Power scheme for implant stimulators on the human or animal body

A power scheme for an implant on a human or animal body comprises: a charging circuit to provide power to deliver controlled stimulation currents to a body tissue; a capacitive storage arrangement connected with the charging circuit and charged by the charging circuit; a shunting arrangement to limit voltage on the capacitive storage arrangement; a driver array configured to transfer charges from the capacitive storage arrangement to the tissue; and an electrode array connected with the driver array and the tissue.

BACKGROUND

The present disclosure relates to implants for humans or animals. In particular, it relates to a power scheme for implant stimulators (also referred to as implants in the present application) on the human or animal body.

2. Related Art

The following paragraphs will introduce some art possibly related to the present invention.

In 1755 LeRoy passed the discharge of a Leyden jar through the orbit of a man who was blind from cataract and the patient saw “flames passing rapidly downwards.” Ever since, there has been a fascination with electrically elicited visual perception. The general concept of electrical stimulation of retinal cells to produce these flashes of light or phosphenes has been known for quite some time. Based on these general principles, some early attempts at devising a prosthesis for aiding the visually impaired have included attaching electrodes to the head or eyelids of patients. While some of these early attempts met with some limited success, these early prosthetic devices were large, bulky and could not produce adequate simulated vision to truly aid the visually impaired.

In the early 1930's, Foerster investigated the effect of electrically stimulating the exposed occipital pole of one cerebral hemisphere. He found that, when a point at the extreme occipital pole was stimulated, the patient perceived a small spot of light directly in front and motionless (a phosphene). Subsequently, Brindley and Lewin (1968) thoroughly studied electrical stimulation of the human occipital (visual) cortex. By varying the stimulation parameters, these investigators described in detail the location of the phosphenes produced relative to the specific region of the occipital cortex stimulated. These experiments demonstrated: (1) the consistent shape and position of phosphenes; (2) that increased stimulation pulse duration made phosphenes brighter; and (3) that there was no detectable interaction between neighboring electrodes which were as close as 2.4 mm apart.

As intraocular surgical techniques have advanced, it has become possible to apply stimulation on small groups and even on individual retinal cells to generate focused phosphenes through devices implanted within the eye itself. This has sparked renewed interest in developing methods and apparati to aid the visually impaired. Specifically, great effort has been expended in the area of intraocular retinal prosthesis devices in an effort to restore vision in cases where blindness is caused by photoreceptor degenerative retinal diseases such as retinitis pigmentosa and age related macular degeneration which affect millions of people worldwide.

Neural tissue can be artificially stimulated and activated by prosthetic devices that pass pulses of electrical current through electrodes on such a device. The passage of current causes changes in electrical potentials across retinal neuronal cell membranes, which can initiate retinal neuronal action potentials, which are the means of information transfer in the nervous system.

Based on this mechanism, it is possible to input information into the nervous system by coding the sensory information as a sequence of electrical pulses which are relayed to the nervous system via the prosthetic device. In this way, it is possible to provide artificial sensations including vision.

Some forms of blindness involve selective loss of the light sensitive transducers of the retina. Other retinal neurons remain viable, however, and may be activated in the manner described above by placement of a prosthetic electrode device on the inner (toward the vitreous) retinal surface (epiretinal). This placement must be mechanically stable, minimize the distance between the device electrodes and the retinal neurons, and avoid undue compression of the retinal neurons.

In 1986, Bullara (U.S. Pat. No. 4,573,481) patented an electrode assembly for surgical implantation on a nerve. The matrix was silicone with embedded iridium electrodes. The assembly fit around a nerve to stimulate it.

Dawson and Radtke stimulated a cat's retina by direct electrical stimulation of the retinal ganglion cell layer. These experimenters placed nine and then fourteen electrodes upon the inner retinal layer (i.e., primarily the ganglion cell layer) of two cats. Their experiments suggested that electrical stimulation of the retina with 30 to 100 uA current resulted in visual cortical responses. These experiments were carried out with needle-shaped electrodes that penetrated the surface of the retina (see also U.S. Pat. No. 4,628,933 to Michelson).

