Patent Publication Number: US-8121703-B1

Title: Dual-range compliance voltage supply for a multi-channel stimulator

Description:
RELATED APPLICATIONS 
     The present application is a divisional application of U.S. patent application Ser. No. 10/459,040, filed Jun. 11, 2003, which application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/388,731, filed Jun. 14, 2002. Both applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     Multi-channel stimulators are used in a number of implantable or partially implantable medical devices. Some of these devices include spinal cord stimulators and partially implantable and fully implantable hearing devices. 
     One challenge with such stimulators is keeping power usage to a minimum to conserve battery life. While increasing battery life may be achieved by extending the size of the battery, that runs counter to the goal of reducing the overall device size which is determined partly by battery size. Conservation of energy in implantable, battery operated devices is an important design goal in order to reduce the overall size of the device. Additionally, given a fixed battery capacity and size, conservation of energy is further desired to prolong the life of the battery. 
     A cochlear device for restoration of hearing is an exemplary device which uses a multi-channel stimulator. Such a device may be fully implantable or partially implantable. In a partially implantable device, there can be two components, an external component containing the battery and an implantable component which contains additional circuitry for processing the stimulation protocol. The power consumed in this processing circuitry, in addition to the power dissipated through the stimulation leads and electrodes, can be substantial. 
     It is desirable to improve the efficiency of such a device so that the battery can be recharged with less frequency. Frequent recharging is inconvenient to the user and, moreover, causes the rechargeable battery to reach its end of life more quickly. 
     A multi-channel spinal cord stimulator for treatment of intractable pain is an exemplary, fully implantable device, wherein the battery is contained inside the device. In this application, prolonging battery life is very important to defer surgery to replace the device. 
     Conventional multi-channel stimulators can be designed to have a single compliance voltage supply that is common to each channel. A “compliance voltage” is the voltage necessary to drive a desired (e.g. programmed) stimulating current through an electrode, which stimulation current is sufficient to cause excitable tissue to be stimulated at the desired intensity. The compliance voltage varies with the impedance of the electrode-tissue interface and the stimulation threshold of the tissue being stimulated. 
     Each channel in a multi-channel stimulator has varying compliance voltage requirements because the electrodes interfacing with the body tissue provide varying electrode/tissue impedances. For purposes of discussion, the electrode/tissue impedance, which is a combination of resistance and capacitance will be hereinafter referred to as a simple resistance. Although compliance voltage varies at each channel, in conventional multi-channel stimulators, a common compliance voltage is used for each of the channels. This electrical configuration wastes available battery power since it is unnecessary to have each channel operate at the same compliance voltage. In particular the compliance voltage is set to the highest setting required to satisfy the channel having the highest requirement. The other channels are also set to the same compliance voltage even though these other channels may actually need a smaller maximum compliance voltage. 
     SUMMARY 
     The systems and methods described herein provide a device for reducing the unnecessary dissipation of energy in a multi-channel stimulator. In this manner, battery life may be prolonged. 
     In accordance with an aspect of the present systems and methods, there is provided an electrical circuit device that allows each stimulation channel to be independently selectable between high and low compliance voltage supplies. Channels which can operate at half or less than half compliance voltage can operate in the lower range to optimize power usage and thereby achieve energy savings. 
     The stimulation circuit for a channel in a multi-channel stimulator has a common voltage power supply with a selectable, dual-range compliance voltage for each channel. The stimulation circuit comprises: first and second electrode contacts, first and second current sources (defined as the first, dual current sources), wherein the first current source has a first connection and a second connection, the second current source has a third connection and a fourth connection and the second and fourth connections are electrically connected. The stimulation circuit further comprises a third and fourth current sources (defined as the second, dual current sources), wherein the third current source has a fifth connection and a sixth connection, the fourth current source has a seventh connection and an eighth connection, wherein the sixth and eighth connections are electrically connected. The stimulation circuit further comprises a bypass switch which, when in a first, closed position (but open to the stimulation circuit), bypasses the third and fourth current sources and electrically connects the second electrode contact to ground, thereby providing a low compliance voltage supply mode and, when the bypass switch is in a second, closed position, permits the first and third current sources to operate together in a push pull configuration and the second and fourth current sources to operate together in a push-pull configuration, thereby providing a high compliance voltage supply mode. The first current source and second current source provide opposite current flow in the stimulation circuit and operate such that only one of the first or second current source operates at one time. The third current source and fourth current source provide opposite current flow through the circuit and operate such that only one of the third or fourth current source operates at one time. 
