Energy efficient wirelessly powered implant stimulator

Abstract

A biphasic implant stimulator includes a direct stimulation supply generator that receives an RF AC input and converts it into a DC voltage ramp for fully-adiabatic stimulation of tissue though electrodes. Current control feedback is used instead of current-sources which reduces energy loss. The current control feedback senses the current in the electrode-tissue load and keeps it constant for the duration of the stimulation. Energy stored in the electrode capacitance is replenished. No bulky inductor is required for operation.

Description

0321*157112PCT 2024­300­2(PCT) ENERGY EFFICIENT WIRELESSLY POWERED IMPLANT STIMULATOR PRIORITY CLAIM AND REFERENCE TO RELATED APPLICATION

[0001] This application claims priority under 35 U.S.C. §119, 35 U.S.C. 365(b) and all applicablestatutes and treaties from prior US Provisional Application Serial Number 63 / 659,006, which was filed on June 12, 2024. FIELD

[0002] A field of the invention concerns implant stimulators, such as neural stimulators andimplants for vagus nerve stimulation, spinal cord stimulation, bladder stimulation, deep­brain stimulation and stimulation of other nerves and organs. BACKGROUND

[0003] Neural stimulators are used to treat patients with conditions such as epilepsy andParkinson’s disease. Neural stimulator implants require serious invasive surgery. Longevity in operation without battery replacement and compact size are advantageous. Age­related conditions such as Parkinson’s and essential tremor are becoming more prevalent with longer life spans. If a battery power source fails, another invasive surgery would be required, and the risk of a second surgery increases as treated persons survive a longer time after an implant.

[0004] Other implanted stimulators treat a variety of conditions. Example conditions treatedwith implanted stimulators include several medical problems such as anxiety, obesity, sleep apnea, chronic pain, central nervous disorders like Parkinson's disease, epilepsy etc.

[0005] Wireless implantable neural stimulators relying on inductive coupling often faceefficiency challenges attributed to cascaded losses in the rectifier, regulator, and charge pump. Traditional constant current stimulators with a constant DC supply dissipate a significant portion ( ) of their stimulation output power across their current sources. B. C. Johnson et al., “An implantable 700μW 64­channel neuromodulation IC for simultaneous recording and stimulation with rapid artifact recovery,” in 2017 Symposium on VLSI Circuits, Kyoto, Japan, 2017. The latter issue has only been partially addressed by generating a dynamic supply across the current sources for semi­adiabatic stimulation. S. Ha, C. Kim, J. Park, G. Cauwenberghs, and P. P. Mercier, “A fully integrated RF­powered energy­replenishing current­controlled0321*157112PCT 2024­300­2(PCT) stimulator,” IEEE Trans. Biomed. Circuits Syst., vol.13, no.1, pp.191–202, Feb.2019. Z. Luo, M. ­ D. Ker, T. ­Y. Yang and W. ­H. Cheng, "A Digitally Dynamic Power Supply Technique for 16­ Channel 12 V­Tolerant Stimulator Realized in a 0.18­ μm 1.8­V / 3.3­V Low­Voltage CMOS Process," in IEEE Transactions on Biomedical Circuits and Systems, vol.11, no.5, pp.1087­1096, Oct.2017. W. Biederman et al., "A 4.78 mm 2 Fully­Integrated Neuromodulation SoC Combining 64 Acquisition Channels With Digital Compression and Simultaneous Dual Stimulation," in IEEE Journal of Solid­State Circuits, vol.50, no.4, pp.1038­1047, April 2015.