The Michelson '933 apparatus includes an array of photosensitive devices on its surface that are connected to a plurality of electrodes positioned on the opposite surface of the device to stimulate the retina. These electrodes are disposed to form an array similar to a “bed of nails” having conductors which impinge directly on the retina to stimulate the retinal cells. U.S. Pat. No. 4,837,049 to Byers describes spike electrodes for neural stimulation. Each spike electrode pierces neural tissue for better electrical contact. U.S. Pat. No. 5,215,088 to Norman describes an array of spike electrodes for cortical stimulation. Each spike pierces cortical tissue for better electrical contact.

The art of implanting an intraocular prosthetic device to electrically stimulate the retina was advanced with the introduction of retinal tacks in retinal surgery. De Juan, et al. at Duke University Eye Center inserted retinal tacks into retinas in an effort to reattach retinas that had detached from the underlying choroid, which is the source of blood supply for the outer retina and thus the photoreceptors. See, e.g., E. de Juan, et al., 99 Am. J. Ophthalmol. 272 (1985). These retinal tacks have proved to be biocompatible and remain embedded in the retina, and choroid/sclera, effectively pinning the retina against the choroid and the posterior aspects of the globe. Retinal tacks are one way to attach a retinal electrode array to the retina. U.S. Pat. No. 5,109,844 to de Juan describes a flat electrode array placed against the retina for visual stimulation. U.S. Pat. No. 5,935,155 to Humayun describes a retinal prosthesis for use with the flat retinal array described in de Juan.

Retinal implants receiving power from an external unit through an inductive power link coupled through coils are known. When the coil sizes and positioning are limited by the physical conditions, the power delivering efficiency can be reduced dramatically, in which case the maximum power to the implant may be limited. On the other hand, a higher amount of power used by the implant also means a worse condition in terms of thermal dissipation.

Both of the situations above require a reduced power demand by the implant stimulator. For applications that need a large number of stimulation channels, such as in the case retinal prosthesis, the efficiency between the output current and the input power becomes critical.

SUMMARY

According to a first aspect of the present disclosure, a power control system for an implant on a human or animal body is shown, comprising: a charging circuit to provide power to deliver controlled stimulation currents to a tissue of the human or animal body; a capacitive storage arrangement connected with the charging circuit and charged by the charging circuit; a shunting arrangement to limit voltage on the capacitive storage arrangement; a driver array configured to transfer charges from the capacitive storage arrangement to the tissue; and an electrode array connected with the driver array and the tissue.

According to a second aspect of the present disclosure, a shunting circuit to regulate capacitor voltage in an implant for a human or animal body is shown, the implant comprising a capacitive storage arrangement, the shunting circuit comprising: a current sensor comprising terminals connected in parallel with the capacitive storage arrangement, the current sensor sinking current from the capacitive storage arrangement when the voltage of the capacitive storage arrangement reaches a voltage value.

According to a third aspect of the present disclosure, a constant current type electrode driver for an implant for a human or animal body is shown, the electrode driver comprising: a driver controller to generate anodic and cathodic stimulation switching signals and generate an output current defining a current amplitude for stimulation; a conversion circuit to convert the output current into an anodic current or a cathodic current; and a switching arrangement to allow selection between the anodic current or the cathodic current.

According to a fourth aspect of the present disclosure, a compliance monitoring circuit to monitor and control compliance voltage of an electrode contacting a tissue of a human or animal body is shown, the electrode being connected with an electrode driver comprising an output MOSFET transistor having a drain-source voltage Vds, a gate-source voltage Vgs and a threshold voltage Vt, the compliance monitoring circuit monitoring a condition Vgs−Vds>Vt and generates an alert signal when such condition is met.

According to a fifth aspect of the present disclosure, a power monitor to monitor charging and draining conditions of i) a capacitive storage arrangement and ii) shunting circuits comprised in an implant for a human or animal body is shown, the power monitor comprising: a current monitor, the current monitor comprising an analog to digital converter digitizing analog inputs from the shunting circuits and outputting an output current level; and a capacitor monitoring circuit, to monitor whether implant power falls below a power value.

According to a sixth aspect of the present disclosure, a method to control power in an implant for a human or animal body is shown, the implant comprising electrodes contacting a tissue of the body, the method comprising: capacitively storing electric charges; providing the electric charges to the electrodes; monitoring when the electric charges are above a high value or below a low value; and controlling the electric charges when above the high value or below the low value.