     It is a feature of the systems and methods described herein to provide electrical circuits that provide dual-range compliance voltages for a bipolar electrode configuration. 
     It is a further feature of the systems and methods described herein to provide electrical circuits that provide dual-range compliance voltages for a monopolar electrode configuration. 
     It is yet another feature of the systems and methods described herein to provide electrical circuits that permit uniphasic or biphasic stimulation of neural cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
         FIG. 1  depicts a functional block diagram of a multi-channel, cochlear stimulation system; 
         FIG. 2  shows a graph of charged-balanced, biphasic stimulation delivered through a stimulating electrode contact; 
         FIG. 3  shows a partial view of a multi-channel stimulation output circuitry depicting N number of electrodes and one case (housing) electrode; 
         FIG. 4  shows, in accordance with the present systems and methods, a schematic diagram of an electrical stimulation circuit, comprised of two partial circuits, and which electrical stimulation circuit uses a push-pull, pair of current sources in high compliance voltage mode and delivers stimulation into a bipolar electrode configuration; 
         FIG. 5  shows, in accordance with the present systems and methods, the identical electrical stimulation circuit depicted in  FIG. 3 , except with one switch bypassing one set of dual current sources, thereby activating the low compliance voltage mode (in a bipolar electrode configuration); 
         FIG. 6  shows, in accordance with the present systems and methods, a schematic diagram of an electrical stimulation circuit, comprised of two partial circuits, which electrical stimulation circuit utilizes a push-pull, pair of current sources in high compliance voltage mode and delivers stimulation into a monopolar electrode configuration; and 
         FIG. 7  shows, in accordance with the present systems and methods, the identical electrical stimulation circuit depicted in  FIG. 5 , except with one switch bypassing one set of dual current sources, thereby activating the low compliance voltage mode (in a monopolar electrode configuration). 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present systems and methods. It will be apparent, however, to one skilled in the art that the present systems and methods may be practiced without these specific details. Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearance of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
     Multi-channel stimulators are used in various implantable medical devices. For example, such multi-channel stimulators can be found in spinal cord stimulation devices for treating intractable pain and cochlear devices for restoration of hearing in the profoundly deaf. As an exemplary application of the present systems and methods, the systems and methods described herein will be discussed in the context of use in a cochlear implant device. Details associated with the operation of a typical cochlear implant system may be found in one or more of the following U.S. patents, each of which is incorporated herein by reference: U.S. Pat. Nos. 6,157,861; 6,002,966; 5,824,022; 5,603,726; 5,344,387; and 4,532,930. 
     Before describing the present systems and methods, it will be helpful to review the operation of a typical cochlear stimulation system. A representative cochlear stimulation system  10  is illustrated in  FIG. 1 . A microphone  12  senses acoustic waves and converts such sensed waves to an electrical signal. The electrical signal is then processed in an appropriate manner by a speech processor (SP)  14 . Such processing may include dividing the signal into different frequency bands and generating an appropriate stimulation control signal for each frequency band. The stimulation control signal(s) is passed on to an implantable cochlear stimulator (ICS)  16  via a radio-frequency communications link  15 . The ICS  16  is connected to an electrode array  20 . The electrode array  20  is inserted into a cochlea  30 . (Note, that the representation of the cochlea  30  shown in  FIG. 1  is meant only as a schematic representation.) 
     The electrode array  20  includes a plurality of spaced-apart electrode contacts  22  thereon. Each electrode contact  22  is electrically connected to the electrical circuitry within the ICS  16  by way of a lead  18 , which lead  18  has a plurality of electrical wire conductors embedded therein as is known in the art. The ICS, in response to the control signal(s) received from the SP  14 , generates an electrical stimulation current on selected groupings of the electrode contacts  22 . 
     The cochlea  30 , as is well known in the art, comprises a snail shaped member having three parallel ducts that spiral around its center bony region, known as the modiolus. One of the spiraling parallel ducts within the cochlea is the scala tympani. The center bony region, or modiolus, is where ganglion nerve cells  32  are located. Each of the ganglion cells  32  is coupled to the auditory nerve  40 , which connects to the brain. 
