltage, voltage,V 4V, reaches4, reaches 5 V,5 whichV, which is typically is typically higher higher than than that that of theof the conventional conventional CCVM CCVM for for any any clock clock frequency.frequency. In addition, thethe boosting boosting time time shortens shortens as as the the clock clock frequency frequency increases. increases.

FigureFigure 7. 7.Typical Typical simulation simulation result result of of the the voltage-boosting voltage-boosting behaviorbehavior forfor eacheach CCVMCCVM and dependency

onon the the clock clock frequency frequency when whenV Vgengenis is set set as as 1.3 1.3 V. V.V V11to toV V44 correspondcorrespond to the measured voltage of the intermediateintermediate or or ﬁnal final storage storage capacitors capacitors at at each each stage, stage, as as shown shown in in Figure Figure3 .3.

FigureFigure8 plots8 plots the the measurement measurement result of of the the vo voltageltage transition transition for for the the prototyped prototyped circuits. circuits. The Thetendencies tendencies are are practically practically the the same same as as those those of of the simulatedsimulated result.result. The voltage-boostingvoltage-boosting performanceperformance is is apparentlyapparently slightly slightly lower lower than than that that obtained obtained from from the simulation the simulation result for result the pMOS- for the pMOS-basedbased CCVM. CCVM. Thus, Thus, V4 isV also4 is alsolower lower than than the thesimulation simulation result result value. value. This This may may be be owing owing to to the the mismatchmismatch between between the the actual actual and and assumed assumed characteristics characteristics of of the thetransistors. transistors. FigureFigure9 plots 9 plots the the dependence dependence of of the the reached reached voltage voltage of ofV V44 at 40 s after the the boosting boosting process process starts, starts, whichwhich is symbolizedis symbolized as Vas4_40s V4_40s, on, onV genVgen(0.85, (0.85, 1.0, 1.0, or or 1.3 1.3 V) V) and and the the clock clock frequency frequency (100 (100 Hz, 300Hz, Hz,300 1Hz, kHz, 1 3 kHz, or3 kHz, 10 kHz). or 10V kHz).4_40s increases V4_40s increases as Vgen asincreases Vgen increases for both for the both CCVMs. the CCVMs. On the On other the hand, other ithand, is almost it is independentalmost independent on the frequency on the frequency in both thein both simulation the simulation and measurement. and measurement. The pMOS-based The pMOS-based CCVM typicallyCCVM typically achieves aachieves higher voltagea higher boosting voltage boosting under any under condition any condition compare compare to the conventional to the conventional CCVM. CCVM.

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Figure 8. Measurement result of the transition of the boosted voltage at each capacitor in the FigureFigure 8. 8. MeasurementMeasurement resultresult ofof thethe transitiontransition ofof thethe boostedboosted voltage at each capacitor in the prototyped circuits. prototypedprototyped circuits. circuits.

Figure 9. Dependence of the reached voltage (V4_40s) at the fourth stage (V4) on Vgen (0.85, 1.0, or FigureFigure 9. 9. Dependence Dependence of of the the reached reached voltagevoltage ((VV4_40s4_40s)) atat the fourth stage ((V44)) onon VVgengen (0.85,(0.85, 1.0,1.0, or or 1.3 1.3 V) V) 1.3 V) and clock frequency (100 Hz, 300 Hz, 1 kHz, 3 kHz, or 10 kHz). V4_40s is deﬁned as the V4 value andand clock clock frequency frequency (100 (100 Hz, Hz, 300 300 Hz, Hz, 11 kHz,kHz, 33 kHz,kHz, or 10 kHz). V4_40s isis defineddefined asas thethe VV44 value value at at 40 40 s s at 40 s after the boosting process starts. afterafter the the boosting boosting process process starts. starts.