According to a seventh aspect of the present disclosure, a circuit is shown, comprising: an electrode driver array; storage capacitors connected with the electrode driver array, wherein charge stored on the storage capacitors is adapted to be transferred to an array of electrodes connected with the electrode driver array; a charging circuit charging the storage capacitors; and a monitoring circuit to continuously monitor voltage on the storage capacitors and to control at least one between the electrode driver array and the charging circuit based on the voltage.

DETAILED DESCRIPTION

FIGS. 1-1,1-2and1-3are a diagram of an implant power control scheme in accordance with an embodiment of the present disclosure.

A retinal implant10receives power from an external unit20through an inductive power link30coupled through coils. Control and status information are exchanged between retinal implant10and external unit20through data link40. While the embodiment ofFIGS. 1-1,1-2,1-3and the following figures is concerned with a retinal implant, the person skilled in the art will appreciate that the same scheme can be used also for other types of implants on the human or animal body, such as cochlear implants or implants to restore neuronal functions impaired due to injuries or diseases.

Power at the implant side is received by implant coil50. Implant coil50is tunable with capacitor C3to the power carrier frequency. The received AC power is converted into DC power by a rectifier circuit60. Rectifier circuits are known per se to the person skilled in the art. In the case at issue, the rectifier circuit60can comprise, by way of example, diode bridges or MOSFET circuits.

The output V+, VM, V− of rectifier circuit60provides the power to deliver controlled stimulation currents to the tissue70. A small portion of the output of rectifier60can be tapped out or diverted to supply circuits for other operations through a regulator circuit75. Such operations can include RF data receiving and transmitting, logic control, signal measurements and so on.

The output of rectifier circuit60continuously charges two capacitive storage arrangements which are shown as C1and C2inFIG. 1-2provided to supply all electrode drivers for bi-phase or multi-phase stimulation. Those capacitive arrangements could also be represented as arrays of capacitors or storage devices based on a capacitive behavior to boost output power.

The voltages of C1and C2are limited by shunt regulator circuits80and90, respectively. The charge stored on C1and C2is transferred to the tissue70through a plurality of electrode drivers100-iforming a driver array100comprised of electrode drivers100-1,100-2, . . .100-i, . . .100-n. The drivers act as a controlled energy transporter to allow stimulation of the tissue70in form of bursts of biphasic (anodic and cathodic) current pulses. Each driver can comprise, for example, a constant current source or sink circuit, as later shown inFIG. 2. Each driver is connected to a respective stimulation electrode245-i(see alsoFIG. 2). The plurality of stimulation electrodes245-iforms a stimulating electrode array245in direct contact with the tissue70, as shown inFIG. 1-2. As also explained above, while the present embodiment makes reference to a retinal tissue, other types of human or animal tissue can be envisaged by the person skilled in the art.

The present disclosure provides for monitoring and control of excessively low capacitor voltage. In particular, when charges are transferred from capacitors C1, C2to the tissue70, the voltage on the capacitors drops. A minimum value for this voltage is defined by a so-called compliance voltage. The compliance voltage is defined by the electrode-tissue impedance at the interface245-70and the stimulating current225flowing between a driver100-iand a respective electrode245-i. If the voltage on C1and C2falls too close to the compliance voltage, the respective driver100-iwill not be able to maintain the required amplitude of the stimulation current255.

In order to prevent this from happening, a compliance monitor circuit110is provided. Circuit110monitors occurrence of the above situation and notifies the external unit20through the back telemetry160-180to lower the stimulation current amplitude255by way of external controller700and the transceiver710and forward data link40, or increase the capacitor voltage accordingly by way of the external controller700and coil driver720.