     It is the function of the cochlear stimulation system  10  to electrically stimulate the ganglion cells  22  with electrical stimulation current representative of the acoustic waves sensed by the microphone  12 . In order to achieve this function, the electrode array  20  is inserted into the scala tympani so that the electrode contacts  22  encircle the modiolus and ganglion cells  32 . Electrical stimulation current flows between selected electrode contacts  22  and hence stimulates the ganglion cells  32  near the selected electrode contacts, as controlled by the ICS  16  in accordance with a programmed or selected speech processing strategy. The speech processing strategy is defined by the control signals received from the SP  14 . The control signals are modulated by the acoustic waves sensed by the microphone  12 , thereby causing the stimulation current to stimulate appropriate ganglion cells as a function of the sensed acoustic waves. For example, low frequency acoustic waves cause ganglion cells near the apical tip of the cochlea to be stimulated, whereas high frequency acoustic waves cause ganglion cells near the basal region of the cochlea to be stimulated. 
     Stimulation of the ganglion cells can be accomplished using two electrode configuration modes. One electrode configuration mode is a “bipolar mode,” which uses two electrode contacts  22  positioned relatively close to each other. In this mode, the load resistance appears between the two electrode contacts  22 . The load resistance is contributed to by the interface between the tissue and electrode contacts  22  and the tissue itself between the electrode contacts. 
     Another electrode configuration is a “monopolar mode,” which employs one of the electrode contacts  22  in the electrode array and an indifferent electrode that is relatively distant from the electrode contacts  22 . In some cases, the indifferent electrode can be the exterior container (the “case”) of the ICS, which container can be made from a biocompatible, electrically conductive metal such as titanium. In the monopolar electrode configuration, the load resistance is contributed to by the interface between the electrode contact  22  and tissue, the interface between the indifferent electrode and tissue, and the tissue itself between the electrode contact and the indifferent electrode. 
     There are two stimulus modes: a uniphasic stimulus and a biphasic stimulus mode. A uniphasic stimulus provides current flow in only one direction through an electrode. A biphasic stimulus, however, provides current flows in both directions through an electrode within a relatively short time period. It is thought that uniphasic stimulation may cause charges to accumulate in the tissue near the stimulating electrode and thereby cause injury to this tissue. In addition, it is also believed that uniphasic stimulation can cause premature degradation of the electrodes. Therefore, most conventional multi-channel stimulators, including ones for cochlear stimulators, use some form of biphasic stimulation. 
       FIG. 2  shows a graph of a biphasic stimulus as a function of time. Stimulus pulse waveforms X and Y are individually uniphasic. But considered together, they are biphasic because the flow of stimulation current through an active electrode is in both directions. A particular biphasic stimulation in which an equal quantity of electrical charge flows in both directions through an electrode is termed a “charged-balanced,” biphasic stimulation. A charge balanced stimulation can be achieved by ensuring that the flow of charge in both directions through a stimulating electrode is equal over time. As represented in the graph of  FIG. 2 , the accumulation of charge is represented by the area (A) and area (B) above and below the zero current flow line, respectively. In this graph the areas above and below the zero line should be equal over a period of time in order to achieve charge balancing. Such charge balancing is believed to prevent injury to cells which are near the stimulating electrode and, furthermore, prevent the stimulating electrode from degrading prematurely. For these reasons, conventional, multi-channel stimulators for cochlear implants and spinal cord stimulation generally employ charged balanced stimulation regimes. 
       FIG. 3  shows, in accordance with the present systems and methods, a circuit diagram of a portion of a multi-channel stimulator circuit connected to N number of electrodes, E 1  . . . E N , and a case or housing electrode E CASE . Each partial circuit, designated as CIR 1 , CIR 2 , CIR 3  . . . CIR N  or CIR CASE , has dual current sources electrically connected to its respective electrode, E 1 , E 2 , E 3  . . . E N  or E CASE . Partial circuits CIR 1 , CIR 2 , CIR 3  . . . CIR N  also include a D.C. blocking capacitor,  120 ,  120 ′  120 ″ or  120 ″. In addition, each partial circuit has a bypass switch  160 ,  160 ′,  160 ″,  160 ″ or  165  for bypassing dual current sources and, instead, connecting the stimulation circuit to ground  170  (“first closed position”) or electrically connecting the stimulation circuit to the dual current sources (“second closed position”). Generally, each partial circuit, CIR 1 , CIR 2 , CIR 3  . . . CIR N  of a multi-channel stimulator, is identical. CIR CASE , however, is different because the device case or housing, which may act as an indifferent electrode, generally has a larger surface area than a stimulating electrode and, moreover, does not include a D.C. blocking circuit since CIR CASE  may be used in combination with one of the partial circuits, CIR 1 , CIR 2 , CIR 3  . . . CIR N , each of which already includes a D.C. blocking capacitor,  120 ,  120 ′,  120 ″ or  120 ″, sufficient to prevent delivery of direct current to stimulated tissue. 