[0006] Other approaches eliminated the current sources to prevent loss across them. Oneapproach couldn't keep the stimulation current constant and controlled. S. K. Kelly and J. L. Wyatt, “A power­efficient neural tissue stimulator with energy recovery,” IEEE Trans. Biomed. Circuits Syst., vol.5, no.1, pp.20–29, Feb.2011. Other approaches required a bulky additional inductor that is not suitable for miniaturized implants. Y. You, R. Tian, Y. Zhang, Z. Chen, W. Lu, and Y. Zhang, “A 15.7­V­compliant 86% peak efficiency current­mode stimulator with dynamic voltage supply for implantable medical devices,” in ESSCIRC 2023­ IEEE 49th European Solid State Circuits Conference (ESSCIRC), Lisbon, Portugal, 2023, pp.321–324. S. K. Arfin and R. Sarpeshkar, “An energy­efficient, adiabatic electrode stimulator with inductive energy recycling and feedback current regulation,” IEEE Trans. Biomed. Circuits Syst., vol.6, no.1, pp.1–14, Feb. 2012. Generally, prior approaches suffer from cascading losses.

[0007] Fig. 1 (Prior Art) illustrates a model for an electrode­tissue interface. D. R. Merrill, M.Bikson, and J. G. R. Jefferys, “Electrical stimulation of excitable tissue: design of efficacious and safe protocols,” J. Neurosci. Methods, vol.141, no.2, pp.171–198, Feb.2005. The interface is modelled as a series resistor. The resistance Zfaradaicis usually quite large and can be ignored for a first order analysis. This gives the series RC model for electrode – tissue – electrode path which acts as an electrical load for the stimulator. Therefore, constant voltage stimulation leads to exponential stimulation current to a first order analysis. This makes precise control over the delivered charge complex. Additional complexity arises because the electrode­tissue impedance changes over time due to faradaic chemical reactions and passive immune response of the body to the implant. Stimulation current passing into the electrodes from the stimulator terminals is ideally constant and can be controlled by controlling either the stimulation current amplitude or the stimulation time.

[0008] When a constant current is injected into this load through terminal 1 for a time durationT while voltage at terminal 2 is maintained at VMID, a ramping voltage is obtained. Conversely,0321*157112PCT 2024­300­2(PCT) when the constant current stimulator generates the voltage waveform at terminal 1, a constant current flows into the load. The efficiency of such a constant current stimulator is determined by the losses incurred to generate the required voltage waveform from its power supply for the given RC load and stimulation current. A wireless powered stimulator is required to generate the appropriate waveform at terminal 1 from the AC input.

[0009] Figs. 2A­2C (Prior Art) illustrates a wireless powered stimulator that has been used togenerate the stimulation waveform. The Figs.2A­2C circuit uses a rectifier followed by a regulator to generate a regulated DC voltage which can be up converted or down converted to generate the stimulation supply. A current source connected to the stimulation supply ensures constant stimulation current. The minimum voltage required for the stimulation supply is dependent on the maximum voltage generated at terminal 1 of the stimulator and minimum voltage across the current source required for its proper functioning. The power train including the rectifier, regulator, and voltage converter suffers from cascaded losses. At the same time, a large portion of the energy is consumed by the current sources. If the tissue resistance is small, the energy wasted even in an ideal current source (minimum voltage required across it is 0V) is at least 50% of the total energy provided by the stimulation supply for constant current stimulation, as seen in Figs.2B and 2C.

[0010] The energy provided by the supply for stimulation, ESTIM, can be written as:

[0011] ( )

[0012] The ESTIM curve plotted against time is shown in Fig. 2B. It is a linear function of time.During cathodic stimulation the energy provided by the supply VDD can be expressed as:

[0013] ,and

[0016] The energy supplied by the VDD during cathodic stimulation is divided into energyconsumed by the current source, energy lost in RS, and energy stored on CDL. Therefore,

[0017] ,0321*157112PCT 2024­300­2(PCT)

[0021] Substituting (3) in (2) yields (1). The energy supplied by VSS during the anodic phase canbe expressed asare of anodic stimulation phase and

[0024]

[0025] The negative sign is because ISTIM is entering VSS. The energy lost in RS and consumed bythe current source is given by:

[0026]

[0028] Substituting (3) in (4):

[0029]

[0030] As seen in Figs. 2B and 2C, during the anodic phase, the energy supplied by the supplyVSSand the energy stored on CDLis lost in RSand is consumed by the current source. More specifically, none of the energy stored on CDLis replenished.