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TheThe dependencedependence ofof thethe boostingboosting quicknessquickness ononV Vgengen and the frequency waswas alsoalso investigated,investigated, asas plottedplotted inin Figure Figure10 10.. The The quickness quickness is definedis defined as theas the required required time time until untilV reachesV4 reaches 63.2% 63.2%(i.e., (i.e., 1 11 /−e) 1/e)of 4 − Vof4_40s V4_40s. When. WhenV Vgengen isis 0.850.85 VV forfor thethe measurementmeasurement results,results, thethe requiredrequired timetime isis significantlysignificantly longerlonger comparedcompared toto thatthat underunder otherother conditions.conditions. This voltage is near the threshold voltage of the transistors. ThisThis lowlowV genVgenprobably probably induced induced an unperfectan unperfect switching switching of transistors of transistors and thus and resulted thus inresulted an ineff ectivein an boostingineffective process boosting owing process to a owing large leakage to a large current. leakage For current. both the For CCVMs, both the the CCVMs, times are the practically times are saturatedpractically when saturated the frequency when the is morefrequency than 1 is kHz. more This th impliesan 1 kHz. that This the amountimplies ofthat the the carried amount charge of perthe unitcarried of time charge is nearly per unit constant of time and is nearly depends constant on the and conductance depends ofon the the transistors conductance when of the the transistors frequency iswhen suffi cientlythe frequency high. The is sufficiently quickness of high. the pMOS-basedThe quickness CCVM of the appears pMOS-based to be slower CCVM than appears that ofto the be conventionalslower than that CMOS-based of the conventional CCVM with CMOS-based the present circuitCCVM constructions. with the present In particular, circuit constructions. this is obvious In atparticular,Vgen = 0.85 this V. is The obvious quickness at Vgen is = improved 0.85 V. The by quickness optimizing is the improved circuit, suchby optimizing as in terms the of circuit, the number such ofas pMOSin terms transistors of the number in each of stage. pMOS Considering transistors in that ea thech stage. ionization Considering tendency that of Mgthe isionization nearly 1.6tendency V and − theof Mg previous is nearly characterization −1.6 V and the result previous of a Mg–Ptcharacterization battery [13 result], we canof a operateMg–Pt battery the CCVMs [13], atwe a can suffi operateciently highthe CCVMs voltage ofat morea sufficiently than 1 V. high The energyvoltagee offfi cienciesmore than ofthese 1 V. The CCVMs energy were efficiencies estimated of from these simulation CCVMs towere be approximatelyestimated from 15–20% simulation at 1.0–1.3 to be Vapproximately as Vgen. 15–20% at 1.0–1.3 V as Vgen.

FigureFigure 10.10. Dependence ofof thethe boostingboosting quicknessquickness ononV Vgengen and the frequency. The quickness isis deﬁneddefined

asas thethe requiredrequired timetime untiluntil 63.2%63.2% ofofV V4_40s4_40s isis reached. reached. 3.2. Characterization of the Ring Oscillator Circuit 3.2. Characterization of the Ring Oscillator Circuit Next, the dependency of the characteristics of the ring oscillator circuit on the RC values was Next, the dependency of the characteristics of the ring oscillator circuit on the RC values was investigated, as shown in Figure 11. It is clearly seen that the frequency and duty ratio decreased as the investigated, as shown in Figure 11. It is clearly seen that the frequency and duty ratio decreased as supply voltage became lower. Finally, when Vgen is below 0.6 V, the oscillation stops. In accordance the supply voltage became lower. Finally, when Vgen is below 0.6 V, the oscillation stops. In with the characteristics, the RC combination of 1.2 kΩ/1 nF yields the highest frequency. As mentioned accordance with the characteristics, the RC combination of 1.2 kΩ/1 nF yields the highest frequency. above, this is preferable for rapid boosting. However, the duty ratio is relatively smaller than other RC As mentioned above, this is preferable for rapid boosting. However, the duty ratio is relatively combinations and will not reach 50% with Vgen generated by the Mg–Pt battery. Ideally, it should be smaller than other RC combinations and will not reach 50% with Vgen generated by the Mg–Pt battery. approximately 50% for an eﬀective boosting process. In addition, unlike a general button battery, the Ideally, it should be approximately 50% for an effective boosting process. In addition, unlike a general output voltage of the gastric acid battery in an actual stomach will vary within a certain level because button battery, the output voltage of the gastric acid battery in an actual stomach will vary within a the acid condition will be unstable. Thus, robust parameters for the duty ratio of nearly 50% are certain level because the acid condition will be unstable. Thus, robust parameters for the duty ratio preferable for actual use. Therefore, the combination of 120 kΩ/10 nF appears to be best in this study. of nearly 50% are preferable for actual use. Therefore, the combination of 120 kΩ/10 nF appears to be best in this study.