     A complete stimulation circuit is formed by combining two partial circuits selected from the set CIR 1 , CIR 2 , CIR 3  . . . CIR N  and CIR CASE . For example, CIR 1  combined with CIR 3  forms a bipolar stimulation circuit having two electrodes E 1  and E 3 . Another example is the combination of CIR 2  and CIR 3  having two electrodes E 2  and E 3  and forming another bipolar stimulation circuit. A monopolar, stimulation circuit may be formed by combining partial circuit, CIR CASE , with one of the circuits in the set, CIR 1 , CIR 2 , CIR 3  . . . CIR N . In such an instance, the device case or housing functions as an indifferent electrode, E CASE , and one selected electrode among the set, E 1 , E 2 , E 3  . . . E N , acts as the stimulating electrode. When two partial circuits are thus combined to form a complete stimulating circuit, two sets of dual current sources are in the stimulation circuit. In operation, however, only two current sources may operate at any one time. For example, current sources  115  and  110 ′ may operate together at one time to produce stimulus current waveform X in  FIG. 2 , as measured at E 1 . Or current sources  110  and  115 ′ may operate together to produce stimulus current waveform Y in  FIG. 2 , as measured at E 1 . Such arrangement of paired current sources is termed a “push-pull” arrangement. 
       FIG. 4  shows, in accordance with the present systems and methods, a schematic circuit diagram of a complete stimulation circuit which utilizes a push-pull pair of current sources in a high compliance voltage mode. It can be seen that CIR 1  and CIR 2 , shown in  FIG. 3 , may be combined together to provide a complete bipolar circuit, as shown in  FIG. 4 . Rails  105 ,  106  and  105 ′,  106 ′ provide electrical connection points for a common supply voltage.  FIG. 4  shows, for purposes of illustration, a rail  105  which can be connected to a voltage of V+ relative to ground and rail  106 ′ which can be connected to a voltage of V−. To provide current flowing in the opposite direction through the stimulation circuit, rail  106  can be connected to a voltage V− and rail  105 ′ can be connected to a voltage V+. 
     As shown in  FIG. 4 , electrode contacts  22  and  22 ′ may be part of an electrode array, for example, as depicted in  FIG. 1 . The complete stimulation circuit has two pairs of dual current sources, two D.C. blocking capacitors  120 ,  120 ′ and two bypass switches  160 ,  160 ′, either of which may be used to bypass dual current sources  115 ,  110  or  115 ′,  110 ′ and connect, instead, to ground  170 . Resistor  125  simplistically represents the load resistance presented by the body tissue between the two electrode contacts  22  and  22 ′. The four current sources may be current mode, digital-to-analog converters (DACs). Current sources  115  and  110 ′ operate together in a “push pull” arrangement to direct current flow in one (uniphasic) direction through the circuit and through the electrode contacts  22  and  22 ′ such that electrode contact  22  functions as a cathode and electrode contact  22 ′ functions as an anode. Similarly, in a different time interval, the pair of current sources  110  and  115 ′ operate together to direct current flow in the opposite direction through the stimulation circuit and through the electrode contacts  22  and  22 ′, such that electrode contact  22 ′ is a cathode and electrode contact  22  is an anode. It can be appreciated that current sources  110  and  115 ′ are operating only when current sources  115  and  110 ′ are turned off and vice versa. A software program may be used to dynamically turn on and off the push pull pairs of current sources in timed intervals. 
     It can be seen that by turning alternately on and off, in timed intervals, the push-pull pairs of current sources  110 ,  115 ′ and  115 ,  110 ′, the stimulation current flow through the electrode contacts  22  and  22 ′ can be made biphasic, as shown in  FIG. 2 . An interval timer may enable current sources  110  and  115 ′ together in one timed interval, while current sources  115  and  110 ′ are disabled, then, in the next interval, current sources  115  and  110 ′ may be enabled together, while current sources  110  and  115 ′ are disabled, and so on. 