[0031] Figs. 3A­3C show another prior approach that uses a dynamic voltage supply stimulator.See, S. Ha, C. Kim, J. Park, G. Cauwenberghs, and P. P. Mercier, “A fully integrated RF­powered energy­replenishing current­controlled stimulator,” IEEE Trans. Biomed. Circuits Syst., vol.13, no.1, pp.191–202, Feb.2019. The supply is generated to track the voltage on terminal 1 with enough gap to keep the current sources in saturation. Overhead power is used to generate the dynamic stimulation supply, which limits application to applications where high stimulation currents are required (>500uA). Energy loss can be significant when stimulation currents are high. With stimulation current of 5mA, for a minimal saturation voltage of 100 mV, power loss across the current source alone will be 500uW, which can be considerably high for implantable devices. Since the dynamic stimulation supply generator requires a regulated supply, the stimulation power train suffers from cascading losses of the rectifier, regulator and charge pump.

[0032] The energy provided by the supply for stimulation, ESTIM, can be written as:

[0033] ( ),

[0034] ESTIM curve plotted against time is shown in Fig. 3B. For the duration of stimulation, ESTIMis a parabolic function of time. During cathodic stimulation the energy provided by the supply VDDcan be expressed as:0321*157112PCT 2024­300­2(PCT)

[0036] where TC1 and TC2 are the start and end time of cathodic stimulation respectively and

[0037]

[0038] Assuming the voltage across the current source ICS is maintained at VCS. Then the supplyVDD,DYNcan be written as

[0039] ,across the electrode­tissue load. Hence,

[0045] (6) in (5) and solving the integral,

[0047] ,stimulation is divided into energyconsumed by the current source, energy lost in RS, and energy stored on CDLsimilar to the case of constant voltage supply, as mentioned earlier. The energy supplied by VSS,DYNduring the anodic phase can be expressed as:

[0049] , ,and

[0052] The negative sign is because ISTIM is entering VSS,DYN. VSS,DYN can be written as

[0053] ,,

[0056] As seen in Figs. 3B and 3C, during the anodic phase, some of the energy stored on CDL isreplenished to VSS,DYNwhile some of it together with the energy supplied by the VSS,DYNis lost across the current source and in RS. This result is different from the constant voltage supply case because the voltage across the current source is constant in dynamic supply while it is0321*157112PCT 2024­300­2(PCT) varying in constant voltage supply case. However, only a small amount of energy is replenished owing to varying voltage across the current source during the energy replenishment.

[0057] To eliminate the energy loss in current sources adiabatic supply has been described.See, S. K. Arfin and R. Sarpeshkar, “An energy­efficient, adiabatic electrode stimulator with inductive energy recycling and feedback current regulation,” IEEE Trans. Biomed. Circuits Syst., vol.6, no.1, pp.1–14, Feb.2012. In this type of stimulator, instead of using the current sources to ensure constant stimulation current, a current mode feedback loop is used to drive a forward­boost­reverse­buck converter that generates a ramping voltage waveform at terminal 1 which will result in a constant stimulation current to flow into the electrode – tissue load. This approach suffers from requiring a bulky inductor, which makes is unsuitable for implantable applications. It also requires a DC supply at the input of the boost converter which means that the overall power train of the stimulator would still suffer from cascading losses if wireless power transfer is used. SUMMARY

[0058] A preferred biphasic implant stimulator includes direct stimulation supply generatorthat receives an RF AC input and converts it into a DC voltage ramp for fully­adiabatic stimulation of an implanted electrode. Current control feedback is used instead of current­ sources which reduces energy loss. The current control feedback senses the current in the electrode­tissue load and keeps it constant for the duration of stimulation. Energy stored in the electrode capacitance is replenished. No bulky inductor is required for operation.