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(a)(a) (b)(b)

FigureFigureFigure 11.11. 11. DependencyDependency Dependency ofof of ((a a()a) the)the the frequencyfrequency frequency andand and ( b( b()b) duty )duty duty ratio ratio ratio of of of the the the ring ring ring oscillator oscillator oscillator circuit circuit circuit on on on the the theV V genVgengen andandand RCRC RC parameters.parameters. parameters. 3.3. Demonstration of the Voltage Boosting with the Mg–Pt Gastric Acid Battery and CCVMs 3.3.3.3. Demonstration Demonstration of of the the Voltage Voltage Boosting Boosting wi withth the the Mg–Pt Mg–Pt Gastric Gastric Acid Acid Battery Battery and and CCVMs CCVMs Actual voltage boosting was performed with an artiﬁcial gastric acid juice and the CCVMs. A pair ActualActual voltage voltage boosting boosting was was performed performed with with an an artificial artificial gastric gastric acid acid juice juice and and the the CCVMs. CCVMs. A A of Mg–Pt electrodes with diameters of 2 mm was formed by masking these electrode plates with a pairpair of of Mg–Pt Mg–Pt electrodes electrodes with with diameters diameters of of 2 2 mm mm was was formed formed by by masking masking these these electrode electrode plates plates with with water-resistant masking tape, as shown in Figure 12. These electrodes were connected with the energy aa water-resistantwater-resistant maskingmasking tape,tape, asas shownshown inin FiguFigurere 12.12. TheseThese electrodeselectrodes werewere connectedconnected withwith thethe harvesting domain of the system, as shown in Figure1. In this experiment, the same capacitors were energyenergy harvesting harvesting domain domain of of the the system, system, as as shown shown in in Figure Figure 1. 1. In In this this experi experiment,ment, the the same same capacitors capacitors used for the CCVM as shown in Figure3. Then, the electrodes were dipped in an artiﬁcial gastric juice werewere used used for for the the CCVM CCVM as as shown shown in in Figure Figure 3. 3. Then, Then, the the electrodes electrodes were were dipped dipped in in an an artificial artificial gastric gastric (pH: 1.4). The resultant generated power was supplied to the CCVMs and ring oscillator circuit. juicejuice≈ (pH: (pH: ≈ ≈1.4).1.4). The The resultant resultant generated generated power power was was supplied supplied to to the the CCVMs CCVMs and and ring ring oscillator oscillator circuit. circuit.

FigureFigureFigure 12. 12.12. AA pairpair ofof Mg–PtMg–Pt electrodeselectrodes withwith diametersdiameters ofof 22 mmmm asas aaa gastric gastricgastric acid acidacid battery batterybattery for forfor the thethe demonstrationdemonstrationdemonstration ofof of energyenergy energy harvestingha harvestingrvesting viavia via thethe the CCVMs.CCVMs. CCVMs. In our previous experiment [13], the output power of a pair of Mg–Pt thin ﬁlms with 6 mm2 InIn ourour previousprevious experiment experiment [13],[13], thethe outputoutput popowerwer ofof a a pairpair ofof Mg–Pt Mg–Pt thinthin films films withwith 66 mmmm2 2 squares measured 500–1000 µW in a gastric acid at the output voltage of more than 1 V. Thus, the squaressquares measured measured 500–1000 500–1000 μ μWW in in a a gastric gastric acid acid at at the the output output voltage voltage of of more more than than 1 1 V. V. Thus, Thus, the the output power of the electrode in this study was estimated to be 250–500 µW. Figure 13 shows the outputoutput power power of of the the electrode electrode in in this this study study was was estimated estimated to to be be 250–500 250–500 μ μW.W. Figure Figure 13 13 shows shows the the boosted voltage transitions. The pMOS-based CCVM achieves a high voltage charge at 5.3 V within boostedboosted voltage voltage transitions. transitions. The The pMOS-based pMOS-based CCVM CCVM achieves achieves a a high high voltage voltage charge charge at at 5.3 5.3 V V within within 15 s. The frequency and duty ratio of CLK are approximately 70 kHz and 60%, respectively. By contrast, 1515 s.s. TheThe frequencyfrequency andand dutyduty ratioratio ofof CLKCLK areare approximatelyapproximately 7070 kHzkHz andand 60%,60%, respectively.respectively. ByBy the conventional CMOS-based CCVM provides a lower voltage of 4.8 V in the same manner as for the contrast,contrast, the the conventional conventional CMOS-based CMOS-based CCVM CCVM provi providesdes a a lower lower voltage voltage of of 4.8 4.8 V V in in the the same same manner manner above-described results. asas for for the the above-described above-described results. results.

