Abstract:
An implantable device includes a stimulation electronic circuit, a battery, a receiver configured to receive energy from a source external to the implantable stimulation device, and a battery charger circuit configured to use the energy to charge the battery and power the stimulation electronic circuit, the battery charger circuit including a load switch for connecting/disconnecting the battery, the load switch being controlled by the stimulation electronic circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 11/215,946, filed Aug. 30, 2005, which is incorporated here by reference in its entirety, and to which priority is claimed. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to neurostimulation systems and, in particular, to a battery charger circuit for a battery powered implantable neurostimulation system. 
     BACKGROUND ART 
     Implantable neurostimulation systems generate electrical stimuli to body nerves and tissues for the therapy of biological disorders, such as Spinal Cord Stimulation Systems to treat chronic pain, Cochlear Stimulation Systems to treat deafness, and Deep Brain Stimulation Systems to treat motor and psychological disorders, etc. All of the implantable stimulation systems need energy to operate and generate stimulation. The energy usually comes from an external power source or from an implanted battery. For neurostimulation systems powered by the external device, an external power device is needed to be continuously worn to power the implanted devices. Some neurostimulation systems utilize an implanted battery to provide power for stimulation. An example of one such neurostimulation system is disclosed in U.S. Pat. No. 6,516,227. The rechargeable spinal cord stimulation system with an implanted lithium-ion battery achieves longer operation lifetime and smaller device size. The implanted battery needs to be recharged occasionally to maintain sufficient energy to power the stimulation electronic circuitry. In U.S. Pat. No. 6,516,227, an off-the-shelf, linear regulation battery charger integrated circuit (IC) available from Linear Technology as part number LTC1731-4.1 is used to receive power through inductive coupling from an external charging system and to provide proper charge current and voltage for the battery. 
     In addition, as described in U.S. Pat. No. 6,516,227, if the battery has been discharged below a certain voltage level or completely discharged to zero volts, the rechargeable spinal cord stimulation system will temporarily shut off the stimulation. It may take up to two hours to charge the battery to a capacity that allows stimulation to be resumed again. This means patients may have to wait some time until the battery is charged enough to provide stimulation again. 
     U.S. Pat. No. 5,769,877 discloses an implantable device with a capacitive replenishable power source that is able to replenish and simultaneously deliver stimulating pulses to targeted tissues. 
     U.S. Pat. No. 6,272,382 discloses a fully implantable cochlear stimulator (ICS) system with an implanted rechargeable battery and an external battery charger (EBC). As described in this patent, in the event the implanted battery within the implantable speech processor (ISP) module malfunctions, or for whatever reason cannot be used, or the user or clinician (or other medical personnel) does not want to use it, it is still possible, through use of the EBC to provide operating power to the ISP module and ICS module so that they may continue to function for their intended purpose (e.g., stimulating and/or sensing). By having such a backup option available, the patient may delay battery-replacement and/or corrective surgery indefinitely. 
     U.S. Pat. No. 6,826,430 discloses a fully implantable cochlear prosthesis that includes an implantable housing with a charger 33. As described in this patent, an RF coil 30 receives power and may transmit back telemetry data. The received power is rectified by diode D 1  and powers a linear Lithium Ion Battery Charger 33 to charge an implanted battery 34. A Battery Protection Circuit 35 protects the battery from conditions such as over charge and over-discharge, automatically disconnecting the source or load when necessary. The system can still operate from an external source through the coil 30 if the battery is disconnected. A Buck Converter Circuit(s) 36 derives the necessary power supply voltages from the battery voltage that are required for operation of the prosthesis. 
     It would be desirable to be able to provide a battery charger circuit that permits externally transmitted power to be used for charging an implanted battery and powering a stimulation circuit, and which is controlled by the stimulation circuit and can be used to power the stimulation circuit when the implanted battery is disconnected, defective, or determined to not be suitable for use. 
     It would further be desirable to be able to provide a battery powered implantable neurostimulation system with a battery charger circuit that is easily integrated with modern integrated circuits technology, such as CMOS N-well integrated circuits technology. 
     It would further be desirable to be able to provide a battery powered implantable neurostimulation system with a battery charger circuit that is configured to charge an implanted battery depending upon a specification characteristic of the battery. 
     SUMMARY OF THE INVENTION 
     The battery charger circuit described herein is part of an implantable battery powered neurostimulation system. In an example embodiment, the battery charger circuit provides energy to charge the battery from an external source, and can also simultaneously power a stimulation electronic circuit independent of the battery voltage. Therefore, and beneficially, the patient does not need to wait several hours until the battery is sufficiently charged for stimulation. 
     For some implantable stimulation systems, while the fitting/programming process is executed, it may be necessary to disconnect the rechargeable battery in order to protect the patient in the event of a failure. The battery charger circuit described herein provides a mechanism for electrically disconnecting the battery and for powering the stimulation electronic circuitry using an energy source external to the implanted system. In an example embodiment, if the battery loses its charging capability over time, the battery charger circuitry can be used as an alternative power source to power the stimulation electronic circuitry through inductive coupling from an external charging system so that the implantable neurostimulation system is not required to be explanted. 
