Abstract:
The output stage of a tissue stimulating apparatus, for example a cochlear implant prosthesis, operating at a low supply voltage ( 35 ) incorporates a multiplier circuit ( 54, 62, 63, 64 ) for ensuring that voltage compliance is maintained in the event that high intensity stimulations are required. The multiplier circuit makes use of compliance monitoring so that multiplication is only used as required. Also described is a method for operating a tissue stimulating apparatus incorporating a multiplier circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to a neural or muscular tissue stimulating prosthesis capable of delivering a high current stimulation signal to a nerve, or brainstem, of a patient and, more particularly, to a cochlear prosthesis with a power supply having an output which can be selectively boosted to a high level. 
     2. Description of the Prior Art 
     Though the subject invention will find application with many types of tissue stimulating device it will be described in relation primarily to cochlear prosthesis systems. These prostheses are used to provide therapy to patients suffering from certain hearing impairing conditions. Frequently such systems are of a “two-part” design in that they comprise two sections: an internal or implanted section, and an external section. The external section includes a microphone for receiving ambient sounds and converting them to electrical signals. Power to the external section is provided by a battery. The electrical signals are processed and sent to the implanted section. The implanted section then generates excitation signals to excite the aural nerve of the patient by means of appropriately positioned stimulation electrodes. 
     Most commonly, the external section of a two-part cochlear prosthesis is inductively coupled by a transcutaneous RF link to the implanted section. The energy of the electrical signals in the RF frequency range is rectified and stored by a power supply located in the internal section. It is that power supply which provides the energy required to power the internal section and to generate the stimulus signals. 
     More recently there has been a trend in cochlear prosthesis design towards the use of totally implantable prostheses. In such devices the entire cochlear prosthesis, including a battery, is implanted. Obviously it is highly desirable that a totally implanted cochlear prosthesis be of as small a size as possible. In order to achieve the necessary miniaturisation it is important that the power supply, and so by necessity the battery, be of a small size. 
     To minimise the power requirements of the implanted section of a cochlear prosthesis, whether it be of the totally implanted or of the two-part type, it is desirable to operate it at as low a voltage as possible. One problem however with this approach is that a minimised voltage may present difficulties for the circuitry which is to apply the stimulation currents. In particular, a low operating voltage has hitherto reduced the maximum available amplitude of the stimulating signals that may be generated. An undesirable result is that the dynamic range of the stimulation signals conveyed to the patient is reduced so that loud sounds are perceived by the patient as being quieter than they should be. 
     Another problem, which is relevant only to cochlear prostheses of the two-part type, is that power supply voltage within the internal section is sensitive to the relative position and spacing of the coils used for the inductive coupling of the internal and external sections. When this positioning is not correct, the intercoil coupling is not optimal, and therefore the available power in the implanted section drops resulting in a limitation of the amplitude of the stimulation current that can be generated into the electrodes. 
     The problem of insufficient power being available to deliver the appropriate stimulations is especially acute for cochlear prostheses using biphasic stimulation current pulses. These pulses consist of two consecutive phases of opposite polarities with the first phase having a higher peak voltage amplitude than the second phase, due to the capacitive component of the load. If the power supply for the internal section has an inadequate voltage level (i.e., the power supply has a compliance problem), the current during the first phase of a pulse is smaller than required while the current during the second, lower voltage phase, remains unchanged thereby resulting in an unbalanced stimulation pulse. 
     In order to resolve these problems it has been proposed that, when sufficiently high voltage levels are not available, the duration of the biphasic pulse be increased to compensate, so that the charge delivered during each current phase remains approximately constant. However, the use of longer stimulation pulses inherently reduces the maximum stimulation rate of the device and so is undesirable. 
     OBJECTIVES AND SUMMARY OF THE INVENTION 
     In view of the above-named disadvantages of the prior art, it is an objective of the present invention to provide a tissue stimulating system with improved power supply in order to eliminate non-compliant episodes, i.e. episodes in which stimulation pulses are applied which are of less-than-desired current. 
     A further objective is to provide a tissue stimulating system which selectively increases the voltage available to the stimulation electrodes to a level sufficient to provide suitable cochlear stimulation. 
     Other objectives and advantages of the invention shall become apparent from the following description. 