[0059] A preferred embodiment includes an AC RF input to an LC tank, a regulating rectifier anda reconfigurable switched capacitor converter (SCC) configured to convert the AC RF input into a DC voltage ramp. Electrodes are configured to apply simulation current to nerves or an organ. A feedback loop is configured to sense stimulation current in the electrodes and drive the regulating rectifier to modify DC voltage to keep the stimulation current constant at a set reference value throughout a stimulation phase. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Fig. 1 (Prior Art) illustrates a model for an electrode­tissue interface.

[0061] Figs. 2A­2C (Prior Art) illustrates a wireless powered stimulator that uses a constant DCsupply and current sources to generate a stimulation waveform for an electrode­tissue interface.0321*157112PCT 2024­300­2(PCT)

[0062] Figs. 3A­3C (Prior Art) show another prior approach that uses a dynamic voltage supplystimulator and current sources.

[0063] Fig. 4A is a block diagram that shows a preferred biphasic implant stimulator.

[0064] Figs. 4B and 4C illustrate the energy supplied the Fig. 4A simulator during anodic andcathodic phases.

[0065] Fig. 5 is a detailed block diagram of a preferred implementation of the direct stimulationsupply generator of Fig.4A.

[0066] Figs. 6A­6E show operation phases of an H­bridge switch of the Fig. 5 direct stimulationsupply generator.

[0067] Fig. 7A shows a preferred implementation of the regulating rectifier of the Fig. 5 directstimulation supply generator.

[0068] Fig. 7B shows a preferred implementation of a digital control circuit of the Fig. 5 directstimulation supply generator.

[0069] Fig. 8 shows a preferred implementation of a Reconfigurable Switched CapacitorConverter of the Fig.5 direct stimulation supply generator.

[0070] Fig. 9 shows a preferred implementation of an H­bridge and current sensor of the Fig. 5direct stimulation supply generator.

[0071] Fig. 10 shows a preferred implementation of the integrator of the Fig. 5 directstimulation supply generator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0072] A preferred embodiment is a biphasic implant stimulator that receives power wirelesslyand provides electrical stimulation to tissue or organs through electrodes. It includes a one­step conversion from AC wireless power input to DC current output all in a fully­integrated chip and achieves higher efficiency than any other in the literature known to the inventors. An AC input is converted via an LC tank, a regulating rectifier and a n ratio (e.g., n = 11) reconfigurable switched capacitor converter (SCC) into a DC voltage. A feedback loop senses the stimulation current in the electrodes and drives the regulating rectifier to modify the output voltage to keep the stimulation current constant at a set reference value throughout the stimulation phase. A voltage ramp is generated directly from an RF input by operating the rectifier, which is upgraded to support inherent regulation, in tandem with a fully­integrated SCC.0321*157112PCT 2024­300­2(PCT)

[0073] A preferred neural stimulator is more compact and more efficient than the state of theart known to the inventors. The RF­DC rectifier is operated in tandem with a switched capacitor convertor to efficiently convert the incoming wireless power to a constant DC neural stimulation current. The resulting performance is 27x more efficient than the prior art discussed in the background. This performance has been validated in a prototype chip.

[0074] A preferred embodiment is a biphasic neurostimulator that directly converts an RF inputinto a voltage ramp for fully­adiabatic stimulation. Current mode control maintains a constant stimulation current. This eliminates the need for a current source and its associated losses. This also requires no inductor, and replenishes energy stored in electrode capacitance to further increase overall energy efficiency.

[0075] Preferred embodiments of the invention will now be discussed with respect toexperiments and drawings. Broader aspects of the invention will be understood by artisans in view of the general knowledge in the art and the description of the experiments that follows.