The implanted device10is continuously powered and controlled by the external unit20through the inductive power link30and the data link40, respectively. The external unit20comprises an external controller700, a coil driver720and a data transceiver710. The external controller700can include an information collector such as a camera in the case of retinal prosthesis, a microphone in the case of cochlear prosthesis, or some other form of sensory devices such as pressure, position or touch sensors for various other neuronal-stimulation applications. The external controller700can include a Digital Signal Processing Unit or a similar operation processor to synthesize the sensed information from the sensors and the feedback information from the implant10and generate controls accordingly to command the implanted device to deliver appropriate stimulation (amplitude and timing) to the tissue through the data transceiver710. The data transceiver710ensures that the commands from the external controller700are delivered to the implant10reliably and the feedback from the implant10is received correctly. The data transceiver710communicates with the implant10in predefined communication protocols through its data antenna730. In the meantime, the coil driver720ensures that adequate but not excessive power is delivered to the implant10for the intended stimulation intensity.

Also excessively high capacitor voltage is monitored, to avoid use of unnecessary high power to deliver the same charge. In particular, shunt regulators80and90include a circuital arrangement to program the level of the nominal capacitor voltage to a required value, for further power saving, as later discussed with reference toFIG. 4.

A power monitor circuit120is further provided, to monitor the charging and draining conditions of the capacitors, so that the external unit20can optimize the RF powering condition (see coil driver/monitor720) and also stop stimulation when the implant10cannot be adequately powered. The power monitor circuit120will be explained in greater detail inFIG. 7.

The implant10further comprises an implant controller130and an implant transceiver140. Implant controller130comprises a main controller150and a back telemetry (BT) controller160. Implant transceiver140comprises a data receiver170and a back telemetry (BT) transmitter180.

Implant controller130and implant transceiver140allow the stimulation control and power control to operate on a system level. In other words, they allow the external unit20to appropriately and accurately control the implant powering and stimulation.

The data receiver170receives forward telemetry (FT) data from the external unit20through data link40. FT data is decoded by the main controller150to control the output of the electrode drivers100, the rail voltages (as later shown inFIG. 4) and other operations, such as monitoring of the electrodes and the device status for safety reasons or for conducting system tests, and so on.

BT controller160collects and encodes implant information from the compliance monitor110and the power monitor120, such as implant powering, stimulation, electrode condition and other safety information of the implant, e.g. device failure, electrode failure, excessive power condition etc. BT controller160sends the implant information back to the external unit20through BT transmitter180, so that the system can act on such information.

For example, the output of the power monitor120is fed to the BT controller160, which will include this information in the back telemetry stream. If the external unit20, upon receiving the back telemetry data, determines that the currents flowing through the shunt regulators80,90are too high, it will adjust the input to the coil driver720to lower the level of power driving the coil by an amount that is predetermined by the power control protocol. After the coil power is lowered, the shunt regulator currents will decrease, thus forming a closed control loop.

A possible type of electrode driver100-ito be used in the present disclosure is a constant current type electrode driver. Current type electrode drivers output current pulses whose timing, duration and amplitude can be controlled through input commands. See, for example, “A Neuro-Stimulus Chip With Telemetry Unit For Retinal Prosthetic Device” by Liu, W., Vichienchom, K., Clements, M., DeMarco, S. C., Hughes, C., McGucken, E., Humayun, M. S., De Juan, E., Weiland, J. D., Greenberg, R., Solid-State Circuits, IEEE Journal of, Volume 35, Issue 10, October 2000, pages 1487-1497.

In this respect,FIG. 2shows a more detailed circuital diagram of one of the electrode drivers100-iofFIG. 1, the electrode driver being a constant current electrode driver.

Electrode driver100-icomprises a driver controller190. The driver controller190takes commands from the ‘StimControls’ signals195coming from the main controller150and translates those signals into anodic and cathodic stimulation switching signals200and210that control pulse duration and timing. Driver controller190also provides an output current220that defines the current amplitude of the stimulation phases. MOSFET transistors M16and M18convert current220into anodic current230. MOSFET transistors M18, M15, M11and M12convert current220into cathodic current240. MOSFET transistors M13, M14and M17act as output switches.

As already explained with reference to previously discussedFIGS. 1-1through1-3, electrode driver100-ireceives power from storage capacitors C1and C2(see voltage signals V+, V− shown inFIG. 2). Storage capacitor C1provides power for anodic pulses. Storage capacitor C2provides power for cathodic pulses.

