TR2023007811T2 - POWER SYSTEM FOR CONSTANT CURRENT NEURAL STIMULATORS - Google Patents

Abstract

The present invention relates to a power system for constant current neural stimulators that provides adaptive supply voltage for neural stimulators to reduce power dissipation by taking advantage of the varying voltage compatibility of constant current stimulation.

Description

DESCRIPTION POWER SYSTEM FOR CONSTANT CURRENT NEURAL STIMULators Technical Field of the Invention The present invention relates to a power system for constant current neural stimulators that provides adaptive supply voltage for neural stimulators to reduce power dissipation by taking advantage of the varying voltage compatibility of constant current stimulation. Prior Art (Prior Art) Neural stimulation has been used to treat or cure those with disorders of certain limbs or organs. Cochlear implants are an example of devices that use neural stimulation because they mimic the functioning of the cochlea to provide hearing in people with sensorineural hearing loss. Additionally, in applications where neural stimulation devices are used, a pacemaker, retinal implant, or neural stimulation may be given to relieve pain. Various methods such as voltage mode, charging mode and current mode can be adopted to realize neural stimulation. Voltage mode is the most efficient mode, but does not provide control over the total charge delivered. This can be controlled with charging mode, but this mode requires large capacitors for adequate charge dissipation. Current mode is the most popular from the USZOl30314129Al document, thanks to its safety and wide load range despite its low efficiency. In the literature, biphasic current pulses are used for constant current stimulation, as shown in Figure 1(b). A two-phase current is used to reduce power dissipation and maintain the charge balance of the neuron. Stimulation is done with electrodes, and the electrode voltage difference corresponding to current stimulation is also shown in Figure 1(b). This voltage waveform is caused by the electrode tissue interface or load impedance, which can be modeled as shown in Figure 1(a). The bulk resistance (Rbulk) models the material between two electrodes, while the electrode-tissue interface is modeled by a capacitor (Csurf). Leakage resistance (Rleakage) is used to model the redox reactions occurring on the electrode surface connected in parallel with Csurf [7]. Additionally, Rleakage is typically ignored due to its larger value [5], [6], [7], [8]. The generation of the two-phase pulse can be done with different applications. The following are examples of various applications where multiple current sources are used, as shown in Figure 2(a) of document US7519428B1, a recharge capacitor 955 is used as shown in Figure 2(b) of document US9079032B2, or as shown in Figure 3 A similar simple switch configuration is used [5]. As mentioned previously, neural stimulators perform their functions either on organs or limbs and, in most cases, in volume-limited scenarios. This brings with it the problem of low capacity battery. Moreover, in most cases these batteries are surgically implanted inside the human body, so they need to be replaced when their performance drops critically due to many charging cycles or they cannot be charged. In order to reduce the discomfort and problems associated with the replacement operation, making these implant devices as efficient as possible has led to studies on this subject. One particular aspect of the neural stimulator that has been focused on is the voltage matching issue. For the load impedance given in Figure 1(a), the voltage generated across the electrodes can be calculated for the specific stimulation current by equation (1) and thus the required voltage match can be determined. For example, for 9 kg and 20 nF load impedance, the calculated voltage for 1 mA and 50 us stimulation current and duration is 11.5 V. However, for a stimulation current of 500 uA, this value drops to 5.75 V. In both scenarios, if it were to be powered by 12 V, power would be wasted in the latter case. This aspect of neural stimulation is exploited to reduce power dissipation. In the next section, several studies on this topic are summarized. Vcomp = IstimRtoplu + -1- IstimAt Unlike some applications, the work presented in USZOl303l4129Al varies the stimulation current itself rather than varying the supply voltage. If the high voltage monitoring block on the stimulator detects that the output voltage on the electrode is lower than a predefined threshold voltage, the stimulation current is changed for the next stimulation. Change, waveform shape, pulse width, amplitude, etc. It can be done for. A specific example is to reduce the pulse width and increase the amplitude of the stimulation current. In this way, the gap between the electrode voltage and the supply voltage is reduced and less power is consumed. As previously mentioned, multiple current sources are used in document US7519428B1. In the case of high voltage compatibility, the electrodes are connected to a current source and sink with positive and negative supply respectively. This is done interchangeably to create a two-phase pulse. When low voltage compatibility is required, one