[0076] Fig. 4A shows a preferred biphasic implant stimulator 402. A direct stimulation supplygenerator 404 and receives an RF AC input and converts it into a DC voltage ramp for fully­ adiabatic stimulation of tissue via an implanted electrode 406. Current control feedback 408 is used instead of an on­chip current source, which reduces energy loss. The current control feedback 408 uses a voltage sensed across the electrode 406 to maintain a constant stimulation current. Energy stored in the capacitance of the electrode 406 is replenished. This uses adiabatic stimulation, but unlike the S. K. Arfin and R. Sarpeshkar discussed in the background, does not require the bulky inductor.

[0077] The energy provided by the supply for stimulation, ESTIM, can be written as:

[0078]

[0079] The ESTIM curve plotted against time is shown in Figs. 4B and 4C. For the duration ofstimulation, ESTIMis a parabolic function of time. During cathodic stimulation the energy provided by the supply VSTIMcan be expressed as:

[0080] ,

[0081] where TC1 and TC2 are the start and end time of cathodic stimulation respectively and

[0083] Since no current sources are used, the voltage across the electrode­tissue load is alsoVSTIM. Therefore,0321*157112PCT 2024­300­2(PCT)

[0088]

[0089] Substituting (10) in (9) and solving the integral

[0090] ,

[0091] Thus, the energy supplied by VSTIM during cathodic stimulation is divided into energy lostin RSand energy stored on CDL. As reflected in Fig.4B and 4C, the energy supplied by VSTIMduring the anodic phase can be expressed as:

[0092] ,and

[0094]

[0095] The negative sign is because ISTIM is entering VSTIM. VSTIM can be written as

[0096] ,

[0098] ,

[0099] Thus, in principle all the energy stored in CDL can be replenished except for the energydissipated in Rs. This is the least power consuming stimulation technique. This can be implemented by generating adiabatic voltage supply directly from wireless power AC input without requiring bulky off­chip components.

[0100] Fig. 5 shows details of a preferred implementation of the direct stimulation supplygenerator 404. Wireless power is received in an off­chip parallel tuned LC tank 500 which is connected to a power train 503 via an off­chip capacitor 502 CAC. The power train 503 of the direct stimulation supply generator 404 includes a voltage­doubling rectifier 504 and a fully­ integrated Reconfigurable Switched Capacitor Converter (R­SCC) 506. To generate an adiabatic voltage supply directly from a wireless power AC input with high efficiency in a small form factor, the rectifier 504 is operated in tandem with the R­SCC 506. The rectifier 504 has a control input VREGwhich can be modulated to convert the wireless AC input into a small fully­integrated continuously­scalable voltage ramp at an output of the rectifier 504 which is connected to the0321*157112PCT 2024­300­2(PCT) input of the R­SCC 506. As the ramp at the output of rectifier 504 is generated though linear regulation, keeping it small ensures high efficiency of the power train. The voltage ramp is generated directly from the RF input. A current mode control loop 508 modulates the control input of the rectifier 504,REG, based on the comparison of the sensed stimulation current, VSEN_ISTIM, with a set reference current equivalent, VREF_ISTIM, to convert the wireless transmitted RF input into a small continuously­scalable voltage ramp at the rectifier output,RECT. This ramp is then expanded by the SCC 506. The R­SCC 506 can be configured to operate in any of its 11 ratios – {1 / 4, 1 / 3, 2 / 5, 1 / 2, 2 / 3, 1, 3 / 2, 2, 5 / 2, 3, 4}, using a 4­bit digital control signal MODE(SCC)0­3. A higher number or smaller number of ratios can be used, as will be appreciated by artisans. Generally, the number of ratios should be at least high enough to avoid discontinuities in the voltage ramp when switching ratios. Factors to be considered in selecting the number of ratios for a specific design include available chip area, power budget. In the example implementation, 11 ratios provide 5 up­conversion ratios, 5 reverse down­conversion ratios and 1 unit ratio. These 5 ratios were selected to make sure the voltage ramp does not have any discontinuity when R­SCC 506 is dynamically configured from one ratio to another ratio. The R­SCC ratio is determined dynamically by the current­mode feedback control loop 508 to obtain a continuous voltage ramp at its output, which is also connected to terminal 1510 the electrode­tissue load. The large number of R­SCC ratios ensure that even a small ramp at the output of the rectifier 504 can be expanded to span from close to 0 to VDD(e.g., 3.3V). As the ramp at the output of rectifier 504 is generated though linear regulation, keeping it small ensures high efficiency of the power train. “Small” is compared to the span of the voltage ramp (0 ­ 3.3V), and any millivolt quantity is small in comparison to the 3.3V range. In the example, a typical small value is 300mV.