Similarly toFIG. 1-2,FIG. 2shows a return electrode250. The return or common electrode250is connected to the junction Vice between C1and C2. The cathodic stimulation currents240, taken from C1, flow from the return electrode250, through the tissue70, to the V− rail of C1. The anodic stimulation currents230, taken from C2, flow from the V+ rail of C2, through the tissue70, to the return electrode250. The electrical properties of the electrode-tissue interface can be modeled to a certain degree of accuracy by the simplified RC network shown in the shaded tissue circle.

FIG. 3is a timing diagram showing the relationship between the stimulation timing, stimulation currents, surplus currents and voltages on capacitors C1and C2.

During the cathodic stimulation phase260, the cathodic stimulation switching signal210(first graph onFIG. 3from the top) is ON, thus turning ON both MOSFETs M13and M14(see alsoFIG. 2). On the other hand, the anodic stimulation switching signal220(second graph onFIG. 3from the top) is OFF during that time interval, thus keeping the MOSFET M17and the anodic current230OFF. Meanwhile, output current signal255(third graph onFIG. 3from the top) carries the programmed cathodic amplitude control current270(FIG. 3), so that a current240(FIG. 2) proportional to signal220(FIGS. 2 and 3) will flow from the return electrode250to V− through MOSFETs M14, M12and the tissue70(FIG. 2), causing a stimulation. When cathodic current240flows, charge is drawn from C1, causing a dip ΔV280on the capacitor voltage VC1, as also shown in the VC1graph ofFIG. 3.

The charge Q delivered during a stimulation phase is
Q=Cathodic Current 240×Duration Switching Signal 210 ON

To the capacitor C1, a loss in charge Q means a drop in voltage ΔV
ΔV=Q/C1=(Cathodic Current 240×Duration Switching Signal 210 ON)/C1

If the capacitor C1is being constantly charged with a current Icharge, the voltage on C1at the end of the stimulation will be
ΔV=(Cathodic current 240−Icharge)×Duration Switching Signal 210 ON/C1

During the anodic stimulation phase290, the anodic stimulation switching signal200(second graph onFIG. 3from the top) is ON, thus turning ON MOSFET M17, while stimulation switching signal210keeps the cathodic circuit OFF during that time interval. Output current signal255carries the programmed anodic amplitude control current300(FIG. 3), so that a current230(FIG. 2) proportional to signal255(FIGS. 2 and 3) for the anodic phase will flow from V+ to the return electrode250through MOSFETs M16and M17and the tissue70(FIG. 2), releasing the charge collected on the stimulating electrode245during the cathodic phase260and achieving electrical chemistry balance. When anodic current230flows, charge is drawn from C2, causing a dip ΔV310on the capacitor voltage VC2, as also shown in the VC2graph ofFIG. 3. Similarly to what discussed above, the voltage drop ΔV on capacitor C2is
ΔV=Q/C2=(Anodic Current 230×Duration Switching Signal 200 ON)/C2

It should be noted that when power is supplied to the drivers100, the charges on capacitors C1and C2are drawn only during the stimulation pulses (see time intervals260and290ofFIG. 3), but are being injected by the power charging circuit60all the time. Therefore, the charging current can be controlled to inject sufficient charges to the capacitors as needed by the stimulation, to minimize the value of surplus currents IshL and IshH to achieve good power efficiency. The charging current is controlled through control of the power driving the external coil and the shunt regulator current value included in the back telemetry data as the output of the control closes the power control loop.

As also shown inFIGS. 1-2and2, the capacitor voltages V+, V− are the supply rails to the electrode drivers100-i. According to a further embodiment of the present disclosure, the capacitor voltages can be limited. Limitation of those voltages can be applied for two reasons. A first reason is that the electronic chip on which the implant10is operated should operate under a specific voltage limit for safety reasons. A second reason is that the power needed to inject a certain amount of charge in the capacitor is proportional to the capacitor voltage itself. Therefore, limiting the capacitor voltage to a level just satisfying the need of electrode compliance voltages will also allow power to be saved.