of the electrodes is always connected to ground and the other electrode is connected to a current source in one phase and a current sink in the other phase. In document US9079032B2, the voltage of an electrostimulation energy storage capacitor is monitored and the load impedance is calculated for a given stimulation current by observing how the storage capacitor is discharged. Additionally, a recharge capacitor is connected between the load and ground so that the opposite phase of the stimulation current can be generated. The design in document US9731116B2 also uses the output capacitor of a charge pump, which is charged to a certain voltage prior to stimulation for a certain period of time determined by the controller. For high stimulation currents where the output capacitor is discharged too much, the charge pump is reactivated to charge the output capacitor once more so that voltage compatibility is maintained and correct stimulation can be achieved. The above designs either use knowledge of previous stimulation currents or require complex circuits. Additionally, they only operate at conservative voltage values. The device presented here monitors and adapts to the electrode voltages resulting from constant current stimulation to follow the electrode voltage difference with a certain headroom to ensure adequate voltage matching. An example ideal waveform that is being tried to be obtained is shown in Figure 4. Towards the end of a stimulation period, the supply voltage is pulled down to a predefined high voltage value to prepare for the next stimulation period. Additionally, necessary load balancing methods can be used to prevent neurons from being damaged by potential accumulation in this range. The following sections describe the overall system and the adaptive voltage feeder. Objects of the Invention and Brief Description The present invention relates to a power system that meets the above-mentioned requirements for constant current neural stimulators, eliminates all disadvantages and provides some new advantages. The present invention discloses a power system that provides adaptive supply voltage for neural stimulators to reduce power dissipation by taking advantage of the varying voltage compatibility of constant current stimulation. The voltages of the stimulation electrodes are monitored to determine the required supply voltage. The actuation frequency and number of actuation charge pump stages are then suitably adapted to provide voltage matching for the stimulation circuit. Additionally, a step-down converter is used to power the control circuit to further reduce power loss. An energy harvester and appropriate interface circuit are used to charge the battery for long-term uninterrupted operation. Explanations of Figures Explaining the Invention The figures used to better explain this invention, which has a power system developed for constant current neural stimulators, and their explanations are as follows: Figure 1: Two-phase current pulse used to stimulate neurons (Prior art). Figure 2: Example applications of two-phase pulse generation using (a) multiple current sources US7519428B1 and (b) a charge capacitor (Prior Art). Figure 3: An example switch configuration for two-phase pulse generation (Prior Art). Figure 4: Example of constant supply voltage and electrode voltage monitoring supply voltage Figure 5: Block diagram of the overall neural stimulation system. Figure 6: Block diagram of the charge pump side of the design. Figure 7: Block diagram for the second part of the design. Figure 8: Single stage charge pump with boot capacitor. Figure 9: Single-stage charge pump with charge reuse and boot capacitors. Figure 10: Two-phase stimulation current, corresponding electrode voltage differences and individual electrode voltages. Figure 11: Magnitude comparison circuit for electrode voltages. Figure 12: Differential amplifier used for electrode voltage subtraction. Figure 13: Example waveform for the absolute value of the charge pump output and electrode voltages for the same load when the battery voltage (bottom) varies from 3.7 V to 3.3 V at 200 us. Figure 14: Example waveform for the absolute value of the charge pump output and electrode voltages for the changing load when the battery voltage (bottom) varies from 3.7 V to 3.3 V at 200 us. Figure 15: Working method of the invention. Detailed Description of the Invention In order to better explain a power system developed for constant current neural stimulators with this invention, details are presented below. The block diagram of the overall system can be seen in Figure 5. The adaptive voltage supply and step-down converter are operated from a battery cell. In some examples, the battery may be rechargeable or non-rechargeable. In the rechargeable state, in addition to being able to charge the battery from an external source such as a wireless power transmitter, the battery can be charged using an energy harvester using appropriate interface circuitry. This will extend the time that devices can be operated before needing to be charged. In some examples, the energy harvester may not be included in the system and the battery will be charged from an external source, or the battery may not be charged at all. Besides