Patent Publication Number: US-9427591-B2

Title: Charging system for an implantable medical device employing magnetic and electric fields

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of U.S. patent application Ser. No. 14/696,819, filed Apr. 27, 2015 (allowed), which is a divisional application of U.S. patent application Ser. No. 13/164,005, filed Jun. 20, 2011 (now U.S. Pat. No. 9,044,616), which is a non-provisional application of U.S. Patent Application Ser. No. 61/360,536, filed Jul. 1, 2010. Priority is claimed to these applications, and they are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to improved battery charging for an implantable medical device. 
     BACKGROUND 
     Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system. 
     As shown in  FIGS. 1A and 1B , a SCS system typically includes an Implantable Pulse Generator (IPG)  100 , which includes a biocompatible device case  30  formed of a conductive material such as titanium for example. The case  30  typically holds the circuitry and battery  26  necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG  100  includes one or more electrode arrays (two such arrays  102  and  104  are shown), each containing several electrodes  106 . The electrodes  106  are carried on a flexible body  108 , which also houses the individual electrode leads  112  and  114  coupled to each electrode. In the illustrated embodiment, there are eight electrodes on array  102 , labeled E 1 -E 8 , and eight electrodes on array  104 , labeled E 9 -E 16 , although the number of arrays and electrodes is application specific and therefore can vary. The arrays  102 ,  104  couple to the IPG  100  using lead connectors  38   a  and  38   b , which are fixed in a non-conductive header  36 , which can comprise an epoxy for example. 
     As shown in  FIG. 2 , the IPG  100  typically includes an electronic substrate assembly  14  including a printed circuit board (PCB)  16 , along with various electronic components  20 , such as microprocessors, integrated circuits, and capacitors mounted to the PCB  16 . Two coils (more generally, antennas) are generally present in the IPG  100 : a telemetry coil  13  used to transmit/receive data to/from an external controller  12 ; and a charging coil  18  for charging or recharging the IPG&#39;s battery  26  using an external charger  50 . The telemetry coil  13  is typically mounted within the header  36  of the IPG  100  as shown, and may be wrapped around a ferrite core  13 ′. 
     As just noted, an external controller  12 , such as a hand-held programmer or a clinician&#39;s programmer, is used to wirelessly send data to and receive data from the IPG  100 . For example, the external controller  12  can send programming data to the IPG  100  to dictate the therapy the IPG  100  will provide to the patient. Also, the external controller  12  can act as a receiver of data from the IPG  100 , such as various data reporting on the IPG&#39;s status. The external controller  12 , like the IPG  100 , also contains a PCB  70  on which electronic components  72  are placed to control operation of the external controller  12 . A user interface  74  similar to that used for a computer, cell phone, or other hand held electronic device, and including touchable buttons and a display for example, allows a patient or clinician to operate the external controller  12 . The communication of data to and from the external controller  12  is enabled by a coil (antenna)  17 . 
     The external charger  50 , also typically a hand-held device, is used to wirelessly convey power to the IPG  100 , which power can be used to recharge the IPG&#39;s battery  26 . The transfer of power from the external charger  50  is enabled by a coil (antenna)  17 ′. For the purpose of the basic explanation here, the external charger  50  is depicted as having a similar construction to the external controller  12 , but in reality they will differ in accordance with their functionalities as one skilled in the art will appreciate. 
     Wireless data telemetry and power transfer between the external devices  12  and  50  and the IPG  100  takes place via inductive coupling, and specifically magnetic inductive coupling. To implement such functionality, both the IPG  100  and the external devices  12  and  50  have coils which act together as a pair. In case of the external controller  12 , the relevant pair of coils comprises coil  17  from the controller and coil  13  from the IPG  100 . In case of the external charger  50 , the relevant pair of coils comprises coil  17 ′ from the charger and coil  18  from the IPG  100 . 
     When data is to be sent from the external controller  12  to the IPG  100  for example, coil  17  is energized with an alternating current (AC). Such energizing of the coil  17  to transfer data can occur using a Frequency Shift Keying (FSK) protocol for example, such as disclosed in U.S. Patent Application Publication 2009/0024179. Energizing the coil  17  produces a magnetic field, which in turn induces a voltage in the IPG&#39;s coil  13 , which produces a corresponding current signal when provided a closed loop path. This voltage and/or current signal can then be demodulated to recover the original data. Transmitting data from the IPG  100  to the external controller  12  occurs in essentially the same manner. 
     When power is to be transmitted from the external charger  50  to the IPG  100 , coil  17 ′ is again energized with an alternating current. Such energizing is generally of a constant frequency, and may be of a larger magnitude than that used during the transfer of data, but otherwise the basic physics involved are similar. 