     Capacitors  120  and  120 ′ are internal, blocking capacitors which function to block direct current. These blocking capacitors prevent the possible occurrence of direct current being applied through electrode contacts  22  and  22 ′ which can harm tissue near these contacts. As shown in  FIG. 4  bypass switch  160  is in a “second closed” position to connect the stimulation circuit to the dual current sources  110  and  115 . Similarly, bypass switch  160 ′ is in a “second closed” position connecting the stimulation circuit to the dual current sources  110 ′ and  115 ′. With the bypass switches  160  and  160 ′ thereby in such positions, the voltage difference or the compliance voltage between rails  105 ′ and  106  is effectively V minus −V=2V, which is the high compliance voltage mode. The voltage difference or compliance voltage between rails  105  and  106 ′ is similarly V minus −V=2V. In the ideal circumstance, the stimulation circuit can supply a maximum current of Imax into a tissue resistance of R=2V/Imax and an operating power consumption of P=2V*I stim , where I stim  is the stimulation current delivered to body tissue at a specific time. Switching the bypass switch  160 ′ between a first, closed position, i.e., connecting the stimulation circuit to ground  170 , and a second, closed position, connecting the stimulation circuit to dual current sources  115 ′ and  110 ′ (while switch  160  remains in a second, closed position, connecting dual current sources  110  and  115 ), as well as enabling and disabling current source pairs, ( 110 ,  115 ′) and ( 115  and  110 ′), can be accomplished with appropriate software programmable controls. It can further be seen that the same effect may be accomplished by holding bypass switch  160 ′ in a second, closed position, which connects dual current sources  115 ′ and  110 ′ to the stimulation circuit, while bypass switch  160  selects between connecting the stimulation circuit to ground  170  or dual current sources  110  and  115 . 
       FIG. 5  shows, in accordance with the present systems and methods, an identical circuit diagram as provided in  FIG. 4 , except that the stimulation circuit is switched to a low compliance voltage mode. In this mode, the bypass switch  160 ′ connects the stimulation circuit to ground  170  and, hence, bypasses the dual current sources  110 ′ and  115 ′, which can be turned off, as they are not needed. Bypass switch  160  is placed in the second, closed position to permit dual current sources  110  and  115  to be connected to the stimulation circuit. The compliance voltage of the stimulation circuit as seen from rail  105  to ground  170  is V−0=V and from the ground to rail  106  is 0 minus −V=V. In the ideal circumstance, the circuit can supply a maximum current Imax into a tissue resistance of R=V/Imax. The operating power consumption of the stimulation current is P=V*I stim  because the compliance voltage is halved from 2V to V. The maximum load resistance through which a given I stim  is supplied is also halved. 
     In operation, a biphasic stimulation with the circuit of  FIG. 5  may be delivered by applying V− to rail  106  and enabling current source  115  in one interval, while current source  110  is turned off. This provides a uniphasic, stimulus pulse through electrode contacts  22  and  22 ′ in one circuit direction, for instance, as shown by curve X in  FIG. 2 . Then, in the next interval, current source  115  is disabled and, at the same time, current source  110  is enabled, while rail  105  has V+ applied, providing another uniphasic, stimulus pulse but in the opposite direction through the circuit, for example, shown as curve Y in  FIG. 2 . These two uniphasic pulses, flowing through the circuit in opposite directions at different times, may be combined to provide a charged-balanced, biphasic stimulation regime. 
       FIG. 6  shows, in accordance with the present systems and methods, a schematic diagram of an electrical stimulation circuit which uses push-pull, current sources in a high compliance voltage mode. The circuit delivers current stimulation into a monopolar electrode configuration, where electrode  22  is the active electrode and electrode  23  is the indifferent electrode, usually the case or housing of an implantable medical device. The complete circuit is a combination of two partial circuits CIR 1  and CIR CASE . CIR 1  is connected to the electrode contact  22  via a D.C. blocking capacitor  120 . Indifferent case electrode  23  is connected to either ground  170  or dual current sources  150  and  155  via a bypass switch  165 . As shown, electrode  23  is not connected to a D.C. blocking capacitor of its own, because partial circuit CIR CASE  will always be combined with at least one partial circuit in the set CIR 1 , CIR 2 , CIR 3  . . . CIR N , which already includes a D.C. blocking capacitor. Load resistor  125 ′ represents a simplified resistance provided by the tissue between the electrode  22  and indifferent electrode  23 . 