[0101] Since the rectifier 504 can only source current and not sink current – as sinking currentwould mean that the energy is being transferred back to the external transmitter – an H­bridge 512 is included to provide bipolar biphasic stimulation. The output of the R­SCC 506 is connected to terminals 1 (510) and 2 (514) of the electrode­tissue load via the H­bridge 512. Each of four switches 5141­5144of the H­bridge can be controlled independently by external digital signals. In the experimental prototype, external control signals were used but in a commercial implementation, the control can be imbedded in the stimulator. Control of the switches 5141­5144can be through digital signals derived from an on­board microprocessor, wireless transceiver, SPI (serial peripheral interface) bus, or other typical forms of digital0321*157112PCT 2024­300­2(PCT) communication. The switches 5142and 5144of the H­bridge 512 are connected to a single off­ chip capacitor CMID(4.7uF) that stores a midrail voltage VMID(e.g., 1.65V). This capacitor serves two purposes 1) It acts as the energy storage unit for the on­chip circuits 2) It stores the energy replenished from the electrode­tissue load capacitance CDL.

[0102] The current flowing through the electrode­tissue load also flows through one of theswitches 5142and 5144of the H­bridge 512. The bottom switch 5142and 5144can be implemented as a digitally controlled 8­bit current mirror. Current­mode feedback control is implemented by sensing the current mirrored by the H­bridge switches 5142and 5144using a current sensor 517 including a transimpedance amplifier TIA 518. A sensed current VSEN_ISTIMis then compared via a comparator 520 with a reference voltage VREF_ISTIMthat is used to set the stimulation current amplitude. A digital output VCMPof the comparator 520 is integrated using a 4­bit digitally tunable integrator 522 to generate an analog voltage VINTfor an input of a bootstrap circuit 524. The bootstrap circuit 524 provides a regulating control input VREGto the rectifier 504. In commercial implementation an auxiliary power management unit (PMU) can be included on­chip or off­chip to charge CMIDfrom RF input and use the energy stored on CMIDto power the on­chip electronic circuits. The auxiliary PMU was however not included in the prototype of the Fig.5 direct stimulation supply generator 404 as it can be implemented using standard circuit techniques.

[0103] Figs. 6A­6E shows the operation of the Fig. 5 direct stimulation supply generator 404.The system operates in 4 phases: A, B, C, and D. In phase A, all H­bridge switches 5141­5144are open and no current flows. Both terminals 1 (510) and 2 (514) of the electrode­tissue load are floating. In phase B, switches 5142and 5143are closed. The stimulation current flows through the power train 503 into the electrode­tissue load from terminal 1 (510) to terminal 2 (514) and back into CMID, which has been pre­charged to VMID. This is the energy provision phase, as CDLis charged to voltage VPKby the end of this phase.

[0104] During this phase terminal 2 (514) is connected to CMID and its voltage remains close toVMID. Since CMIDis much larger than CDL, it can be assumed that its voltage does not change significantly during stimulation cycle. Also, terminal 1 (510) is connected to VSTIMand has a waveform as shown in Fig.5E. In phase C, switches 5141and 5144are closed. The stimulation current flows in the electrode­tissue load from terminal 2 to terminal 1 and back into CMID. During this phase terminal 2 is connected to VSTIMand terminal 1 (510) is connected to VMID. As current flows in the electrode­tissue load, CDLis discharged and energy stored on CDLis0321*157112PCT 2024­300­2(PCT) replenished to CMIDwhich is available for use by other circuit loads. In practice, due to stimulation current mismatch, up to 99.5% of the energy can be replenished. Finally, in phase D, the terminals 1 and 2 of the electrode­tissue load are shorted by closing switches 5142and 5144to remove any residual charge.