     The IPG  100  can also communicate data back to the external charger  50  by modulating the impedance of the charging coil  18 . This change in impedance is reflected back to coil  17 ′ in the external charger  50 , which demodulates the reflection to recover the transmitted data. This means of transmitting data from the IPG  100  to the external charger  50  is known as Load Shift Keying (LSK), and is useful to communicate data relevant during charging of the battery  26  in the IPG  100 , such as the capacity of the battery, whether charging is complete and the external charger can cease, and other pertinent charging variables. LSK communication from an IPG  100  to an external charger is discussed further in U.S. Patent Application Publication 2010/0179618. 
     As is well known, inductive transmission of data or power can occur transcutaneously, i.e., through the patient&#39;s tissue  25 , making it particularly useful in a medical implantable device system. During the transmission of data or power, the coils  17  and  13 , or  17 ′ and  18 , preferably lie in planes that are parallel, along collinear axes, and with the coils as close as possible to each other. Such an orientation between the coils  17  and  13  will generally improve the coupling between them, but deviation from ideal orientations can still result in suitably reliable data or power transfer. 
     Although the burden on the patient to charge the IPG seems minimal, the inventors recognize that some percentage of the patient population does not have the skills necessary to operate the charger  50 . For example, some patients may be physically impaired and thus unable to place a charger  50  at the appropriate location over the IPG  100 . Furthermore, even in patients that are able, it may be difficult for the patient to tell where the IPG  100  is located, or what an appropriate alignment would be between the charger  50  and the IPG  100 . In short, the need for the patient&#39;s involvement in the charging process can be problematic, and the inventors here introduce a solution that can allow patients to recharge their implants with no or little participation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  show an implantable medical device, and the manner in which an electrode array is coupled to the IPG in accordance with the prior art. 
         FIG. 2  shows the relation between the implantable medical device, an external controller, and an external charger. 
         FIG. 3  shows the addition of a base station charger to the system of  FIG. 2 . 
         FIGS. 4A and 4B  show the base station charging an IPG using an E-field and a B-field, respectively, in accordance with an embodiment of the invention. 
         FIGS. 5A-5E  depict various physical embodiments of base station of  FIG. 3 . 
         FIG. 6  shows an IPG electrode being used as an antenna for E-field reception in accordance with an embodiment of the invention. 
         FIG. 7  shows a schematic of the circuitry within the base station in accordance with an embodiment of the invention. 
         FIGS. 8A and 8B  show schematics of the circuitry within the IPG for interfacing with the base station in accordance with an embodiment of the invention. 
         FIGS. 9 and 10  show alternative schematics of a base station and the IPG having an additional communication channel and hardware. 
     
    
    
     DETAILED DESCRIPTION 
     The description that follows relates to use of the invention within spinal cord stimulation (SCS) system. However, it is to be understood that the invention is not so limited, and could be used with any type of implantable medical device system. 
     The inventors address the problem of recharging a battery in an implant by providing a external device that can passively recharge the battery without patient involvement. The external device is referred to as a base station  200 , and is shown in conjunction with a traditional external controller  12  and external charger  50  in  FIG. 3 . The base station  200  can be hand held similar to devices  12  or  50 , but in the disclosed embodiments is described as equipment configured to be placed at a fixed location, such as under a bed, on or next to a wall, etc. In other words, the base station  200  would normally be located somewhere where the patient would be expected to spend a significant amount of time—time which can be spent recharging the battery. The base station  200  could be battery-powered, but would more likely be plugged into a wall socket. 
     Base station  200  in one embodiment can generate an electric field and a magnetic field (E-field and B-field) that couple with an antenna and a receiving coil within the IPG  100  to generate a charging current for charging the IPG battery  26 . No handling or manipulation on part of the patient is necessary; the implant battery is passively charged whenever the patient is within range of either the magnetic or electric charging fields generated by base station  200 . Charging using the B-field occurs when the IPG is at a relatively short distance from the base station  300  (e.g., less than 1 m), while charging using the E-field occurs at longer distances (e.g., &gt;1 m). Back telemetry from the IPG  100  to the base station  200  can be used to inform the base station  200  as to whether charging should occur via B-field or E-field, and B-field charging is preferred if possible for its ability to transfer higher amounts of power to the IPG  100 , as will be explained here. 
       FIGS. 4A and 4B  illustrate both E-field and B-field modes of operation of the base station  300 .  FIG. 4A  shows base station  200  using an antenna  204  for generating a radiating E-field  302 . The E-field  302  is sensed by an antenna  150  in IPG  100  to generate an alternating current, which is rectified to produce DC power to recharge the battery, as will be described in further detail later. Because these antennas  204  and  150  primarily interact with the electrical component of the electromagnetic field,  FIG. 4A  illustrates only the E-field  302 .  FIG. 4B  shows the base station  200  using a coil (antenna)  206  for generating an inductive B-field  304 . Coil  18  in IPG  100  couples with the B-field  304  to generate an alternating current, which is rectified to produce DC power. Such B-field charging is similar to the charging scheme implemented in a traditional external charger  50  ( FIG. 2 ), and uses similar circuitry, although circuitry in the base station  302  has been modified as discussed herein. For example, the base station&#39;s circuitry allows for selection of the charging mode—E-field  302  or B-field  304 —to transfer energy to the IPG  100 . As discussed further below, the B-field  304  is typical a lower frequency (e.g., on the order of 100 kHz) than the E-field  302  (e.g., on the order of 1 MHz to 10 GHz). 
     As noted, and as depicted in  FIG. 4 , E-field charging will be used for longer distances, while B-field charging will be used for shorter distances. The strength of an E-field, such as E-field  302 , typically reduces proportional to the square of the distance between the transmitting antenna  204  and the receiving antenna  150 . In contrast, strength of an inductive magnetic field, such as B-field  304 , typically reduces proportional to the cube of the distance between the generating coil  206  and the receiving coil  18 . Therefore, for larger distances, transferring energy using an E-field is more efficient that using a B-field. 
     Before discussing the circuitry and operation of the base station  200 , various physical embodiments of base station  200  are discussed, as shown in  FIGS. 5A-5E . As noted earlier, base station  200  can be positioned on a wall or floor, i.e., placed near a bed, against a wall, in a corner, or at any other convenient location generally close to an expected location of a patient.  FIG. 5A  shows base station  200  having a serpentine wire antenna  204  connected to a circuit module  208 . As will be discussed later, circuit module  208  includes circuitry such as microcontrollers, amplifiers, transceivers, etc., for operating base station  200 . Serpentine antenna  204  is energized by the circuit module  208  for radiating the E-field  302 . Alternatively, an inductively-loaded antenna such as the one shown in  FIG. 5B  can be used in place of the serpentine antenna. Generally, both the antennas  204  illustrated in  FIGS. 5A and 5B  would be quarter-wavelength monopole antennas. A quarter-wavelength antenna ideally has a length equal to one-fourth of the wavelength of the E-field being radiated. For example, a quarter-wavelength antenna  204  of length 0.25 m would be used for transmitting a wavelength of 1 meter (which corresponds to a frequency of approximately 300 MHz). Because of the serpentine shape of the antenna of  FIG. 5A , and because of the inductive loading of the antenna of  FIG. 5B , these antennas can be made smaller than the optimal quarter-wavelength length. Base station  200  can be equipped with a parabolic reflector (not shown) placed behind the antenna  204  to radiate or propagate energy of the E-field in the desired direction. This can be of particular advantage to focusing the radiating E-field towards the patient&#39;s IPG  100 . 
     Base station  200  also includes the coil  206  for generating the B-field  304 , which coil is also coupled to the circuit module  208 . Coil  206  is typically wound on a ferrite core (not shown) to increase the strength of produced inductive field. 
     Because the antennas  204  of  FIGS. 5A and 5B  are vertically oriented, the electromagnetic wave radiated from the antenna  204  is also vertically oriented, i.e., vertically polarized. If the IPG antenna  150  is also vertically oriented, maximum coupling with the vertically oriented E-field  302  will occur. Maximum coupling is favorable because it results in the maximum E-field power transfer, thus providing more energy to recharge the IPG  100 &#39;s battery  26 . Such coupling will diminish as the angle between the polarization of the E-field  302  and the orientation of the receiving IPG antenna  150  increases, with minimum coupling at a 90-degree angle. 
       FIG. 5C  shows another embodiment of a base station  200  that includes a patch antenna  204  coupled to the circuit module  208 . Patch antenna  204  is typically made of a square or rectangular metal plate placed at a certain distance over a ground plane, which may comprise an additional metal plate connected to ground or the floor itself. Patch antennas generally operate as dipole, or half-wavelength antennas, meaning that the antenna is ideally dimensioned to half the wavelength of the transmitted electromagnetic wave. For example, if the patch antenna  204  is used to generate an E-field  304  at 300 MHz, the length of patch antenna would ideally be equal to 0.5 m. Patch antenna  204  of  FIG. 5C  will radiate or propagate energy of an E-field vertically upwards, i.e., in a direction that is generally normal to the plane of the patch antenna, and is therefore useful for placement underneath a patient&#39;s bed for example to allow for IPG charging while the patient is sleeping. Polarization of the E-field generated by the patch antenna is determined by the location of the contact(s)  210  as shown in  FIG. 5C , which is the location where the patch antenna is coupled to the circuit module  208 . Again, maximum coupling between the E-field generated by the patch antenna  204  and the IPG antenna  150  will occur when the direction of the IPG antenna  150  is the same as the direction of the polarization of the E-field. Note that the base station of  FIG. 5C  can also be placed vertically, as shown in  FIG. 5D . Such orientation could be mounted on a wall, and might be more advantageous to recharge an IPG in a patient sitting in a nearby chair for example. Antenna  204  can also comprise a slot antenna. 
       FIG. 5E  shows an embodiment of base station  200  that combines the embodiments shown in  FIGS. 5C and 5D , and thus provides both horizontal and vertical polarization of the produced E-field. Because the produced E-field is polarized in two directions, it will more likely couple to the antenna  150  in the IPG  100 , which antenna  150  orientation may not be known exactly or can vary as the patient moves. In this embodiment, the base station  200  includes two patch antennas  204   a  and  204   b  respectively placed in horizontal and vertical planes, allowing the energy of E-field to be radiated or propagated in both upward and sideways directions. Patch antenna  204   a  also includes two contact points  210   a  and  210   c , which allows the base station  200  to select the desired polarization. Patch antenna  204   a  can be simultaneously energized at both contact points  210   a  and  210   c  to generate a circularly-polarized E-field, typically by energizing points  210   a  and  210   c  90-degrees out of phase. Such circularly polarized field minimizes constraints on the orientation of the IPG antenna  150  for maximum coupling. Patch antenna  204   b  may likewise contain two contact points and produce a circularly-polarized E-field, although this is not shown in  FIG. 5E  for clarity. 
     The base station  200  of  FIG. 5E  also contains two cols  206   a  and  206   b . Like the antennas  204   a  and  204   b , the coils  206   a  and  206   b  are orthogonal, and produce B-fields which are orthogonal, which minimizes constraints on the orientation of the IPG coil  18  ( FIG. 4B ). A rotating B-field can also be produced using the two coils  206   a  and  206   b . See, e.g., U.S. Patent Application Publication 2009/0069869, which is incorporated herein by reference in its entirety. 
     Base station  200  can also select the antenna  204   a  or  204   b , or coil  206   a  or  206   b , that provides maximum power transfer to the IPG  100 , and use only that antenna or coil. This selection can be based on assessing coupling information for each antenna and coil orientation, which information can be telemetered from the IPG  100 , or can be deduced based on the production of the E-field or B-field at the base station  200 . See, e.g., U.S. Patent Application Publication 2008/0172109, which is incorporated herein by reference. 
       FIG. 6  shows further details of a suitable E-field antenna  150 , and in this embodiment antenna  150  comprises one of the electrode leads  112  used in array  102  as the antenna  150 . For example, the wire connecting to electrode E 1  is used as the antenna  150 . Wires to other electrodes (E 2 , E 3 ) can also be used, but because selecting the longest wire advantageously reduces the transmission frequency, the wire to electrode E 1  has been chosen. (Of course, a signal wire connecting to electrodes on array  104  can also be chosen). Because electrode leads  112  and  114  provide individual wires of varying lengths, a wire whose length is closest to the ideal length for a particular E-field receiving/transmitting frequency can be readily selected. Note that using an electrode lead for the antenna  150  does not affect stimulation produced at the affected electrode because the frequency of the E-field  302  received or transmitted by antenna  150  is at least a few orders of magnitude higher than the frequency of signals sent to electrodes. For example, the E-field  302  is typically on the order of 1 MHz to 10 GHz, while the frequencies of pulses sent to the electrode via a signal wire are in the range of tens of Hz to hundreds of Hz. Moreover, the magnitude of the AC signal on the signal wire resulting from E-field transmission or reception is typical very small (e.g., mV) compared to the magnitude of the stimulation pulses (e.g., Volts). Of course, the IPG  100  can also include a dedicated antenna, separate from the electrode leads  112  and  114 , for transmitting and receiving the E-field  302  to and from the base station  200 . Such an antenna can be placed in the header  36  or in the metal case  30  of the IPG  100 . 
     Discussion now turns to the circuit module  208  in the base station  200  used to transfer and receive energy to and from the IPG  100 . As shown in  FIG. 7 , microcontroller  212  controls the operation of the transmission circuitry and receiver circuitry, as well as controlling other operations in the base station  200  not discussed here. As is typical, microcontroller  212  can include both volatile memory (e.g., RAM) and non-volatile memory (e.g., Flash, EEPROM) for storing and implementing the functionality described herein. Transmission circuitry includes a digitally-controlled signal generator  214  and power amplifier  216 . Receiver circuitry includes two receiver circuits, LSK Rx  220  and RF Rx  228 . Switch  222  couples the transmission and receiver circuitry to either the antenna  204  or the coil  206 , depending on whether E-field or B-field charging has been selected. 
     For transferring energy using coil  206  via B-field  304 , microcontroller  212  controls the signal generator  214  to generate a signal with a transmission frequency of f B =80 kHz for example. Signal generator  214  will typically generate a sinusoidal signal at the specified frequency, but can also generate waveforms with a varying duty-cycle. 
     For transferring energy using antenna  204  via E-field  302 , microcontroller  212  controls the signal generator to generate a signal with a transmission frequency of f E . f E  can range from about 1 MHz to 10 GHz, and whether higher or lower frequencies are used for f E  involve tradeoffs. Transmitting at higher frequencies allows higher energy to be transmitted to the IPG  200 , and at longer distances. However, high frequency signals are attenuated by the body tissue. Lower frequencies have less attenuation, but can require a longer antenna  150  in the IPG  100  for optimal quarter-wavelength tuning Antenna length is mitigated slightly by the permittivity of the tissue, which is primarily water. Because the length of the antenna  150  will scale in inverse proportion to the square root of the permittivity of the tissue (water), the required length of the antenna  150  can be shortened significantly, which will allow f E  to be lowered. In any event, a lack of precise tuning and the reality of signal attenuation can be mitigated by proper antenna circuitry design and by adjusting the power of the E-field transmission, and it is not strictly necessary that an antenna  150  in the IPG  100  be exactly one-quarter of the wavelength of f E . In useful embodiments, f E  can comprise a frequency selected from the Industrial, Scientific, and Medical (ISM) band in one example, and could comprise frequencies of 13.56 MHz, or 27.12 MHz, or 2.45 GHz for example, even if the antenna  150  in the IPG  100  is not dimensioned to resonate optimally at such frequencies. 
     The output of signal generator  214  is fed to the input of power amplifier  216 , which amplifies its input signal by a magnitude controlled by the microcontroller  212  using a gain control signal. In reality, separate amplifiers  216  may be used depending on the frequency (f E  or f B ) chosen, but this is not shown in  FIG. 7  for simplicity. Initially, the microcontroller  212  may set a default gain for power amplifier  216  via the gain control signal, which signal can be increased as necessary. 
     The output of the power amplifier  216  is ultimately sent to either of antennas  204  or coil  206  via appropriate impedance matching circuitry  218  and  230 . Impedance matching circuits are well known in the art, and can include transformers, passive RLC networks, stepped transmission lines, etc. Which of the antenna  204  or coil  206  are chosen is determined by a control signal K 1  issued from the microcontroller  212 , which equals a logic ‘1’ when B-field charging is used, and a logic ‘0’ when E-field charging is used. When K 1 =1, switch  222  couples the transmission and receiver circuitry to coil  206  via its impedance matching circuitry  230 . When K 1 =0, switch  222  couples the circuitry to antenna  204  via its impedance matching circuitry. 
     In the embodiment of  FIG. 7 , base station  200  includes two receiver circuits for receiving back-telemetry data from the IPG  100  during recharging of the IPG battery  26 . LSK receiver  220  receives load-shift-keyed data via coil  206 , while RF receiver  228  receives modulated data via antenna  204 . Like switch  222 , these receivers  220  and  228  are controlled by K 1 , such that only one of them is enabled at a time, depending on whether the base station is operating in B-field or E-field mode. LSK telemetry is well known, and involves modulating the resistance of the receiving coil  18  in the IPG to produce detectable reflections at the transmitting coil  206 , as is explained further below with reference to  FIG. 8A . 
     Charging information back telemetered from the IPG  100  can include the IPG&#39;s battery voltage (V BAT ) and data indicative of the coupling between the base station and the IPG. V BAT  informs the microcontroller  212  of the present voltage of the IPG battery  26  during charging to allow the microcontroller  212  to either modify the power of the antenna  204  or coil  206  broadcasting the charging energy, or to suspend charging altogether once the battery  26  is full. 
     Coupling data received from the IPG  100  indicates the amount of energy that the IPG is receiving, and will depend upon several factors, such as transmission power, distance between the base station  200  and the IPG  100 , relative orientations of the transmitting/receiving elements (antenna  204  and antenna  150 ; or coil  206  and  18 ), etc. In one embodiment, coupling data can comprise the voltages V DCE  and V DCB  respectively output by the B-field and E-field rectifiers  154  and  164  in the IPG  100 , as will be discussed later with reference to  FIG. 8A . In another embodiment, coupling data can comprise a voltage drop across charging circuitry  156  ( FIG. 8A ). See, e.g., U.S. Pat. No. 8,744,592, which is incorporated herein by reference in its entirety. When the base station  200  receives such coupling data during charging, it can control the gain of power amplifier  216  via the gain control signal. For example, if the output voltage of rectifiers  154  or  164  ( FIG. 8A ), V DCE  or V DCB , in the IPG  100  reduces below a predetermined value, microcontroller  212  can increase the gain of power amplifier  216  so that the magnitude of the produced E-field  302  or B-field  304  increases. How to adjust the gain control signal for a particular received value of the coupling data can be determined by experimentation or simulation, and can be stored as a look up table in memory associated with the microprocessor  212 . 
       FIG. 8A  shows an embodiment of the circuitry in the IPG  100  for receiving the charging energy broadcast by the base station  200 , and for back telemetering charging information to the base station  200 . Antenna  150 , which again can comprise one of the signal wires as discussed earlier with respect to  FIG. 6 , is coupled to a multiplier and rectifier  164  through an impedance matching circuit  168 , and receives the E-field  302  generated by the base station  200 . The rectifier  164  generates a DC voltage, V DCE , which is used to charge the battery  26 .  FIG. 8B  illustrates an example circuit that can be used for rectifier  164 , which is known in the art as a half-wave series multiplier or a Villard cascade. The rectifier  164  comprises a number of capacitor-diode stages, with four such stages shown in  FIG. 8B . The number of stages dictates the multiplier that will be applied to the AC input voltage, Vin, to produce DC voltage V DCE , such that four stages will essentially produce a V DCE  that is four times the peak voltage of Vin. Diodes  174 - 177  are preferably zero threshold or low threshold diodes such as Schottky diodes, which will allow for the rectification and multiplication of small AC voltage, Vin, produced at the output of the antenna  150  (tens to hundreds of mVs). V DCE  is fed to the charging circuit  156 , which monitors and controls the battery&#39;s  26  charging process. 
     Referring again to  FIG. 8A , IPG  100  also includes a charging coil  18  connected to a rectifier  154  via an impedance matching circuit  152 . This coil  18  receives the B-field  304  generated by the base station  200 . Coil  18  may also receive a B-field from a more traditional external charger  50 , such as was discussed in  FIG. 2 , and in this regard, the improved circuitry of  FIG. 8A  does not disrupt the use of such legacy system designs. Impedance matching circuit  152  matches the impedance of the coil  18  with the input impedance of the rectifier  154  to allow for maximum power transfer. Rectifier  154  can be a single diode half-wave rectifier, a full-wave bridge rectifier, or other rectifiers well known in the art. Because the AC voltages induced on the coil  18  by the B-field are generally quite large (on the order of Volts), the rectifier can use traditional diodes. Output of the rectifier  154 , V DCB , is fed to the charging circuit  156 . 
     Both V DCE  and V DCB  are fed to a comparison circuit  223  to be compared to threshold voltages V thE  and V thB , respectively. Generally speaking, comparison circuit  223  informs the microcontroller  158  when charging energy is being received either at the antenna  150  (via E-field charging) or at the coil  18  (via B-field charging). As shown, comparison circuit  223  can include two comparators for comparing the DC voltages produced by each of the rectifiers  164  and  154 , V DCE  and V DCB , to reference voltages V thE  and V thB . If either DC voltage exceeds its associated reference voltage, its comparator will digitally indicate that fact to the IPG  100 &#39;s microcontroller  158  as a logic ‘1’ at inputs X and Y. V thE  and V thB  can be experimentally determined, and can be made adjustable, but in any case are generally set to a significant level to discern true power reception from mere noise. Note that because V DCE  will often be much less than V DCB , reference voltage V thE  will likewise generally be much smaller than V thB . In an alternative arrangement, if the microcontroller  158  includes or is associated with analog-to-digital converters, then V DCE  and V DCB  can be directly fed to such analog inputs, allowing the microprocessor  158  to assess the magnitude of those voltages digitally. 
     Microcontroller  158  can interpret input signals X and Y and issue control signals B and E accordingly, which control signals indicate to the remainder of the circuitry whether the microcontroller  158  is recognizing and allowing charging of the battery  26  to occur via B-field or E-field reception. The following truth table shows the generation of these control signals B and E based on input signals X and Y, and shows the preference of the IPG  100  to charge via B-field reception if that route is available. 
                                                     X   Y   B   E                          0   0   0   0           0   1   0   1           1   0   1   0           1   1   1   0                        
Allowing B-field charging to take precedence over E-field charging (i.e., B=1 when X is asserted, regardless of Y) is preferred because the rectified voltage produced via B-field reception, V DCB , would generally be much greater than the rectified voltage produced via E-field reception, V DCE . Allowing charging circuitry  156  to then choose V DCB  over V DCE  as its input voltage will allow such circuitry  156  to charge the battery  26  faster. Conversely, and as discussed further below, charging of the battery using V DCE  is used as a last resort, and can occur passively. Charging circuitry  156  is well known in the art, and is capable of handling input voltages of different values, such as would be provides by V DCE  and V DCB . Although shown as comprising two different inputs to charging circuitry  156 , it should be understood that V DCE  and V DCB  can be selected as a single input to the circuitry  156  using a switch controlled by control signals B and E (not shown). Of course, assertion of neither of control signals B or E would signify that the IPG  100  is not recognizing the receipt of any charging field from the base station  200  (or any other source such as the external charger  50 ), and will behave accordingly.
 
     As discussed previously, the IPG  100  can back telemeter to the base station  200  charging information, such as the battery voltage (V BAT ) and coupling data, and such telemetry can also be controlled via control signals B and E. In this regard, and as shown in  FIG. 8A , IPG  100  contains a RF transmitter/receiver  166  enabled by control signal E, and an LSK transmitter  160  enabled by control signal B. In other words, the IPG  100  decides through this scheme to communicate back to the base station  200  using the means (B-field or E-field) already established as reliable at the IPG  100  based on the fields it has received. LSK transmitter  160 , if chosen using control signal B, uses the charging information to be telemetered to modulate a transistor  168  connected in parallel with coil  18 . As noted earlier, this produces reflection in the coil  206  used in the base station to produce the B-field  304 , which data can then be demodulated at the LSK receiver  220  in the base station ( FIG. 7 ) to recover the charging information. Should the RF transmitter/receiver  166  be chosen via control signal E, the charging information will be modulated using a protocol suitable for broadcast via the E-field antenna  150 , such frequency shift keying (FSK), phase shift keying (PSK), amplitude shift keying (ASK), etc. Such RF back telemetered data would then be received at the RF receiver  228  in the base station  200  ( FIG. 7 ). Circuit  166  can also include corresponding demodulation circuits for receiving data from the base station  200 , and in this regard, base station  200  can include an RF data transmitter coupled to antenna  204 . However, such RF data transmission circuitry is not shown in the base station of  FIG. 7 , because in a simple embodiment of the technique, the E-field antenna  204  only broadcasts E-fields for the purpose of charging the IPG&#39;s battery  26 , as explained further below. 
     RF transmitter/receiver  166  can operate at a frequency, f E ′, that is different from the transmission frequency of the E-field, f E . Choosing a different frequency for f E ′ can prevent interference with the E-field  302  broadcast from the base station, and may allow for data reception at the base station E-field antenna  204  which is simultaneous with such broadcast. If a different frequency f E ′ is chosen for back telemetry, it may be advisable that such frequency not differ greatly from f E ; if an E-field at frequency f E  is successfully received at the IPG  100 , then it would be likely that transmission at a slightly different frequency f E ′ would likewise be received at the base station  200  without significant attenuation, etc. However, this is not strictly necessary, and f E  can be significantly different from f E ′. Alternatively, E-field  302  and transmission of data from the RF transmitter/receiver  166  can be time multiplexed, in which case f E  can equal f E ′. 
     Having described the charging circuitry of both base station  200  and IPG  100 , discussion now turns to describing an exemplary method for charging the IPG  100  using the base station  200 . In this example, the base station  200  automatically produces a charging field when turned on, and in particular, microcontroller  212  ( FIG. 7 ) initially selects B-field charging as default. Using a B-field  304  as default means of charging is preferred if it can be accomplished, because it can generally provide more energy to the IPG  100 , and hence can charge the battery  26  faster. Therefore, microcontroller  212  outputs K 1 =1 to set the base station for B-field charging: i.e., to set the signal generator  214  to output a frequency of f B =80 kHz; to activate the switch  222  to couple the transmission circuitry to the coil  206 ; and to enable LSK receiver  220 . At this point, the base station  200  is broadcasting the B-field  304 , with the hope that an IPG  100  will receive this broadcast, and will acknowledge receipt by broadcasting either some sort of acknowledgment, or the charging information discussed earlier. Accordingly, microcontroller  212  waits for a certain period of time (e.g., one minute) to receive back telemetry data at the LSK receiver  220 . During this “B period,” the base station  200  can adjust the strength of the B-field  304  via the gain control signal with the hope of producing a B-field  304  that will eventually be large enough to be recognized by the IPG  100 . For example, the base station may start with gain control at its smallest setting, and ramp the gain until it reaches a maximum level nearer to the end of the B period. 
     If IPG  100  is within range of the base station  200 , its charging coil  18  ( FIG. 8A ) will receive the B-field  304 . Assuming the B-field reception is strong enough, i.e., if V DCB &gt;V thB , input X to the IPG&#39;s microcontroller  158  will be asserted. As discussed earlier, microcontroller  158  then acknowledges that B-field charging has commenced, and will set up the IPG  100  for charging by assertion of control signal B, which will enable charging circuitry  156  to chose V DCB  as its input, and enable LSK transmitter  160 . At that point, and as is typical in IPGs configured for B-field charging, the LSK transmitter  160  will start telemetering charging information (V BAT ; coupling data, etc.) back to the base station  200  via coil  18 . Such charging information produces reflection in the base station&#39;s coil  206 , and is decoded at the LSK receiver  220 . Receipt of such data (or some other form of acknowledgment) informs the base station  200  that the IPG  100  is receiving the transmitted B-field  304 , and that the base station  200  should stay in B-field default mode by continuing to assert K 1 =1. Moreover, the base station  200  can begin to interpret the received charging information, and modify the produced B-field  304  as necessary, i.e. by changing its magnitude via the gain control signal, and/or by changing its duty cycle. See, e.g., the above-incorporated &#39;733 application. 
     If the battery  26  is fully charged, microcontroller  212 , based on the reported value of V BAT , can cease generating the B-field. At this point, the base station  200  can default to E-field charging, as discussed further below. Providing lower-power E-field charging can be beneficial should the IPG  100 &#39;s battery  26  start to drain during use. If the IPG  100  however will not benefit from E-field charging because its battery  26  is full, it can simply disable charging circuitry  156  for example. 
     If the IPG  100  goes out of the range of the base station  200  or was never within range to start with, input X in the IPG  100  will equal ‘0’. As a result, microcontroller  158  in the IPG  100  will not acknowledge receipt of a B-field (or an E-field at this point), and so control signals B and E will be disabled, such that IPG  100  will not send any form of acknowledgement back to the base station  200 . Eventually, e.g., once the one-minute B period has expired, the microcontroller  212  in the base station  200  will conclude that B-field charging cannot be accomplished, and will now default to E-field charging. Accordingly, microcontroller  212  now asserts K 1 =0, which sets the signal generator  214  to output a frequency of f B =300 MHz; activates the switch  222  to couple the transmission circuitry to the antenna  204 ; and enables RF receiver  228 . 
     In one embodiment, the base station  200  at this point will simply continue broadcasting the E-field  302  so long as it is powered on and without any communication from the IPG  100 , that is, regardless of whether the IPG  100  can acknowledge and use the E-field for charging. This embodiment can be viewed as a simple, passive way to provide E-field charging: i.e., the low-power E-field  302  is produced, and it is hoped, but ultimately unknown, whether the E-field is of use to the IPG  100 . Such an embodiment is simple, because it doesn&#39;t require any communications from the IPG  100  to the base station. Hence, RF transmitter/receiver  166  in the IPG  100 , and RF receiver  228  in the base station  200 , can be dispensed with. However, because this communication route is useful and provides additional flexibility in tailoring the generated E-field, it is discussed further below. 
     If the IPG  100  is within range of the E-field  302 , but outside of the range of the B-field  304 , signal input Y at the IPG&#39;s microcontroller  158  will be set to ‘1,’ assuming E-field reception is strong enough, i.e., if V DCE &gt;V thE . At that point, microcontroller  158  will set up the IPG  100  for charging by assertion of control signal E, which will enable charging circuitry  156  to chose V DCE  as its input, and enable RF transmitter/receiver  166 . The battery  26  will then begin charging, but as discussed above at a slower rate due to the relatively small value of V DCE . RF transmitter/receiver  166  can then transmit the charging information back to the base station  200  via its antenna  150 . At the base station  200 , such charging information is received at antenna  204 , decoded at RF receiver  228 , and used appropriately by the microcontroller  212 . For example, the microcontroller  212  can use the charging information to modify the strength of the generated E-field  302  via the gain control signal for example. Alternatively, the microcontroller  212  could suspend generation of the E-field  302  if V BAT  informs that the battery  26  is fully charged. However, and as discussed above, in another embodiment, the base station  200  can simply continue to generate the lower-power E-field  200  even if the battery is currently fully charged, in the off chance that the battery  26  depletes and will eventually be able to use the E-field for charging once again. 
     Charging of a patient&#39;s IPG battery  26  by E-field is a significant benefit due to its relatively long effectiveness (e.g., &gt;1 m), and even though it is imparts a relatively low amount of power to the IPG  100 , such power can still be put to use to recharge the battery  26  if the patient is in the vicinity of the E-field, even in passing. 
     In a preferred embodiment, the base station  200  can periodically assess whether B-field charging is available, and can switch to that mode if so. This is reasonable because an IPG  100  initially outside of the range of the base station  200  may come within range, because the patient has moved, is now lying down in bed, etc. Accordingly, periodically, e.g., every 15 minutes or so, the base station  200  can revert to the B period discussed above: it can assert K 1 =1 for a period of time to enable B-field charging, and adjust the strength of the B-field  304  to see if the IPG  100  acknowledges B-field reception. If so, the base station  200  can continue production of the higher-power B-field  304 . If not, the base station  200  can once again start generating the lower-power E-field  302  at the expiration of the B period. 
       FIGS. 9 and 10  respectively show alternative embodiments for the base station  200  and IPG  100 , where the back telemetry is carried out using hardware and a communication channel that is separate from those used to charge the IPG. Thus,  FIG. 9  shows the base station with an additional antenna  232 , and  FIG. 10  shows the IPG having an additional antenna  13 . In this example, the antennas  232  and  13  are shown as coils, and communicate by magnetic induction. This is convenient, and is considerate of legacy system design, because the antenna  13  already exists in the IPG  100  and is traditionally used to communicate with an external controller  12  ( FIG. 2 ) as discussed in the Background. As explained earlier, such communication between antennas  232  and  13  could occur using an FSK protocol, and thus FSK transceivers  221  and  124  are shown coupled to antennas  232  and  13 . Such communication can be bi-directional, or one-way from the IPG  100  to the base station for the purpose of telemetering the charging information. By using the pre-existing coil  13  in the IPG  100  to also communicate with the base station  200  during charging, system functionality can be expanded without the need to modify existing communication circuitry in the IPG  100 . However, is it not strictly necessary to use the pre-existing communications coil  13  in the IPG, and instead a separate dedicated RF or magnetic-induction antenna could be added to the IPG  100  and base station  200  instead. Because this means of transmission between the IPG  100  and the base station is not tied to the communication channels used for charging, note that the FSK transceivers  221  and  124  can be enabled using control signals (FSK) having no connection to the control signals used in the base station  200  or IPG  100  indicative of whether those devices are operating in a B-field or E-field mode (i.e., FSK is independent of control signal K 1  in the base station, or control signals B and E in the IPG  100 ). 
     Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. It will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.