As also explained with reference toFIGS. 1-1,1-2and1-3, the voltages of C1and C2are limited by shunt regulator circuits80and90, respectively. In particular, shunt regulator circuits80and90can limit the capacitor voltages to a preprogrammed value, during charging. After the voltage on capacitors C1and C2reaches the safety voltage limit, circuits80and90will bypass (shunt) the surplus charging current and the capacitor voltages will not rise further. The shunting circuits80and90may also comprise a current tap out, which provides a quantitative indication of the surplus current. Such indication can be used to estimate the power condition of the implant10. Shunting circuits80and90can also comprise a rail control mechanism, which allows the external unit20to set the capacitor voltage limit through binary control bits.

FIG. 4shows in greater detail the internal structure shunt regulator circuits80,90, in accordance with a further embodiment of the present disclosure. The shunting circuit comprises a current sensor and a current sink. In the embodiment ofFIG. 4, the current sensor is comprised of a diode stack D1-D11in series with a sampling resistor R1, and the current sinking circuit is comprised of an npn transistor amplifier Q1. During operation, the first terminal VH of the current sensor is connected with either V+ or VM (seeFIGS. 1-1,1-2and1-3), while the second terminal VL of the current sensor is connected with VL either VM or V− (seeFIGS. 1-1,1-2and1-3). Therefore, during operation, the two terminals of the current sensor are connected in parallel with the capacitors C1, C2. When the voltage across each capacitor reaches a value that is greater than the sum of the “knee” voltages of PN junction diodes D1-D11, the current through the diodes increases rapidly, so as the voltage across sampling resistor R1and the base current of transistor Q1. The common emitter transistor Q1will then sink a larger current through its collector that is determined by the current gain of the transistor. The sensitivity of the current sensor is controlled by resistor R1. The person skilled in the art will understand that transistor Q1can be replaced by any other arrangement suitable for the same purpose. For example, if CMOS technology is used, the current gain of a single transistor may be limited, and Q1could be replaced with a Darlington pair and/or a transistor array to increase the current gain for a better shunt effect. The person skilled in the art will also understand that the number of diodes D1-D11can be any number, depending on the highest rail voltage specified.

The shunting circuit ofFIG. 4also comprises a transistor Q2and a current mirror MOSFET circuit M1-M4. In particular, Q2and M1-M4provide a current tap out Ishunt for quantitative measurement of the surplus current flowing through the shunting circuit ofFIG. 4. The output signal Ishunt is sent to power monitor120, as already shown inFIGS. 1-1,1-2,1-3and also later shown inFIG. 7.

The embodiment ofFIG. 4also provides for the presence of a rail control mechanism along connection RC between main controller150and shunting circuits80,90(seeFIGS. 1-1,1-2and1-3). In particular, control of a rail voltage can be obtained by selectively shorting a certain number of diodes of the diode chain in the shunting circuit using MOSFET switches (see switches S1-S3inFIG. 4). As shown inFIG. 4, the connection of the switches S1-S3with the diodes D1-D11provides a direct binary encoding, allowing the rail voltage limit to be conveniently programmed without the need of an encoding interface. It should be noted that the resolution of the rail voltage control is one diode voltage drop which is logarithmically proportional to the diode current. In the embodiment shown inFIG. 4, only a small portion of the surplus current will flow through the diode chain D1-D11. Therefore, the diode voltage drop is close to the turn-on voltage for the operating diodes, and the error caused by the ON resistance of the switches for shorted diodes is insignificant.

When the electrode voltage reaches the compliance limit due to increased electrode impedance, the output current255of the electrode driver cell (see, e.g.,FIG. 2) will deliver less amount of current than what is set to be. This situation will affect the output accuracy of the implant10and is also likely to result in unbalanced current pulses that are not favorable to the electrodes245or the tissue70. The electrode voltages, however, are determined by the in vivo electrode-tissue impedances which can vary greatly over time or among different electrodes, thus making any preset limit inefficient.

A further embodiment of the present disclosure addresses the above issue by providing a compliance monitoring circuit as already mentioned with reference to circuit110ofFIG. 1-3, which circuit is shown inFIG. 5in greater detail. Although the compliance monitoring circuit110ofFIG. 1-3has been shown separately from the driver array100, such circuit, portions of which are represented inFIG. 5with reference numerals320-iand325, can be embedded into each current driver cell100-i. Therefore, according to a further embodiment of the present disclosure, there will be one compliance monitoring circuit per driver. Generally speaking, the compliance monitor320-i/325monitors the voltage of electrode245to see if such voltage has reached the compliance limit. When such limit is reached, the compliance monitor generates a signal CompAlert340(FIGS. 1-3and5) to alert the BT controller160and/or the external unit20, so that a decision as to whether the rail voltage should be increased or the stimulation current decreased can be made.