the adaptive voltage generator, a step-down converter is used to reduce the power dissipation of the control circuit. In some examples, this step-down converter may consist of a switched capacitor converter to achieve a fully integrated design. In some examples, it may be a resonant or multilevel switching transducer containing both capacitive and inductive elements. In some cases, for the step-down converter, a linear converter can be used to reduce size and complexity. Additionally, as mentioned, adaptive voltage supply is used in this system. The main task of adaptive voltage supply is to provide the correct amount of voltage to ensure voltage compatibility to the switch matrix used to generate the required stimulation signal for the stimulation electrodes. To do this, it monitors the voltage at the stimulation electrodes and generates a supply voltage that follows this voltage with a certain voltage headroom, as shown in Figure 4. The next section will explain the adaptive voltage generator in more detail. Adaptive Voltage Supply General description of the Adaptive Voltage Generator: The general design is represented by two block diagrams. The first, shown in Figure 6, is the charge pump side of the design. As mentioned, the charge pump voltage follows the electrode voltage. Monitoring is accomplished in two ways. The first is the control of the number of charge pump stages used, and the second is the frequency control. Controlling the number of stages allows rough control of the output voltage. For example, for an input voltage of 3.3 V and an output voltage of 3 to 6 V, only one stage is operational, while for 9 to 12 V, all three stages must be used. This was adopted due to the natural decrease in charge pump efficiency when the output voltage deviates from the discrete value provided by the charge pump [9]. For an input voltage of 3.3 V, a 3-stage charge pump that quadruples the input voltage will be most efficient when supplying voltages closer to 13.2 V. Thus, to avoid dramatic efficiency loss at low voltage, the voltage range is divided into three parts where different voltage levels can be provided. This is summarized in Table 1. In this table, Bit_1 and Bit_2 are control signals that determine the number of running stages. These control signals are provided from the second part of the design, which will be explained later. This version of the adaptive voltage generator uses a three-stage charge pump, but the number of stages can be increased depending on the required voltage to be supplied to the neural stimulator. Accordingly, the control circuit, drivers, level shifters and the second part of the design, which will be explained, must also be changed. In another example, other topologies of charge pumps can be used. Similar to this configuration, the topology of the design in these examples may also need to be configured. In other examples, it may be sufficient to use pulse width modulation (PWM) or pulse frequency modulation (PFM) control, as described below. Additionally, precise control of the output voltage is ensured by controlling the output impedance of the charging pump, which is achieved by frequency control. For this, an error amplifier and voltage controlled oscillator (VCO) are used. The error amplifier generates a control voltage for the VCO by comparing the split charge pump output voltage with a reference voltage also provided by the second part of the design. In some examples, ripple control may be used to enable and disable the clock signal fed to the non-overlapping clock generator to obtain a regulated voltage. Table 1: Output voltage ranges corresponding to control bits and number of charge pump stages operating. Bit_2 Bit_1 Voutput Operation O 0 3.3 0 O 1 3.3 - 6 V 1 1 O 6 - 9 V 2 l 1 9 - 12 V 3 The block diagram for the second part of the design, which is voltage sampling and control signal generation, is given in Figure 7. EL is the channel electrode and EL_COM is the common electrode. In the first part, electrode voltages are sampled and fed to the subtraction circuit. In the subtraction part, by comparing their sizes, it is determined which electrode has the larger potential, as it varies depending on the phase of the two-phase pulse. This is to ensure proper operation of the differential amplifier used to perform the subtraction. In other examples, magnitude comparison may not be necessary if a circuit is used that can subtract accurately regardless of which electrode voltage is subtracted from which voltage. As a result, the electrode voltages are subtracted and offset by a certain amount and then fed to a flash ADC mapped to two bits. These are Bit_1 and Bit_2, also shown in Figure 6. Comparison voltages for hysteresis comparators are created with a resistive section to drive the correct number of charge pump stages for the desired voltage compatibility. Hysteresis comparators determine whether the charge pump stage is operational. For example, for an input voltage of 3.3 V, a two-stage charge pump can provide an output voltage of 9.9 V at no load. Therefore, considering the load conditions, the design should work such that the third stage does not operate until the required output voltage reaches 9 V. Beyond this value, the third stage