     In an example embodiment, an implantable device includes a stimulation electronic circuit, a battery (e.g., a Lithium-Ion battery), a receiver (e.g., a RF power receiver) configured to receive energy from a source external to the implantable stimulation device, and a battery charger circuit configured to use the energy to charge the battery and power the stimulation electronic circuit, the battery charger circuit including a load switch for connecting/disconnecting the battery, the load switch being controlled by the stimulation electronic circuit. In an example embodiment, the battery charger circuit is configured to power the stimulation electronic circuit when the battery is disconnected from the stimulation electronic circuit. In an example embodiment, the battery charger circuit is configured to simultaneously charge the battery and power the stimulation electronic circuit. In an example embodiment, the battery charger circuit includes a load voltage regulator which allows the stimulation electronic circuit to be powered by the battery charger circuit alone without the battery. In an example embodiment, the battery charger circuit includes a charging circuit (e.g., including current limited voltage regulators) configured to charge the battery in multiple charge modes. In an example embodiment, the load switch is configured to provide an interface compatible with CMOS N-well integrated circuit technology. In an example embodiment, the load switch is implemented as a PMOS transistor. In an example embodiment, a substrate of the PMOS transistor is electrically connected to a voltage output of the receiver when the external source is providing energy to the receiver, and to a voltage output of the battery when the external source is not present. 
     In an example embodiment, an implantable device includes a stimulation electronic circuit, a battery, a receiver configured to receive energy from a source external to the implantable stimulation device, and a battery charger circuit configured to use the energy to charge the battery and power the stimulation electronic circuit, the battery charger circuit including a load switch for connecting/disconnecting the battery, the load switch being configured to provide an interface compatible with CMOS N-well integrated circuit technology. In an example embodiment, the battery charger circuit is configured to power the stimulation electronic circuit when the battery is disconnected from the stimulation electronic circuit. In an example embodiment, the load switch is implemented as a PMOS transistor. In an example embodiment, the load switch is controlled by the stimulation electronic circuit. In an example embodiment, the battery charger circuit includes a load voltage regulator which allows the stimulation electronic circuit to be powered by the battery charger circuit alone without the battery. In an example embodiment, the battery charger circuit includes a charging circuit configured to charge the battery in multiple charge modes. 
     In an example embodiment, an implantable charger for a battery of an implantable stimulation device includes a receiver configured to receive energy from a source external to the implantable stimulation device, and circuitry configured to use the energy to charge the battery depending upon a specification characteristic of the battery, and to power the implantable stimulation device when the battery is disconnected from the implantable stimulation device. In an example embodiment, the specification characteristic is a trickle charge voltage threshold. In an example embodiment, the circuitry is configured to charge the battery in multiple charge modes. In an example embodiment, the multiple charge modes include a trickle charge mode which charges the battery with a constant current until a voltage of the battery rises above a threshold. In an example embodiment, the multiple charge modes include a normal charge mode which charges the battery with a constant current when a battery voltage is above a trickle charge voltage threshold and below a constant charge voltage threshold, and thereafter with a constant voltage until a charge current decreases to an end of charge limit, which may be, e.g., substantially zero. In an example embodiment, the charger for a battery of an implantable stimulation device further includes a delay cell to smooth a transition between two of the charge modes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Detailed description of embodiments of the invention will be made with reference to the accompanying drawings: 
         FIG. 1  illustrates an example embodiment of an implantable neurostimulation system; 
         FIG. 2  illustrates an example embodiment of the battery charger circuit of  FIG. 1 ; 
         FIG. 3  illustrates an example embodiment of the charging circuit of  FIG. 2 ; 
         FIG. 4  illustrates an example embodiment of the load voltage regulator of  FIG. 2 ; 
         FIG. 5  illustrates an example embodiment of the load switch of  FIG. 2  and its control circuitry; and 
         FIG. 6  is an example charge profile plot showing charge current and battery voltage over time. 
     
    
    
     DETAILED DESCRIPTION 
     The following is a detailed description for carrying out embodiments of the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the example embodiments of the invention. 
     Referring to  FIG. 1 , in an example embodiment, an implantable neuro stimulation system  100  includes an external portion  102  and an internal portion  104  positioned on opposite sides of a tissue barrier  106  (such as a layer of skin). In use, the internal portion  104  is implanted within the patient beneath the tissue barrier  106  (in a manner generally determined by the nature of the stimulation to be provided). In this example embodiment, the internal portion  104  includes a RF power receiver  108 , a battery charger circuit  110 , a stimulation electronic circuit  112 , a battery protection circuit  114  and a battery  116  (e.g., a rechargeable Lithium-Ion battery) configured as shown. In this example embodiment, the external portion  102  includes an external power supply  118  with a RF power transmitter  120 . Power is generated by the power supply  118  and applied to the battery charger circuit  110 . More specifically, incoming energy induces AC voltage in a coil of the RF power receiver  108 . The RF power receiver  108  converts the induced AC voltage to a fixed DC Voltage as Vcoil. As described below in greater detail, the battery charger circuit  110  provides a charging function in addition to powering the stimulation electronic circuit  112 . The battery protection circuit  114  monitors battery voltage and current, and in the event of a fault, disconnects the battery  116  from the circuit. 