Generally speaking, ignoring the short channel effect of the MOSFET, the drain output current of the electrode driver is set by a gate-source voltage Vgs but not a drain-source voltage Vds as long as the output MOSFET is in the saturation region, i.e., when the drain-source voltage Vds is larger than the gate-source voltage Vgs subtracting the threshold voltage Vt of the device. The drain current falls off rapidly as the drain-source voltage decreases further. This condition to hold the drain output current constant can be written as:
Vds>=Vgs−Vt(1)

In the electrode driver circuit, the above condition of drain-source voltage Vds of the output MOSFETs, together with the power supply rail voltage Vp, defines the output compliance limit of the electrode stimulator. For example, if the power supply voltage is 10 volts, the threshold voltage Vt is 1 volt, and the gate-source voltage Vgs is set at 1.5 volts, then the compliance limit can be defined as Vcomp=Vp−(Vgs−Vt)=9.5 volts. The compliance monitor circuit according to the present disclosure detects the condition of failing to meet (1), i.e., it detects the condition that fulfils:
Vds<Vgs−Vt(2),
orVgs−Vds>Vt(3)

The circuit shown inFIG. 5is a direct realization of the condition of equation (3). In a cathodic phase first stimulation protocol, the voltage on the electrode245tends to shift towards a negative value relative to the return electrode250because of the capacitive component in the electrode impedance. Therefore, the sink part (MOSFETS M11-M14) of the current driver is more susceptible to reaching the compliance limit.

In the circuit ofFIG. 5, MOSFET M12is the output transistor that is responsible for maintaining a stable current output. Vgs represents the gate-source voltage difference of M12, while Vds represents the drain-source voltage difference of M12. The compliance monitor circuit320monitors the difference Vgs−Vds of M12by way of a subtracting circuit350whose output is compared with the threshold voltage Vt reference360through comparator370. The output of comparator370drives an open drain nMOS transistor M21whose output CC is fed to node Vc, where the outputs of the compliance monitors of all drivers100-1,100-2. . . ,100-i, . . .100-nare line OR-ed through MOSFET M100.

In normal operation, equation (3) is not fulfilled, and the output of comparator370is LOW, shutting off M21so that its output CC does not affect node380. However, if the output voltage of the electrode245reaches the compliance limit, equation (3) is satisfied and the output of comparator370becomes HIGH, turning on M21, so that its output CC pulls down the voltage on node380to LOW. The voltage change on node380will be sensed by a compliance logic circuit390to generate the CompAlert signal340discussed above.

FIG. 6is a timing diagram for signal210, output current255, Vds, Vgs−Vds, CC and CompAlert, showing the behavior of those signals during operation of the compliance monitoring circuit discussed with reference toFIG. 5.

According to a further embodiment, the present disclosure also provides for power monitoring circuits120to monitor the charging and draining conditions of the capacitors, as already explained with reference toFIGS. 1-1,1-2and1-3. In particular, the power monitoring circuits monitor the surplus currents flowing through the shunting circuits80,90and the voltages on the capacitors C1, C2. Monitoring the surplus current allows a flexible, quantitative control of the inductive power loop, i.e. the loop including the coil driver, the coil pair, and the power storage capacitors.

For example, the power level can be predefined (by way of a feedforward method) or adaptive to the load requirement (by way of feedback information). In addition, monitoring of the storage capacitor voltages prevents the implant device10from operating with an insufficient power condition that may compromise safety.

With reference toFIG. 4, applicants have already discussed how the surplus current is tapped out as Ishunt through Q1and M1-M4in the shunting circuits80,90.FIG. 7shows a current monitor480comprising an analog-to-digital converter (ADC)490that digitizes the analog Ishunt inputs IShH and IshL received from shunting circuits80and90(see alsoFIG. 1-2). The converted digital output I_Level is sent to BT controller160and eventually reported to the external unit20.