should also be operational. Assuming the voltage divider in Figure 6 provides 1/5 of the charge pump output voltage, a reference voltage of 1.8 V will be required for the error amplifier and oscillator to maintain the output voltage at 9 V. As noted, if the results of the subtraction in Figure 7 exceed 1.8 V, the third stage should start operating when the third hysteresis comparator changes its output to HIGH and the third stage begins operating. As can be seen in Figure 7, a resistive divider is used to generate the 1.8 V required for the hysteresis comparator to compare the reference voltage. This is to enable the design to operate under varying supply voltages, as the output voltage at which the third stage starts operating will no longer be 9 V. As mentioned, the subtraction voltage obtained by offset is also the reference voltage for the charge pump, allowing it to follow the electrode voltage. This part of the adaptive voltage supply allows it to be used with any constant current neural stimulation device because it tracks the electrode voltage in real time. In cases where more than three charging stages are used, the flash ADC should be replaced and the bitmap changed accordingly. An adaptive voltage supply generator that reduces the average supply voltage and therefore low average power consumption is provided. Adaptive voltage supply generator in which the required voltage compliance is achieved roughly by controlling the number of charge pump stages, where the maximum number of charge pump stages is determined by the voltage range of the stimulation circuit. Adaptive voltage supply generator, where the required voltage matching is fine-tuned by pulse frequency modulation (PFM) control, the reference for which is provided by the monitoring circuit with a certain headroom to provide the scaled electrode voltage. Thanks to the scaled electrode voltage, it enables real-time monitoring of the electrode voltage when used as a reference for the regulation system. Charge Pump: One stage of the charge pump circuit is given in Figure 8 [10]. It uses two charge transfer capacitors. Node B or Bi is charged in one phase to Vi.1 and then amplified by the input voltage. That is, when (In is HIGH, Bi is equal to Vi-1, and when (In is HIGH), it is amplified by the input voltage. Since the charge pump is dual configuration, the same process applies to B. In addition to the bootstrap method, the bottom of the on-chip MIM capacitors [10] A charge reuse scheme has been adopted to reduce the effects of plate interference, this version is shown in Figure 9: The charge pump in this design is a three-stage version of the circuit given below to get 12 V at the output from a 3.3 V input. Regulation: As mentioned, the charge pump output Two different methods of regulation are used. Coarse regulation is achieved by varying the number of operational charge pump stages. This is done with a flash ADC using three hysteresis comparators. The outputs of these comparators are mapped to two bits, Bit_1 and Bit_2, which determine the number of processing stages as described previously. Comparison voltages for these comparators are created by dividing the input voltage by the same ratio as the output voltage of the charge pump, resulting in the fine regulation described below. As mentioned before, establishing the comparison voltage in this way allows the adaptive voltage source to operate at varying battery voltages, e.g. If the desired output voltage is between two and three times the input voltage, it uses two stages of the charge pump. PFM control method is adopted for precise regulation of charge pump voltage. The operational transconductance amplifier, used as the error amplifier, compares the split output of the charge pump with a reference voltage. According to its output, the control voltage of the oscillator is adjusted so that the switching frequency can change according to the load current. In this way, the output voltage is kept at the desired value by adapting the output impedance of the charging pump. In other examples, ripple-based editing may also be used. Non-overlapping Clock and Drives: Non-overlapping phases are needed for the charging pump to function properly. This is necessary to eliminate short circuit losses of the charge pump. If transistors that should not operate at the same time are open during switching due to timing delays, a short circuit path will occur and this will reduce efficiency. Thus, to prevent this, a conventional non-overlapping generator is used and the clock signals are fed to the switches via a drive chain. In the block diagram shown earlier, it was mentioned that the electrode voltage at its terminals is used as a reference voltage for the error amplifier and to control the number of charge pump stages in operation. The important parts are explained here. Magnitude Comparison: Two-phase stimulation and electrode voltage difference are repeated in Figure 10. In this figure, the individual voltages of the electrodes are also shown. In one phase of stimulation, one electrode voltage is greater and vice versa for the other phase. Therefore, to achieve correct subtraction, magnitude comparison must be made to determine which voltage is being subtracted from