     In particular, a cochlear prosthesis system constructed in accordance with this invention includes a means for receiving and processing ambient sounds to generate processed signals which are applied to the aural nerve through an electrode array. Importantly, the generation of the output signal being delivered to an electrode is monitored and if it is determined to be insufficient a voltage multiplier scheme is used to boost the voltage of the power supply to a high level temporarily, thereby ensuring that the output current can reach the required value. 
     According to a first aspect of the invention there is provided a tissue stimulating system of the type wherein a power supply and at least one programmable current source are provided for generating a stimulation current of predetermined amplitude, said system further comprising: 
     a booster circuit for selectively boosting the supply voltage of said power supply when said supply voltage is insufficient to allow said current source to provide a predetermined stimulation current. 
     According to another aspect of the invention there is provided a cochlear prosthesis system including: 
     a microphone for picking up ambient sounds; 
     signal processing circuitry coupled to the microphone for determining stimulation signals corresponding to said ambient sounds; 
     an electrode array for applying said stimulation signals to the nervous system of a patient; 
     a power supply for providing energy for the generation of said stimulation signals; 
     an energy storage device arranged and constructed to selectively supplement said power supply; 
     a sensor for monitoring the generation of said stimulation signals to determine if said power supply requires supplementation by said energy storage device in order to enable generation of said determined stimulus signals; and 
     a multiplier switching circuit for selectively switching said energy storage device so that it supplements the voltage provided by the power supply in order to ensure that stimulation signals as determined by the signal processor are generated. 
     Finally, according to yet another aspect of the invention there is provided a method for improving the compliance of the stimulation current output stage of a tissue stimulating system, said output stage including at least one programmable current source and power supply, said method including the steps of: 
     a) determining if the voltage across each said current source is sufficient to allow generation of a stimulation current of desired amplitude; 
     b) in the event that said voltage is determined to be insufficient in step a), placing a charged energy storage device in series with said power supply so as to increase the potential voltage across each said programmable current source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a block diagram of a prior art two-part cochlear prosthesis system; 
     FIG. 1B shows a block diagram of a totally implanted cochlear prosthesis system; 
     FIG. 2 shows a block diagram of a prior art output stage of a two-part cochlear prosthesis system; 
     FIG. 3 shows a block diagram of an output stage of a cochlear prosthesis system, either of the two-part or totally implanted type, constructed in accordance with the present invention wherein the standby power supply is in the idle mode; 
     FIG. 4 shows a block diagram of the output stage of a cochlear prosthesis system constructed in accordance with this invention wherein the standby power supply is in the active mode to boost the primary power supply; 
     FIG. 5 shows a pair of desired stimulation current waveforms, the second waveform resulting in an otherwise non-compliant condition; 
     FIG. 6 shows the current that is required to flow in the current source to generate the desired stimulus current of FIG. 5; 
     FIG. 7 shows the corresponding voltages at nodes n 1  and n 2  of FIGS. 3 and 4; 
     FIG. 8 shows the corresponding voltage waveforms across the current source; 
     FIG. 9 shows a generalised block diagram of an embodiment of the invention; and 
     FIG. 10 shows an alternate embodiment of the invention. 
     FIG. 11 depicts a further embodiment of the invention. 
     FIG. 12 depicts a flowchart of the operation of the embodiment of FIG. 11 in one mode. 
     FIG. 13 depicts a flowchart of the operation of the embodiment of FIG. 11 in another mode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1A, an example of a tissue stimulating device being a cochlear prosthesis is depicted. A cochlear prosthesis system  10  of the two-part type consists of an internal or implanted section  12  and an external section  14 . The external section  14  includes a microphone  16 , a signal processor  18 , an encoder  20 , and a data and power transmitter  23  as well as a patient map memory  22  and external section power supply  24 . Briefly, ambient sounds are picked up by microphone  16  and sent to the signal processor  18 . The signal processor detects various components from these sounds and adjusts them according to patient map memory  22  which stores data concerning the patient&#39;s perceptual response to stimulation by the cochlear prosthesis. The adjusted data is sent from signal processor  18  to data encoder  20 . The encoder converts the data from the signal processor into a serial stream of binary data. A radio frequency signal is amplitude modulated by this data in the power transmitter  23  and transmitted via an RF link to the internal section  12 . The RF link consists of two coupled coils  28 ,  30 . 