[0105] Fig. 7A shows a preferred implementation of the regulating rectifier 504 implementedas a voltage doubler with a preferred implementation of the bootstrap circuit 524. The rectification function is enabled by the power transistors MPand MNtogether with the off­chip cap CAC. The regulation function is enabled by controlling the voltage between the gate and drain, VREG, of MPand MNwith the help of the bootstrap circuit 524. To avoid reverse conduction, VREGneeds to be kept below the threshold voltage, VTHof MPand MN. This is facilitated by a digital control circuit 550 (Fig.5) that includes a comparator 702 in Fig.7B that shorts the capacitors CAand CB, when VREGexceeds VDG_MAX, and triggers a counter 704 that holds the conversion ratio (MODE) of the R­SCC is configured into a higher conversion ratio to continue generating a voltage ramp to keep the stimulation current constant. The bootstrap circuit 524 can use, for example, 1MHz, 0 – 3.3V non­overlapping clocks clkA and clkB. In a prototype, CAand CBhave been implemented on chip as a combination of MOSCAP and MIMCAP.

[0106] Fig. 8 shows a preferred implementation of the R­SCC 506. In the prototype, switchesS1­3 are implemented as NMOS while the rest are implemented as NMOS and PMOS in parallel as their drain and source voltages range from 0 – 3.3V.1MHz 3.3V non­overlapping clock is used to drive the switches. The flying capacitors have been implemented on chip as a combination of MOSCAP and MIMCAP. The example 1 MHz clock is generated by dividing the 13.56MHz clock recovered from the wireless input by the clock recovery circuit.

[0107] Fig. 9 shows a preferred implementation of the H­bridge 512 and the current sensor517. The H­bridge switches are implemented as NMOS and PMOS in parallel. The switches 5142and 5144are implemented as 8­bit DACs. The current sensing is performed by mirroring the stimulation current flowing through the switches 5142and 5144in N:1 ratio. The mirrored current is then fed into the TIA 518 to obtain VSEN_ISTIM, which is given by:

[0108] 0321*157112PCT 2024­300­2(PCT)

[0109] where RFB is the feedback resistor used in the TIA 518. N can be set automatically anddigitally via on­chip digital registers which are written using a SPI as per the stimulation current ISTIMto get appropriate sensitivity to ISTIM. When ISTIMis small compared to its maximum value, which is 2.5mA in the prototype implementation of the Fig.5 direct stimulation supply generator 404, N should be kept small – compared to its maximum value – commensurate with ISTIMto allow for small variations in ISTIMgetting reflected in VSEN-ISTIM,and vice­versa. If N is set to a small value for a large ISTIM, small variations in ISTIMcan lead to saturation of the output stage of the TIA. For example, in the prototype implementation of current sensor 517 of Fig.9, N can vary from 10 to 1250. When ISTIMis 2mA, N can be set to 1000 and when ISTIMis 200uA, N can be set to 100. Due to mismatch – coming from process variations – between the current mirrors of H­bridge switches 5142and 5144of Fig.5, even for the same N, the stimulation current averaged over the duration of the phase when switch 5142is turned on – cathodic stimulation phase – could be different from that of the phase when switch 5144is turned on – anodic stimulation phase. This difference can be compensated for by setting the N appropriately. When there is a difference between the average stimulation currents of cathodic and anodic phases, a positive or negative residual voltage can be observed across the terminals of the electrode­tissue load of Fig.1 after a biphasic stimulation where the duration of cathodic stimulation phase is same as the duration of anodic stimulation phase. For example, if N of the current mirrors of switches 5142and 5144are same but average stimulation current during cathodic stimulation phase is more than the average stimulation current during the anodic phase, N of the current mirror implemented on the switch 5144can be incremented until the average stimulation current of both the phases become same and the residual voltage across the electrode­tissue load reaches below the water­window of 25mV. This can be used to achieve charge balancing.