which voltage. The circuit used for this purpose is given in Figure II. Subtraction: For subtraction, a difference amplifier is applied as shown in Figure 12. A folded cascode structure is used for the riser. An offset is given to the difference between the electrodes to provide headroom between the switch matrix and the charge pump output voltage. Stimulations: Stimulation results for several scenarios for the charge pump output voltage following the electrode voltage are provided in this section. Figure 13 shows two consecutive stimulation processes in which the same load was used for the same stimulation current, as shown in Table 2. The only thing that changes is the supply voltage. As seen in the lower waveform, the supply voltage varies between 3.7 V and 3.3 V in the 200 us time interval. However, the electrode voltage and the adaptive supply voltage remain the same and thus it has been shown that the system can operate under changing supply voltages. Figure 14 also shows two successive stimulation processes in which the supply was reduced from 3.7 V to 3.3 V, but this time the loads were also different and their values are given in Table 3. Table 2: Stimulation conditions for the waveforms in Figure 13 stimulation # Ryuk Cyük 1. Electr0t 8 k!! 30 nF 2.Elektr0t 8 k!! 30 nF Table 3: Stimulation conditions for the waveforms in Figure 14 stimulation # Ryload Cyload 1. Electr0t 8 k!! 30 nF 2.Elektr0t 6.5 kg 40 nF A monitoring circuit for measuring electrode voltage compatibility and controlling the voltage source generator, wherein the monitoring circuit includes a voltage subtraction circuit with a sign correction method applicable to any neural stimulation system and also includes an operational Contains digitization circuitry for varying the number of charge pump stages. A power system that provides adaptive supply voltage for constant current neural stimulators, comprising: It is configured to be connected to an energy source for extracting the desired voltage levels and reducing the average supply voltage, and said adaptive voltage supply generator will produce the required voltage compatibility with energy sources having varying voltage levels and provide this to the switching matrix used to generate the required stimulation signal for the stimulation electrodes. An adaptive voltage supply generator, configured as follows, a monitoring circuit for measuring electrode voltage compliance and controlling the voltage supply generator, and said monitoring circuit comprising: 0 continuously providing the voltage difference of the electrodes to the supply generator, capable of monitoring this voltage and monitoring this voltage during stimulation. a voltage subtraction circuit that allows it to adapt to any changes that occur, a digitization circuit for varying the number of charge pump stages that operate, a charge pump that monitors the electrode voltage by frequency control and control of the number of charge pump stages used, a charge pump to power the monitoring circuit and detect power loss. a step-down converter configured to be connected to the energy source to provide power to the control circuit to reduce power dissipation of the control circuit, and wherein said step-down converter is used to reduce the power dissipation of the control circuit, a battery charged using an energy harvester using the interface circuit or external source. A battery can be a rechargeable battery. Other aspects of the invention; The system further includes an energy harvesting system configured to charge the rechargeable battery, and said energy harvesting system interface circuitry to recharge the battery and clear ambient energy, adaptively configured to be controlled by the number of charge pump stages for required voltage compatibility. voltage supply generator 0 Includes adaptive voltage supply generator using pulse frequency modulation (PFM) control to fine-tune required voltage compatibility, 0 Charge pump circuit includes level shifters to handle switches connecting the charge pump stages to output, 0 Adaptive voltage generator, three o Adaptive voltage supply and step-down converter uses a battery as an energy source to operate, o Step-down converter consists of a switched capacitor converter, o Step-down converter is a resonant converter, multi-level switching converter or linear converter, o The external source is a wireless power transmitter. To contribute to the battery level by taking energy from the environment where the device is located, sweep the ambient energy with a piezoelectric or similar energy generator system. It is the working method of the system and the method includes the following; Generation of control signals through the control circuit for starting the charge pump and maximizing its output, Generation of control signals through the control circuit for the switch matrix for starting the stimulation process, Activating the voltage sampling circuit and generating the scaled electrode voltage with offset, Reference for pulse frequency modulation control of the scaled electrode voltage Using it as voltage and creating signals for stage number control, Decreasing the charging pump output from its maximum value to the value determined by the scaled electrode voltage