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V V FigureFigure 13. Voltage-boostingVoltage-boosting behavior behavior at at 1V– 1–4Vfor4 for each each CCVM CCVM in the in Mg–Ptthe Mg–Pt gastric gastric acid battery acid battery dipped dippedin an artiﬁcial in an artificial gastric acidgastric juice. acid juice.

Therefore, we successfully demonstrated that the pMOS-based CCVM based on single-well Therefore, we successfully demonstrated that the pMOS-based CCVM based on single-well CMOS technology with two-phase clocks can provide a higher voltage and also a smaller circuit CMOS technology with two-phase clocks can provide a higher voltage and also a smaller circuit occupied area compared to the conventional CMOS-based CCVM. The saved area on the IC can be occupied area compared to the conventional CMOS-based CCVM. The saved area on the IC can be implemented with other functional circuits or contribute to miniaturizing the chip size. This CCVM implemented with other functional circuits or contribute to miniaturizing the chip size. This CCVM balances well the boosting performance and cost and, thus, will be usable in energy harvesting based balances well the boosting performance and cost and, thus, will be usable in energy harvesting based onon a a gastric gastric acid acid battery. battery. In our previous study, wewe successfullysuccessfully demonstrateddemonstrated voltage voltage boosting boosting with with a aconventional conventional CMOS-based CMOS-based CCVM CCVM and and Mg–Pt Mg–Pt gastric gastric acid acid batterybattery inin aa dog’sdog’s stomach [[22].22]. Therefore, Therefore, thethe pMOS-based pMOS-based CCVM CCVM promises promises to to de demonstratemonstrate superior performance. 4. Conclusions 4. Conclusions In this study, we prototyped an all-pMOS-based CCVM operated by two-phase clocks with In this study, we prototyped an all-pMOS-based CCVM operated by two-phase clocks with single-well CMOS technology for an energy harvesting application based on a gastric acid battery used single-well CMOS technology for an energy harvesting application based on a gastric acid battery in an ingestible device. In the evaluation experiment, the proposed pMOS-based CCVM successfully used in an ingestible device. In the evaluation experiment, the proposed pMOS-based CCVM boosted the input voltage and achieved a higher output voltage than the conventional CMOS-based successfully boosted the input voltage and achieved a higher output voltage than the conventional CCVM under any predetermined condition, due to the elimination of the body effect of the nMOS CMOS-based CCVM under any predetermined condition, due to the elimination of the body effect transistors. In addition, this pMOS-based CCVM was demonstrated to be able to significantly save of the nMOS transistors. In addition, this pMOS-based CCVM was demonstrated to be able to the occupied footprint on an IC. This is advantageous for implementing other functional circuits on significantly save the occupied footprint on an IC. This is advantageous for implementing other the saved area or reducing the chip size. This superior boosting performance was also confirmed by functional circuits on the saved area or reducing the chip size. This superior boosting performance an experiment using a Mg–Pt gastric acid battery in an artificial gastric juice. We believe that this was also confirmed by an experiment using a Mg–Pt gastric acid battery in an artificial gastric juice. pMOS-based CCVM with a low-cost standard single-well CMOS process has tremendous potential to We believe that this pMOS-based CCVM with a low-cost standard single-well CMOS process has become a useful circuit for energy harvesting based on gastric acid power generation. tremendous potential to become a useful circuit for energy harvesting based on gastric acid power generation.Author Contributions: S.Y. designed and simulated the voltage-boosting circuits. He also measured the data and wrote the manuscript. H.M. built the setup for the characterizations. T.N. proposed the concept, designed the Authorcircuits, Contributions: contributed to theS.Y. technical designed discussions, and simulated and reviewedthe voltage-boos the manuscript.ting circuits. He also measured the data andFunding: wrote Thisthe manuscript. work was supportedH.M. built bythe JST setup COI for Grant the characterizations. Number JPMJCE1303 T.N. proposed and by the the Kato concept, Foundation designed for thePromotion circuits, ofcontributed Science. to the technical discussions, and reviewed the manuscript. Acknowledgments: This work was supported by NAITO DENSEI KOGYO CO., LTD. Funding: This work was supported by JST COI Grant Number JPMJCE1303 and by the Kato Foundation for PromotionConflicts of of Interest: Science. The authors declare no conflict of interest.

Acknowledgments:References This work was supported by NAITO DENSEI KOGYO CO., LTD.

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