[0110] Fig. 10 shows a preferred implementation of the integrator 522. The preferredimplementation is continuous time comparator with PMOS input transistors. Based on the output of the integrator 522, a tunable current is either sunk into an on­chip integrating capacitor, CINT, or sourced from it. One plate of CINTis connected to the CMIDso that the minimum voltage at VINTis VMIDand the sink current sources of the integrator are always in saturation. The voltage across the CINTacts as input of the bootstrap circuit 524 connected to the regulating rectifier 504.

[0111] While specific embodiments of the present invention have been shown and described, itshould be understood that other modifications, substitutions and alternatives are apparent to0321*157112PCT 2024­300­2(PCT) one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.

[0112] Various features of the invention are set forth in the appended claims.

Claims

0321*157112PCT 2024­300­2(PCT) Claims 1. A wirelessly powered implant simulator, comprising: a direct stimulation supply generator thatreceives an RF AC input and converts it into a DC voltage ramp for adiabatic stimulation of an implanted electrode using current control feedback that senses the current in the electrode­ tissue load and keeps it constant for the duration of a stimulation event.

2. The implant stimulator of claim 1, wherein the supply generator, which uses a regulatingrectifier configured and optionally a reconfigurable switched capacitor converter (SCC) to convert the AC RF input into a DC voltage; wherein the electrode is configured to apply simulation current to nerves, organs, or biological tissue; wherein the current control feedback loop is configured to sense stimulation current in the electrode and drive the regulating rectifier to modify DC voltage to keep the stimulation current constant at a set reference value throughout a stimulation phase.

3. The implant stimulator of claim 2, wherein a voltage ramp is generated directly from the AC RFinput by operating the rectifier in tandem with the SCC.

4. The implant stimulator of claim 2, the regulating rectifier, the SCC and the feedback loop arefully integrated in single IC chip.

5. The implant stimulator of claim 4, comprising no current source on the IC chip.

6. The implant stimulator of claim 4, comprising H­bridge switches that apply voltage across theelectrode.

7. The implant stimulator of claim 2, wherein current control feedback loop modulates a controlinput of the rectifier.

8. The implant stimulator of claim 7, wherein the current control feedback loop modulates thecontrol input based on a comparison of sensed stimulation current with a set reference current equivalent.

9. The implant stimulator of claim 8, an output of the rectifier is generated through linearregulation.

10. The implant stimulator of claim 2, comprising an H­bridge circuit configured to ensure that therectifier can only source current and not sink current.

11. The implant stimulator of claim 10, comprising a bootstrap circuit configured to provide aregulating control input to the rectifier.0321*157112PCT 2024­300­2(PCT)12. The implant stimulator of claim 11, wherein a mid­rail of the H­bridge circuit is connected to theelectrodes.

13. The implant stimulator of claim 12, wherein the H­bridge circuit is configured to provide fourphases of operation, including a first phase in which all switches are open and no current flows, a second phase in which switches close to provide stimulation to load connected to the electrodes in one direction and provide energy to charge the electrode capacitor, a third phase in which switches close to stimulate in reverse direction and replenish energy from the electrode capacitance, and a fourth phase in which switches close to short the load and electrodes to remove residual charge.

14. The implant stimulator according to claim 13, wherein the all switches are controlled by settingcurrent mirroring switches of the all switches in an in N:1 ratio, wherein N is set and change such that average stimulation current of an anodic and cathodic phase becomes the same.

15. The implant stimulator of claim 2, wherein the SCC comprises a number of ratios to ensure thatthe voltage ramp does not have any discontinuity when the SCC is dynamically configured from one ratio to another ratio.

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