and monitoring it, Before the stimulation process ends, the charging pump output is pulled to the maximum with the signals coming from the control circuit via the control circuit to provide sufficient voltage for the next stimulation process. Maintaining the charge pump output at maximum until the start signal is given for the next stimulation period. REFERENCES L. Yao, "Stimulator and Method for Processing a Stimulation Signal", United States L. P. Palmer, "Dual-Range Compliance Voltage Supply for a Multi-Channel D. J. Ternes, S. Vanderlinde, R. Vijayagopal, S. C.Boon, "Power Supply Management 2015. J. Chen, “Charge Pump System, Devices and Methods for an Implantable Stimulator,” H. Ulusan, A. Muhtaroglu and H. Kulah, “A Sub-500 uW Interface Electronics for W. Ngamkham, M. van Dongen and W. Serdijn, "Biphasic stimulator circuit for a Wide range of electrode-tissue impedance dedicated to cochlear implants", 2012 IEEE B. Swanson, P. Seligman, and P. Carter, "Impedance measurement of the Nucleus 22- electrode array in patients," Ann. Otol. Rhinol. Laryngol, vol. 104, no. 166, pp. 141- 144, 1995. J. Sit and R. Sarpeshkar, "A Low-Power Blocking-Capacitor-Free Charge-Balanced Electrode- Stimulator Chip With Less Than 6 nA DC Error for 1-mA Full-Scale Stimulation", IEEE Transactions on Biomedical Circuits and Systems, vol. 1, no. 3, pp. M. Makowski and D. Maksimovic, "Performance limits of switched -capacitor DC-DC converters", Proceedings of PESC '95 - Power Electronics Specialist Conference.TR TR

Claims (1)

STEMS 1. It is a power system that provides adaptive supply voltage for constant current neural stimulators, and its feature is that it includes the following; It is configured to be connected to an energy source for extracting the desired voltage levels and reducing the average supply voltage, and said adaptive voltage supply generator will produce the required voltage compatibility with energy sources having varying voltage levels and provide this to the switching matrix used to generate the necessary stimulation signal for the stimulation electrodes. an adaptive voltage supply generator configured as follows, a monitoring circuit for measuring electrode voltage compliance and controlling the voltage supply generator, and said monitoring circuit comprising: a voltage subtraction circuit to enable it to adapt to any change, a digitization circuit to vary the number of charge pump stages operated, a charge pump to monitor the electrode voltage by frequency control and control of the number of charge pump stages used, to power the monitoring circuit and reduce power loss. a step-down converter configured to be connected to the energy source to power the control circuit, and wherein said step-down converter is used to reduce power dissipation of the control circuit. A rechargeable battery. The system according to claim 1, characterized in that the system further includes an energy harvesting system configured to charge a rechargeable battery. It is a system according to claim 1, characterized in that the adaptive voltage supply generator is configured to be controlled by the number of charge pump stages for the required voltage compatibility. It is a system according to Claim 1 and its feature is that the adaptive voltage supply generator is configured for fine tuning of the required voltage compatibility with pulse frequency modulation (PFM) control. It is a system according to claim, characterized in that the charge pump circuit includes level shifters to operate the switches connecting the charge pump stages to the output. It is a system according to claim and its feature is that the adaptive voltage generator uses a three-stage charge pump. The system according to claim 1, characterized in that said adaptive voltage supply and step-down converter use a battery as an energy source. It is a system according to claim 1 and its feature is that the step-down converter in question consists of a switched capacitor converter. It is a system according to claim 1 and its feature is that the step-down converter in question is a resonant converter, multi-level switching converter or linear converter. It is a system according to claim, characterized in that the external source in question is a wireless power transmitter. It is the operating method of the system according to claim 1, and its feature is that the method includes the following; o Generation of control signals through the control circuit for starting the charging pump and maximizing its output, o Generation of control signals through the control circuit for the switch matrix to start the stimulation process, o Activating the voltage sampling circuit and generating the scaled electrode voltage with offset, o Pulse of the scaled electrode voltage Using it as a reference voltage for frequency modulation control and generating signals for stage number control, o Decreasing the charging pump output from its maximum value to the value determined by the scaled electrode voltage and monitoring it, 0 Before the stimulation process ends, the charging pump output is maximized with the signals coming from the control circuit. 0 Maintaining the charge pump output at maximum until the start signal is given for the next stimulation period.