     In this monopolar electrode configuration, electrode contact  22  is near or in contact with the tissue to be stimulated. Indifferent electrode  23 , which is often the medical device case or housing, however, is generally distant from the target tissue and also distant from electrode contact  22 . 
     To enable the high compliance voltage mode (2V), the bypass switch  165  is programmed to the second, closed position. Current source pair  110  and  150  operate together to drive current in one circuit direction, while rail  105  has V+ applied and rail  175  has V− applied. Current source pair  115  and  155  work together in a push-pull arrangement to drive current flow in an opposite circuit direction, while rails  106  and  176  have V− and V+ voltages applied, respectively. 
       FIG. 7  shows, in accordance with the present systems and methods, the identical circuit of  FIG. 6 , except switched to the low compliance voltage mode (V). The electrode configuration is once again monopolar with electrode contact  22  and indifferent electrode contact  23 . To provide the low compliance mode, the bypass switch  165  is in the first, closed position, thereby bypassing dual current sources  150  and  155 . Because dual current sources  150  and  155  are not being used, they may be turned off. Alternatively, bypass switch  160  can bypass dual current sources  110  and  115  and connect to ground, while bypass switch  165  connects the stimulation circuit to dual current sources  150  and  155 . This latter circuit configuration would also produce a low compliance voltage, monopolar electrode configuration. 
     Referring to  FIG. 3 , the voltages at pairs of rails  105 ,  106 ′;  106 ,  105 ′;  105 ,  175 ; and  106 ,  176  are provided by the common supply voltage. Each of the up to N stimulation circuits (channels) can operate independently and be selected to operate between low and high compliance voltages, as previously described. The bypass switches  160  and  165  may be used to control the compliance mode for each stimulation channel. To provide instantaneous switching between low and high compliance voltage modes, the switches  160  and  165  for each stimulation circuit may be independently and dynamically controlled by software. Dynamic switching does not require user involvement and this instantaneous adjustment of compliance voltages can further optimize the power consumed by each channel. 
     The present systems and methods thus allows selection of low and high compliance voltage independently for each stimulation channel connected in parallel to a common supply voltage. In this manner each stimulation channel (or stimulation circuit) may be independently switched to use either low or high compliance modes according to the needs of each stimulation channel. The selection of compliance voltage modes may be automatically stored in long-term memory contained in the stimulator. The stimulation threshold data of each stimulation channel (or stimulation circuit) consisting of two electrode contacts or one electrode contact and an indifferent, case electrode may also be stored in memory for later retrieval and used to select whether a stimulation channel should be set to low or high voltage compliance modes. 
     Stimulation thresholds may differ widely and are a function of differences in the position of an electrode contact relative to the ganglion nerves in the cochlea and to physiological variance in the location of the nerves along the cochlea. Even very slight electrode positional differences can have marked changes in stimulation thresholds. 
     The stimulation thresholds may be measured in various ways, for example, stimulation current may be applied at the two electrode contacts, e.g., the magnitude of the current (with pulsewidth held constant) may be increased until some indication of nerve firing is noticed or measured. In the case of cochlear stimulation, threshold stimulation may be determined by the perception of sound or, alternatively, the stimulator device may have sensors which can detect nerves firing when they are captured by a stimulus that is at or above stimulation threshold. Once such stimulation threshold is obtained for each channel, this threshold information may be kept in memory and later recalled in order to set each channel (or stimulation circuit) to either a low or high voltage compliance modes. The setting of the voltage compliance mode may be done nearly instantaneously, using dynamic switching. 
     Each selected channel may be independently set so that low voltage compliance mode may be used whenever possible. The current draw in the low voltage compliance mode may be halved and, thus, energy use can be reduced. Such energy savings may be substantial. Up to a 20 to 25% savings in energy may be achieved, if half of the channels are operated in low voltage compliance mode. 
     Further, each channel (or stimulation circuit) may provide uniphasic or biphasic stimulation, and selection of monopolar or bipolar electrode configurations. 
     As an exemplary embodiment, a multi-channel stimulation system for cochlear application has been discussed. It can be appreciated, however, that the present systems and methods may be used with any multi-channel stimulation system having a single common supply voltage and where each channel has a different compliance voltage requirements. For example, a spinal cord stimulator for treating intractable pain can have multiple channels and may have a circuit design which utilizes a common power supply. Such a circuit may be amenable to utilizing a dual range compliance voltage as provided by the present systems and methods. 
     The preceding description has been presented only to illustrate and describe embodiments of the invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching.