Abstract:
Combination charging and telemetry circuit for use within an implantable medical device uses a single coil for both charging and telemetry that is controlled via the use of an opto-switch. One or more capacitors are used to tune the coil to different frequencies for receiving power from an external device and for the telemetry of information to and from an external device. The opto-switch is coupled to the resonant circuit, but because its input is electrically decoupled from its output, it easy to control.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a non-provisional of U.S. Provisional Patent Application Ser. No. 61/552,362, filed Oct. 27, 2011, which is incorporated by reference and to which priority is claimed. 
         [0002]    The present application is related to U.S. Pat. No. 8,155,752 (the &#39;752 patent). 
     
    
     FIELD OF THE INVENTION 
       [0003]    This application relates to improved circuitry for an implantable medical device having a single coil for both telemetry and power reception. 
       BACKGROUND 
       [0004]    Implantable stimulation devices generate and deliver electrical stimuli to 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, occipital nerve stimulators to treat migraine headaches, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The present invention may find applicability in all such applications and in other implantable medical device systems, although the description that follows will generally focus on the use of the invention in a Bion® microstimulator device system of the type disclosed in U.S. Patent Publ. No. 2010/0268309. However, the invention can also be used in a Spinal Cord Stimulator (SCS), such as is disclosed in U.S. Pat. No. 7,444,181, for example. 
         [0005]    Microstimulator devices typically comprise a small, generally-cylindrical housing which carries electrodes for producing a desired stimulation current. Devices of this type are implanted proximate to the target tissue to allow the stimulation current to stimulate the target tissue to provide therapy for a wide variety of conditions and disorders. A microstimulator usually includes or carries stimulating electrodes intended to contact the patient&#39;s tissue, but may also have electrodes coupled to the body of the device via a lead or leads. A microstimulator may have two or more electrodes. Microstimulators benefit from simplicity. Because of their small size, the microstimulator can be directly implanted at a site requiring patient therapy. 
         [0006]      FIG. 1  illustrates an exemplary implantable microstimulator  100 . As shown, the microstimulator  100  includes a power source  145  such as a battery, a programmable memory  146 , electrical circuitry  144 , and a coil  147 . These components are housed within a capsule  202 , which is usually a thin, elongated cylinder, but may also be any other shape as determined by the structure of the desired target tissue, the method of implantation, the size and location of the power source  145 , and/or the number and arrangement of external electrodes  142 . In some embodiments, the volume of the capsule  202  is substantially equal to or less than three cubic centimeters. 
         [0007]    The battery  145  supplies power to the various components within the microstimulator  100 , such the electrical circuitry  144  and the coil  147 . The battery  145  also provides power for therapeutic stimulation current sourced or sunk from the electrodes  142 . The power source  145  may be a primary battery, a rechargeable battery, a capacitor, or any other suitable power source. Systems and methods for charging a rechargeable battery  145  will be described further below. 
         [0008]    The coil  147  is configured to receive and/or emit a magnetic field that is used to communicate with, or receive power from, one or more external devices that support the implanted microstimulator  100 , examples of which will be described below. Such communication and/or power transfer may be transcutaneous as is well known. 
         [0009]    The programmable memory  146  is used at least in part for storing one or more sets of data, including electrical stimulation parameters that are safe and efficacious for a particular medical condition and/or for a particular patient. Electrical stimulation parameters control various parameters of the stimulation current applied to a target tissue including the frequency, pulse width, amplitude, burst pattern (e.g., burst on time and burst off time), duty cycle or burst repeat interval, ramp on time and ramp off time of the stimulation current, etc. 
         [0010]    The illustrated microstimulator  100  includes electrodes  142 - 1  and  142 - 2  on the exterior of the capsule  202 . The electrodes  142  may be disposed at either end of the capsule  202  as illustrated, or placed along the length of the capsule. There may also be more than two electrodes arranged in an array along the length of the capsule. One of the electrodes  142  may be designated as a stimulating electrode, with the other acting as an indifferent electrode (reference node) used to complete a stimulation circuit, producing monopolar stimulation. Or, one electrode may act as a cathode while the other acts as an anode, producing bipolar stimulation. Electrodes  142  may alternatively be located at the ends of short, flexible leads. The use of such leads permits, among other things, electrical stimulation to be directed to targeted tissue(s) a short distance from the surgical fixation of the bulk of the device  100 . 
         [0011]    The electrical circuitry  144  produces the electrical stimulation pulses that are delivered to the target nerve via the electrodes  142 . The electrical circuitry  144  may include one or more microprocessors or microcontrollers configured to decode stimulation parameters from memory  146  and generate the corresponding stimulation pulses. The electrical circuitry  144  will generally also include other circuitry such as the current source circuitry, the transmission and receiver circuitry coupled to coil  147 , electrode output capacitors, etc. 
         [0012]    The external surfaces of the microstimulator  100  are preferably composed of biocompatible materials. For example, the capsule  202  may be made of glass, ceramic, metal, or any other material that provides a hermetic package that excludes water but permits passage of the magnetic fields used to transmit data and/or power. The electrodes  142  may be made of a noble or refractory metal or compound, such as platinum, iridium, tantalum, titanium, titanium nitride, niobium or alloys of any of these, to avoid corrosion or electrolysis which could damage the surrounding tissues and the device. 
         [0013]    The microstimulator  100  may also include one or more infusion outlets  201 , which facilitate the infusion of one or more drugs into the target tissue. Alternatively, catheters may be coupled to the infusion outlets  201  to deliver the drug therapy to target tissue some distance from the body of the microstimulator  100 . If the microstimulator  100  is configured to provide a drug stimulation using infusion outlets  201 , the microstimulator  100  may also include a pump  149  that is configured to store and dispense the one or more drugs. 
         [0014]    Turning to  FIG. 2 , the microstimulator  100  is illustrated as implanted in a patient  150 , and further shown are various external components that may be used to support the implanted microstimulator  100 . An external controller  155  may be used to program and test the microstimulator  100  via communication link  156 . Such link  156  is generally a two-way link, such that the microstimulator  100  can report its status or various other parameters to the external controller  155 . Communication on link  156  occurs via magnetic inductive coupling. Thus, when data is to be sent from the external controller  155  to the microstimulator  100 , a coil  158  in the external controller  155  is excited to produce a magnetic field that comprises the link  156 , which magnetic field is detected at the coil  147  in the microstimulator. Likewise, when data is to be sent from the microstimulator  100  to the external controller  155 , the coil  147  is excited to produce a magnetic field that comprises the link  156 , which magnetic field is detected at the coil  158  in the external controller. Typically, the magnetic field is modulated, for example with Frequency Shift Keying (FSK) modulation or the like, to encode the data. For example, data telemetry via FSK can occur around a center frequency of 125 kHz, with a 129 kHz signal representing transmission of a logic ‘1’ and 121 kHz representing a logic ‘0’. 
         [0015]    An external charger  151  provides power used to recharge the battery  145  ( FIG. 1 ). Such power transfer occurs by energizing the coil  157  in the external charger  151 , which produces a magnetic field comprising link  152 . This magnetic field  152  energizes the coil  147  through the patient  150 &#39;s tissue, and which is rectified, filtered, and used to recharge the battery  145 . Link  152 , like link  156 , can be bidirectional to allow the microstimulator  100  to report status information back to the external charger  151 . For example, once the circuitry  144  in the microstimulator  100  detects that the power source  145  is fully charged, the coil  147  can signal that fact back to the external charger  151  so that charging can cease. Charging can occur at convenient intervals for the patient  150 , such as every night. 
         [0016]      FIG. 3A  shows the circuitry within microstimulator  100  that is coupled to coil  147 . Such circuitry is explained in detail in the &#39;752 patent that was incorporated by reference above. Therefore, the circuitry is only briefly explained here. 
         [0017]    As explained in the &#39;752 patent, the circuitry of  FIG. 3A  is beneficial because it uses a single coil L 1  ( 147 ) receiving a magnetic charging field  152  from the external charger  151 , and for transmitting and receiving data telemetry  156  to and from the external controller  155 . (The external charger  151  and external controller  155  are shown in  FIG. 3A  as one integrated unit for simplicity). 
         [0018]    Coil  147  is connected at one end through transistor switch M 1  to a voltage, Vbat, provided by the battery  145  in the microstimulator  100 , which may ranges from 2.5V to 4.2 Volts. Coil  147  is connected at its other end through transistor switch M 2  to ground. Capacitor C 1  is connected in parallel with coil  147 , thus forming a resonant tank circuit, and tunes the tank circuit to a particular frequency for transmitting or receiving data telemetry to and from the external controller  155  (e.g., approximately 125 kHz). A series combination of a capacitor C 2  and transistor switch M 3  are also connected in parallel to coil  147 . Transistor M 3  is turned on during receipt of a magnetic charging field along link  152  from the external charger  151  to tune the tank circuit to the frequency of the magnetic charging field (e.g., approximately 80 kHz). Also connected in parallel with coil  147  is a full bridge rectifier represented by diodes D 1 -D 4  for producing DC voltage Vout. A half bridge rectifier could also be used. Diodes D 1 -D 4  may comprise, for example, Schottky diodes having forward voltage drops of approximately 0.4V. A transistor switch M 4  is also connected between the rectifier circuitry and ground. 
         [0019]    DC voltage Vout is received at storage capacitor C 3 , which smooths the voltage before being passed to charging circuitry  92 . Charging circuitry  92  is used to charge battery source  145  in a controlled fashion. If needed, a Zener diode D 5  or other suitable voltage clamp circuit may be connected across capacitor C 3  to prevent Vout from exceeding some predetermined value, e.g., 6.2V. 
         [0020]      FIG. 3B  shows the status of transistor switches M 1 -M 4  for the energy receive, data receive, and data transmit modes. As shown, to operate in an energy receive mode, the circuit will turn switches M 1 , M 2  and M 4  OFF, and will turn switch M 3  ON. Turning M 3  ON includes capacitor C 2  in parallel with capacitor C 1 , which, in conjunction with the inductance formed by the coil  147 , forms a resonant circuit which is tuned to the frequency of the magnetic charging field. The circuit of  FIG. 3A  may also operate in a data transmit mode during charging by employing back telemetry known as Load Shift Keying (LSK), in which case transistor M 4  is modulated with the data to be transmitted back to the external charger  151 . Switches M 1 -M 4  is typically standard semiconductor switches, such as MOSFET switches. 
         [0021]    For the circuit of  FIG. 3A  to operate in a data receive mode, the circuit will turn switches M 1 , M 3  and M 4  OFF, and will turn switch M 2  ON. Turning M 3  off excludes capacitor C 2  from the resonant circuit, whose tuning is thus governed by coil  147  and capacitor C 1 . With capacitor C 2  excluded, the resonant circuit is tuned to a higher frequency matching the operation of the external controller  155 . Turning M 2  ON grounds the resonant circuit, which provides an input to the receiver, which demodulates the received data (DATA RCV). The receiver can either comprise a differential input as illustrated in solid lines in  FIG. 3A , or can comprise a single-ended non-differential input in which one of the inputs is grounded, as shown by the dashed line in  FIG. 3A . 
         [0022]    As further shown in  FIG. 3B , the circuit of  FIG. 3A  may also operate in a data transmit mode by turning switches M 3  and M 4  OFF, by modulating switch M 2  with a data signal (DATA XMIT), and by turning switch M 1  ON. Under these conditions, the resonant circuit is once again, by virtue of transistor M 3  being OFF, tuned to the higher frequency, and will broadcast a signal to the external control unit  151 / 155  along link  156  accordingly, with the energy for the radiation being supplied from the battery voltage, Vbat, via transistor M 1 . The transmitter receiving the data to be transmitted (DATA XMIT), is shown coupled to transistor M 2 , but could also couple to transistor M 1 . 
         [0023]    Thus, it is seen that by selectively controlling the state of the switches M 1 -M 4 , the circuit of  FIG. 3A  may operate in different modes, using only a single coil  147 . Such modes may be invoked in a time-multiplexed manner, e.g., with a first mode being followed by a second mode, depending upon the particular application at hand. Control signals M 1 -M 4 , as well as DATA XMIT, are ultimately issued by a microcontroller  160  in the microstimulator  100 , and DATA RCV is received by that microcontroller. 
         [0024]    The inventors have noticed that integrating such functionality into a circuit with only one coil  147  presents technical challenges. For one, it is difficult to operate a MOSEFT switch M 1  during the energy receive mode because of the large swing in AC voltage produced at the drain of transistor M 1  (node X in  FIG. 3A ). During the energy receive mode, transistors M 1  and M 2  are off, thus decoupling the coil  147  from Vbat and ground. As a result, the AC voltage across the coil  147  is somewhat indeterminate, but will still be bounded by the various diodes in the circuit. For example, the maximum voltage at node X will be determined by the threshold voltage of diode D 3  and the breakdown voltage of Zener diode D 5 . Assuming low-threshold-voltage Schottky diodes in the rectifier (Vts=0.4V) and a breakdown voltage in the Zener diode D 5  of 6.2V, node X will not exceed 6.6 Volts or so. At node Y, i.e., the drain of switch M 2 , the voltage is clamped by diode D 2 , such that node Y cannot drop below its threshold, i.e., −0.4V. Therefore, in accordance with the nature of the AC circuitry, the voltages at nodes X and Y will vary from −0.4V to 6.6V during the energy receive mode. 