     In addition to coil  30 , internal section  12  includes a tuning capacitor  32  and rectifier consisting of diode  36  and storage capacitor  34 . Coil  30 , tuning capacitor  32 , diode  36  and storage capacitor  34  comprise the power supply for the internal section. The electrode array control circuitry  40  includes electrode switching control circuits  44  and a programmable current source  48 . The data decoder  38  receives the signal transmitted from coil  28  to coil  30  and sends commands to the array control circuitry  40 . In response, stimulation signals are applied by the control circuitry  40  to a cochlear electrode array  46 . The array  46  is disposed along the patient&#39;s basilar membrane (not shown). Except as noted below, the operation of the system  10  is described in U.S. Pat. No. 4,532,930, the contents of which are incorporated herein by reference. 
     Referring now to FIG. 1B, therein is depicted a block diagram of a totally implantable cochlear prosthesis. The operation of the prosthesis depicted in FIG. 1B is largely analogous to that of the two-part device shown in FIG. 1A, although, because there is no separation of the prosthesis into external and internal sections the decoder, encoder, RF link and rectifier circuitry which appear in two-part designs are not needed. The modules shown in FIG. 1B are highly similar to those of the device depicted in FIG.  1 A and therefore the common elements have been identified by the same indicia. The totally implantable prosthesis may be entirely powered by an implanted battery power supply  24 . 
     Referring now to FIG. 2, there is depicted a typical prior art output stage as used in two-part cochlear implants for delivering stimulation signals. The arrangement of FIG. 2 includes a plurality of conductors  37 - 1  to  37 -n and electronic switches E 1  . . . En controlled by switching control  44 . Each of the switches E 1  . . . En are used to selectively connect one of the conductors  37 -n of array  46  to either Vdd or programmable current source  48 . The programmable current source  48  is additionally under command of switching control  44  which sets the amplitude of the stimulations to be generated. Power supply  35  comprises either a tuned circuit, rectifying diode and storage capacitor, as described with reference to FIG. 1 in the case of a two-part prosthesis, or a battery and associated circuitry in the case of a totally implantable cochlear prosthesis. 
     In order to deliver a stimulation pulse via conductor  37 - 2 , for example, the corresponding switch E 2  is set to a first position wherein the conductor  37 - 2  is connected to the positive rail Vdd. The return path is established through one of the other conductors, such as  37 - 3  in which case the switch E 3  is set so that it is now connected to the programmable current source  48 . The current source  48  is set by switching control  44  as discussed above. 
     Current source  48  is set so that currents of a specified amplitude are passed through the selected electrodes. After a predetermined time the two switches E 2 , E 3  are flipped to the opposite connection thereby reversing their connections to the Vdd rail and current source  48  and in consequence completing the delivery of a biphasic current stimulus pulse through the tissue. It should be noted that although the direction of current through the tissue is reversed, the direction of current out of current source  48  remains constant. 
     As discussed above, one problem with this arrangement is that under certain conditions the voltage across power supply  35  may be insufficient to allow the proper operation of current source  48 . With reference to FIG. 2, if the voltage across source  48  falls too low then it will not be possible to attain high current amplitudes. This situation may occur for example when the tissue to be stimulated presents a higher than usual impedance or when it is desired to present an unusually high stimulation. In either case, the potential difference across current source  48  will no longer be sufficient for satisfactory operation of the source. Consequently a non-compliance condition will result. FIGS. 3 and 4 show an improved circuit which remedies this problem. 
     FIGS. 3 and 4 include components necessary for the implementation of the present invention in a first embodiment, being multiplier switches S 1 , S 2  and S 3 , labelled  62 ,  64  and  63 , sensing and multiplier switch control circuit  54  and a standby multiplier capacitor C 1 ,  33 . It will be noted that by closing multiplier switches  62  and  63  as shown in FIG. 3 the standby multiplier capacitor  33  is placed in parallel with the power supply  35  and in that position will become charged. Furthermore by opening switches S 1  and S 3  and closing switch S 2 , as shown in FIG. 4, capacitor  33  is placed in series with the power supply  34  so that the potential difference between node n 1 ,  51  and power rail Vdd is greatly increased. Multiplier control circuit  54  may be implemented using digital logic, alternatively it may be implemented as software running on an implanted microprocessor. 
     The operation of the circuit of FIGS. 3 and 4 shall now be described in conjunction with the waveshapes of FIGS. 5-8. FIG. 5 depicts the plurality of current waveforms  70  and  73  requested by the switching control  44  to be passed through the load by programmable current source  48 . It can be seen that each waveform is biphasic, consisting of two current pulses of equal amplitude and opposite polarity. Thus, lower amplitude biphasic current waveform  70  consists of positive and negative pulses  71  and  72  respectively, and higher amplitude current waveform  73  consists of positive and negative pulses  74  and  75 . 
     Next, FIG. 6 depicts the corresponding current waveforms that must pass through the controlled current source  48  to produce the desired stimulation current waveforms  70  and  73 . The current source must pass two lower amplitude square waves  76  and  77  to generate stimulus pulses  71  and  72  respectively, and two larger amplitude square waves  78  and  79  to generate the stimulus pulses  74  and  75 . The current pulse  78  exceeds the capability of the power supply and necessitates a boost, as will be described. In the specific embodiment shown in FIG. 4, this boost is provided by a voltage multiplier as follows: 
     Referring to FIG. 7 the voltage waveform  80  at node n 2  is depicted measured with reference to Vss; and the voltage waveform  88  at node n 1  is depicted also measured with respect to Vss. It can be seen from the shape of the voltage waveform  80  that the load contains a capacitive component. The level Vc marks the minimum voltage across the current source  48  (measured as the voltage at n 2  with respect to Vss) at which compliance with the desired current waveform of FIG. 6 can be maintained. The voltage Vca is a little higher than Vc and is the trigger control voltage at which a voltage compliance alarm is sensed and voltage multiplier action is commenced. At  85  of the graph in FIG. 7 the voltage  80  at n 2  reaches the trigger control voltage Vca. This condition is sensed and, very shortly after, switches  62  and  63  are opened and switch  64  is closed. The reconfiguration of the switches has the effect of placing capacitor C 1  between the current source  48  and Vss, consequently pulling the voltage  88  at n 1  down to a potential Vdd below Vss. This excursion of the voltage  88  at n 1  is shown as the waveform  89  in FIG.  7 . After its initial drop at  85  the voltage  88  on n 1  subsequently slowly decays towards Vss. 
     In FIG. 8 the graph depicted shows the voltage waveform levels measured at n 2  with respect to n 1 . With reference to FIG. 8 it is seen that the voltage level across the current source  48  is significantly increased at time  85  and consequently the current compliance margin is increased from the one shown at  92 , approaching the voltage compliance limit  86 , to the substantial margin  93 . 
     At the completion of the first of the large amplitude pulses  83 , switches S 1  and S 3  are again closed and S 2  opened. They remain in that state until another non-compliance state is detected. 
     FIG. 9 depicts a generalised block diagram of an embodiment of the invention. In this figure a power supply  100  maintains a power rail  102  at a substantially constant voltage Vdd. A settable current source S L    106  sets the current which flows through the load. In standard operation the current flows through a voltage booster  108  which is internally connected to the low potential rail Vss  110 . The voltage Va at point or node  112  is sensed by a compliance sensor  104 , which compares the same to a settable trigger control voltage. Should the voltage across the load decrease so that Va falls below the trigger voltage then the compliance sensor will set the compliance latch  109  which will activate the voltage booster  108 . The booster pulls down the voltage at point B below Vss so that the potential difference across the current source S L ,  106 , is increased thereby ensuring the satisfactory operation of the current source and the maintenance of the desired current through the load. It will be realised by those skilled in the art that other arrangements are also possible, for example a re-arrangement of the circuit would make it possible to increase the potential difference across the current source by raising the positive supply with respect to Vdd rather than lowering the negative supply with respect to Vss. 
     FIG. 10 shows an implementation of the arrangement depicted in FIGS. 2 and 4 and therefore the common elements have been identified by the same indicia. In this embodiment, the sensor and control circuit  54  consists of a current source S C , (of much lower amplitude than S L ) inverters U 3 , U 4 , a FET switch Q 1  and two NOR gates U 1 , U 2  connected to form a standard flip-flop latch. The trigger control voltage is provided at the gate of the FET Q 1  and its source is used to monitor the voltage at n 2 , as shown. 
     Circuit  54  is designed so that when the voltage at n 2  falls below a level set by the trigger voltage on the gate of Q 1  then the FET Q 1  is turned on and the input to U 3  falls low. Consequently, the latch formed by U 2  and U 1  is set and its output goes high. When the output of the latch goes high switch S 2  closes and switches S 1  and S 3  are opened via inverter U 4 . This operation places C 1  in series with the current source S L  which has the effect of pulling the voltage at n 1 , the negative end of current source S L , down and so prevents S L  from entering a non-compliance state. C 1  remains so connected until the end of the stimulation phase at which time the reset input on U 2  is set high, switches S 1  and S 3  close and switch S 2  opens. The capacitor C 1  is then placed in parallel with the power supply capacitor and recharged. The trigger control voltage (which should be set to the compliance alarm voltage Vca plus the turn on voltage of Q 1 ) can be set by any stable reference voltage e.g., a diode-configured FET or a band-gap reference. 
     In the illustrated embodiment the power supply voltage in the internal section is boosted simply and efficiently thereby ensuring that sufficient energy is provided to deliver proper stimulation to the electrodes in a short period of time so that adequate charge is delivered without the need to increase the duration of the stimulation pulses. Moreover, this boosting occurs only when it is required. At all other times a lower voltage is provided thereby reducing the overall power consumption of the system. The switches S 1 -S 3  can be manufactured using a modern CMOS IC technology so that the delay in boosting the voltage can be only a few microseconds or less. 
     Referring now to FIG. 11 there is depicted a further embodiment of the invention in which the voltage doubler is activated under command of a compliance calculator. The compliance calculator takes its input from analog to digital converter  131  which is coupled to differential amplifier  134 . Differential amplifier  134  determines the voltage Vn drop across the electrode—tissue combination, represented by load  107 , of a particular electrode. Given this value of voltage drop, for a current stimulus of particular amplitude, the impedance of the load is readily calculated by compliance calculator  130 . 
     Referring now to FIG. 12 the determination of the threshold current for each electrode by compliance calculator  130  will be explained. This procedure may be performed by the microprocessor that performs the signal processing tasks of the prosthesis or alternatively by a maths co-processor operating under the command of the signal processing microprocessor. 
     At box  146  the electrode counter variable n is initialised to  1 . At box  148  a known current stimulus I test  is applied via the nth electrode. At box  150  the voltage Vn across the load  107  is measured by differential amplifier  134  and converted to a digital value, by ADC  131 , which is coupled to compliance calculator  130 . At box  152  compliance calculator  130  calculates the load impedance for the nth electrode as Zn=Vn/I test . The compliance threshold current for the nth electrode, I n,t  is then calculated as I n,t =Vmax/Zn where Vmax is the maximum voltage that can be provided across the current source without the aid of voltage boosting. At decision box  156  the electrode counter variable n is tested to see if the calculation procedure has been performed for all of the electrodes. If it has not been so performed then n is incremented at box  154  and the entire procedure repeated for the next electrode until a compliance threshold current has been determined and stored for all of the electrodes. 
     Referring now to FIG. 13 the further operation of the device of FIG. 11 will be explained. At box  136  the controlling microprocessor, for example the same microprocessor that implements signal processor  18  of FIG. 1B, determines the intensity of the next stimulus current I s  and the electrode n via which the stimulus will be delivered. 
     At decision box  138  the amplitude of I s  is tested to determine if it exceeds the precalculated compliance threshold current for the electrode in question, minus an operational margin. If the result of the test at  138  is positive then the need for voltage boosting is indicated. Control then diverts to box  140  wherein the compliance latch  140  is set so that voltage booster  108  is placed into service. The stimulus is then applied at box  142  and the compliance latch reset subsequent to delivery of stimulus at box  144 . 
     Alternatively, if the result of the test at  138  is negative then no requirement for voltage boosting is indicated and the system proceeds through boxes  142  and  144  without setting the compliance latch. 
     Although the invention has been described with reference to several particular embodiments, it is to be understood that these embodiments are merely illustrative of the application of the principles of the invention. Accordingly, the embodiments described in particular should be considered exemplary, not limiting, with respect to the following claims.