Reference can also be made to the flow chart shown in the followingFIGS. 8-1and8-2, where this current data is checked against a preset power level (threshold) (see steps S4and S20ofFIG. 8-1) and a decision is made as to whether increase or decrease the power output of the RF coil driver (see the left portion ofFIG. 8-1).

Meanwhile, the voltages on C1and C2are also monitored by a capacitor monitoring circuit500comprising a subtracting circuit510, comparators520,530and NOR logic540. In normal power conditions, the output CapLevel of the capacitor monitoring circuit is HIGH. If either one of the capacitor voltages VC1, VC2falls below a preset threshold Vref (depending on the implementation of the circuit), the output CapLevel becomes LOW. The LOW value of CapLevel indicates that implant power has fallen below a critical low level required for safe operation. Upon receipt of a CapLevel LOW signal, the external control will have to stop the stimulating operation of the implant and raise the RF power output level until CapLevel reaches again and stays at a HIGH value.

FIGS. 8-1and8-2are a flow chart showing the system power control flow. During start-up step S1the PowerLevel limit (high and low threshold values of the digital output of ADC490inFIG. 7. to be checked against) and the rail control bits (seeFIG. 4) are set. Step S2checks the value of the CapLevel signal discussed inFIG. 7. If the value is HIGH, the implant is under normal operating conditions and the flow proceeds to step S3where the value of the CompAlert signal340ofFIG. 5is checked, to see whether the voltage on electrode245has reached the compliance limit. In case such limit has not been reached the flow proceeds to step S4to check whether the power (represented by the surplus current from ADC490) has exceeded the high threshold of PowerLevel limit set in step S1. Should this not be the case, the flow proceeds to step S20to check whether the power has fallen below the low threshold of the PowerLevel limit. If not (meaning that the implant power is within the expected range), the flow goes back to step S2for continuous monitoring. However, if the answer to the determination made in step S20is affirmative, the flow first checks if the power driving the external coil (RF Power) has reached its maximum level (an external unit specification) in a step S21. If it has not reached the maximum level, the coil drive is increased by an amount predefined in the control protocol in a step S22and the flow goes back to step S2to continue a next round of monitoring of the implant power. If, on the other hand, the power driving the external coil has reached the maximum level so that no further increase of driving power is allowed, a low power alert of the implant is asserted in step S23.

On the other hand, if the answer to the question of step S4is positive, which means that the implant power is excessive, the flow moves to step S5where it is checked if the power driving the external coil is at its minimum level. If the power is not at its minimum, such power is lowered in step S6(lower coil drive) and the flow goes back to step S2. On the contrary, if the RF driving power level is already at its minimum, indicating that the PowerLevel overshoot of step S4could be due to malfunctioning of the external unit, safety measures are activated in step S7. Turning back to step S2, if the value of the CapLevel signal ofFIG. 7is LOW, it indicates a critical low power level that could be due to poor coupling of the coils, insufficient rail voltages, excessive stimulation current, or malfunction in the powering loop that includes the external coil driver, the coils and the implant circuits. When this happens, the flow first proceeds to step S8where it is checked if the power driving the external coil has been at its maximum. If that condition is satisfied, the flow proceeds to step S9where the rail voltage is checked. If also that condition is satisfied (i.e. rail voltage at its maximum value), safety measures are activated in step S10to stop the stimulation operation. If, on the other hand, the rail voltage is not at its maximum value, such voltage is increased at step S11and the flow goes back to step S2. Similarly, if the coil driving power checked in step S8is not at its maximum, such power is increased in step S12and the flow goes back to step S2. Turning to step S3, in case the compliance limit has been reached, the flow moves to step S13where it is checked if the rail control has set the rail voltage to its maximum value. If yes, the electrode impedance is checked at step S14and the maximum stimulation current is recalculated accordingly at step S15. Once this has been done, the flow moves to step S4, already discussed above, to check the power level. Turning to step S13, if the rail voltage is not at its maximum value, such value is increased at step S16and the flow moves to step S4already discussed above.

While several illustrative embodiments of the invention have been shown and described in the above description, numerous variations and alternative embodiments will occur to those skilled in the art. Such variations and alternative embodiments are contemplated, and can be made without departing from the scope of the invention as defined in the appended claims.