         [0025]      FIG. 3C  shows switches M 1  and M 2  in cross section to better appreciate the problems of the prior art circuitry of  FIG. 3A . Assuming switch M 2  is an N-channel silicon MOSFET device as indicated by the p/n doped regions, there is no concern of leakage through that switch in the energy receive mode. The body diode B 2  in switch M 2  will have threshold voltage of about Vt=0.6V or so. Therefore, even at lower voltages present at node Y (i.e., −0.4 V), body diode B 2  will not become forward biased and conduct; again, it is clamped and prevented from going below this voltage by Schottky diode D 2 . (It is assumed that the substrate of switch M 2  is tied to its source and is thus held at ground, i.e., Vsub 2 =0V). Thus, there is no risk of inadvertent leakage from a traditional silicon MOSFET switch M 2 , and that transistor can be turned off as is desired in the energy receive mode. 
         [0026]    By contrast, these same voltages at node X do present a leakage problem in switch M 1 . There is no means for clamping the voltage at node X akin to diode D 2 . Moreover, the substrate of switch M 1  is typically connected to its source, Vbat (i.e., Vsub 1 =Vbat). As such, the body diode B 1  in switch M 1  will leak when the voltage at node X is less than Vbat−Vt. Given typical voltage ranges of 2.5V&lt;Vbat&lt;4.2V, body diode B 1  will become forward biased when node X is at lower voltages, for example from −0.4V to perhaps 3V or so. This at least partially defeats the operation of switch M 1 , which is supposed to be off at all times during the energy receive mode. Worse, because the substrate of switch M 1  is tied to Vbat, such leakage will occur directly from the battery  145 . This is contrary to the desired purpose of the energy receive mode—which is to charge the battery−and also defeats the charging circuitry  92  ( FIG. 3A ), which is meant to isolate the battery during the energy receive mode and control its charging. 
         [0027]    Better means of switching are possible in lieu of a traditional MOSFET switch M 1 , but would involve the use of multiple transistors, and more complicated means of control, such as the use of non-standard signals levels to control such transistors. Thus, a better solution is needed to support the sort of single-coil multi-function communication circuitry of  FIG. 3A , and a solution is provided in this disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  shows a microstimulator of the prior art. 
           [0029]      FIG. 2  shows a microstimulator of the prior art as implanted in a patient, as well as an external controller and an external charger. 
           [0030]      FIGS. 3A-3C  show the communication and charging circuitry in the microstimulator of the prior art, and the various modes in which such circuitry can be operated. 
           [0031]      FIGS. 4A and 4B  show the communication and charging circuitry in a microstimulator in accordance with an embodiment of the invention using an opto-switch, as well as the various modes in which such circuitry can be operated. 
           [0032]      FIG. 5  shows further details of one example of an opto-switch useable in an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Improved circuitry for an implantable medical device is disclosed that utilizes an “opto-switch” (i.e., an optically-isolated switch, such as a photocoupler) that allows for a single coil to be used safely and reliably for both charging and communication. 
         [0034]    Opto-switches use light waves to provide switching control with electrical isolation between the photocoupler&#39;s input (its gating signal) and its output (the two terminal of the switch). An exemplary opto-switch contains a source of light (e.g., a near infrared light-emitting diode (LED)) controlled by an electrical gating control signal; an optical channel for transmitting that light; and a photosensor for receiving that light and shorting the two terminals of the switch. In other words, the gating control signal is optically coupled to the photosensor, thus controlling the opto-switch. The photosensor can be, for example, a photoresistor, a photodiode, a phototransistor, a silicon-controlled rectifier (SCR), or a triac. An opto-switch allows use of a low voltage gating control signal irrespective of the voltages at its output because the control signal is electrically decoupled from those output voltages. 
         [0035]    The inventors have realized that such a non-semiconductor-based switch is beneficial when gating AC signals in a resonant tank circuit, such as in the single-coil multi-function telemetry and energy receive circuitry of  FIG. 3A , and such an improvement is shown in the improved microstimulator  200  of  FIG. 4A . Similar to the single-coil circuitry of  FIG. 3A , the single-coil circuitry of  FIG. 4A  can receive and transmit data, and can receive power for charging the implantable medical device. New to the circuitry of  FIG. 4A  is the use of an opto-switch M 1 ′, which is controlled by gating control signal, Ictrl, which is electrically decoupled from the voltages on the output side of M 1 ′, i.e., from Vbat and node X. As noted earlier, node X is subject to large voltage swings, particularly in the energy receive mode when M 1 ′ is off. By electrically isolating node X from its gating control signal Icntl in the opto-switch M 1 ′, this switch is easier to control. Moreover, the opto-switch M 1 ′ is not subject to leakage regardless of the voltage level at node X. 
         [0036]      FIG. 4B  shows the status of switches M 1 ′, M 2 , M 3  and M 4  when placing the improved microstimulator  200  in data transmit, data receive, and energy receive modes, and to tune the resonance of the tank circuit. These switches operate as before, including the opto-switch M 1 ′, which is OFF during the energy receive mode, OFF during the data receive mode, and ON during the data transmit mode. Because the opto-switch M 1 ′ is different from the MOSFET switches M 2 , M 3 , and M 4 , its gating control signal Ictrl may also differ, but is simple to generate. For example, it may only be necessary to provide a small voltage, Vctrl, at the input of the opto-switch M 1 ′, which voltage is sufficient to forward bias the LED in the opto-switch and turn it on. A simple regulator circuit can be used to derive Vctrl if necessary. As before, the control signals for the switches can come from microcontroller  160 . As the basic operation of the circuitry has not changed from its description in the Background, such operation is not repeated here. 
         [0037]      FIG. 5  shows the internal circuitry of an exemplary opto-switch that can be used for switch M 1 ′, which can comprise for example TOSHIBA® Photocoupler Photo Relay, Toshiba Part No. TLP3231. This opto-switch is relatively small, measuring about 4.2×2.0×1.8 mm in volume, which is suitably small for inclusion inside an implantable medical device. Terminals  1  and  2  of opto-switch M 1 ′ receive the gating control signal, Ictrl, which causes LED  94  to emit light  95  through the optical channel inside the switch. 
         [0038]    This light is received by two serially-connected photo-sensitive MOSFET transistors  97   a  and  97   b  which act as the photosensor. When illuminated, these normally “off” transistors  97   a  and  97   b  are turned “on.” In other words, transistors  97   a  and  97   b  are normally open between Terminals  3  and  4 , but become a short circuit when illuminated by the LED  94 . These transistors  97   a  and  97   b  are thus completely electrically isolated from the gating control signal, Ictrl, making M 1  switch easier to control even though subject to varying AC voltages at its output terminals. 
         [0039]    Moreover, the two transistors in the opto-switch M 1  are coupled “back to back,” resulting in two body diodes B 3  and B 4  which are back to back. Thus, and unlike the traditional MOSEFT switch M 1  of the prior art ( FIG. 3A ), opto-switch M 1 ′ cannot leak to the substrate: regardless of the voltage at node X, at least one of the body diodes B 3  or B 4  will be reversed biased, thus preventing leakage. 
         [0040]    Modifications to the circuitry of  FIG. 4A , and the opto-switch M 1 ′, are possible. For example, it is not strictly necessary that the opto-switch M 1 ′ occur on the high side of the coil  147  proximate to Vbat. With modification to the polarity of the circuit, the opto-switch could also occur on the low side of the circuit proximate to ground. Opto-switches could also be used in lieu of both of traditional MOSFET switches M 1  and M 2 , although as explained earlier using an opto-switch for switch M 2  is not necessary, at least in the context of the particular circuit of  FIG. 4A . It is not strictly necessary that the transmitter be coupled to switch M 2 ; it could also be coupled to the opto-switch M 1 ′ at the high end of the circuit. Finally, while an opto-switch is particularly preferred to provide isolation between the switch&#39;s input and output, the mere use of back-to-back transistors could be used without optical coupling to its gating control input. That is, traditional electrical signals could be provided to the gates of the back-to-back transistors, although this may require the use of more complicated control signals. Still other modifications are possible. 
         [0041]    Although illustrated as useful in the single-coil, multi-function communication circuitry of  FIG. 4A , it should be noted that opto-switches can be used in connection with other types of communication circuitry present in an implantable medical device. In short, an opto-switch may be used in any such communication circuitry in which it is desirable to isolate AC voltages in the tank circuitry from the control for that circuit. One or more opto-switches may for example be used with a resonant tank circuit in which the coil and capacitor are serially connected. 
         [0042]    While the invention herein disclosed has been described by means of specific embodiments and applications, numerous modifications and variations could be made thereto by those skilled in the art without departing from the literal or equivalent scope of the inventions set forth in the claims.