     Referring to  FIG. 2 , in an example embodiment, the battery charger circuit  110  includes a charging circuit  122 , a load voltage regulator  124  and a load switch  126 , configured as shown. In an example embodiment, the charging circuit  122  is configured to provide the required charging current and voltage to charge the battery  116  according to the battery specification. By way of example, if the battery voltage is below a trickle charge voltage (e.g. 2.7V), a fixed trickle charge current Itrickle (usually in the range of C/10, where C indicates the capacity of the battery) is provided on the trickle charge path. If the battery voltage is higher than the trickle charge voltage level, a Constant Current (CC) Inormal is provided on the normal charge path. The load voltage regulator  124  is used to power the stimulation electronic circuit  112  when the battery is disconnected or dead. The load voltage regulator  124  allows the stimulation electronic circuit  112  to be powered by the battery charger circuit  110  alone without the battery  116 . The load switch  126  is used to control the connection/disconnection of the battery  116 . 
     Referring to  FIG. 3 , in an example embodiment, the charging circuit  122  includes two current limited voltage regulators. A first current limited voltage regulator  128  provides trickle charge current Itrickle, and a second current limited voltage regulator  130  provides normal charge current Inormal. In this example embodiment, the charging circuit  122  includes a reference circuit  132  which generates reference voltage VREF and reference current IREF. In this example embodiment, a first current mirror pair CM 1   134  amplifies IREF and generates the trickle charge current Itrickle which is for example C/10, where C indicates the capacity of the battery. In this example embodiment, a second current mirror pair CM 2   136  amplifies IREF and generates the normal charge current Inormal which is for example in the range of C/5 to C/2, where C indicates the capacity of the battery. In operation, charging begins when the Charge Enable signal is asserted, which closes a first switch S 1   138  and a second switch S 2   140 . In this example embodiment, the charging circuit  122  includes a resistor divider  142  formed by resistors RC 1 , RC 2 , and RC 3  configured as shown. The resistor divider  142  senses the battery voltage Vbat and generates sense voltages Vsense 1  and Vsense 2  which are fractions of the battery voltage Vbat determined by the values of the resistors RC 1 , RC 2 , and RC 3 . The voltages Vsense 1  and Vsense 2  are compared with the reference voltage VREF by a first amplifier A 1   144  and a second amplifier A 2   146 , respectively. The outputs of A 1  and A 2  serve as the control signals for a first series-regulating transistor MP 1   148  and a second series-regulating transistor MP 2   150 , respectively. In this example embodiment, the resistor divider  142 , amplifier A 1   144 , series-regulating transistor MP 1   148 , and current mirror pair CM 1   134  form the current limited voltage regulator  128  for trickle charge function. In this example embodiment, the resistor divider  142 , amplifier A 2   146 , series-regulating transistor MP 2   150 , and current mirror pair CM 2   136  form the current limited voltage regulator  130  for normal charge function. 
     At the beginning of the charge cycle in this example embodiment, if the battery voltage is below the trickle charge voltage threshold, the charging circuit  122  (e.g., provided as an IC) goes into a trickle charge mode. In this example embodiment, the charging circuit  122  goes into a normal charge mode after the battery voltage rises above the trickle charge voltage threshold. By way of example, and referring to  FIG. 6 , when the battery voltage Vbat is below the trickle charge voltage threshold (e.g., 2.7V), the charging circuit  122  goes into trickle charge mode and charges the battery at a charge current Icharge of 5 mA. The charging circuit  122  goes into the normal charge mode after the battery voltage rises above the trickle charge voltage threshold and charges the battery at a charge current Icharge (e.g., 40 mA). When the battery voltage is charged to a constant charge voltage threshold (e.g., 3.9V), the charging circuit  122  goes into constant voltage mode until the charge current decreases to an end of charge limit, which may be, e.g., substantially zero. 
     Referring again to  FIG. 3 , it should be appreciated that the charging circuit  122  can be configured in other ways, for example, to switch between charge modes using a different threshold which can be (but is not necessarily) derived from the battery specification. Other criteria can also be used to determine how and when the charging circuit  122  will transition from one charge mode to another. Moreover, the charging circuit  122  can be configured to provide only a single charge mode, or to provide more than two different charge modes. 
     In this example embodiment, in the normal charging mode, the battery  116  is charged in two further modes, namely, a Constant Current (CC)-mode when the cell voltage is above a trickle charge voltage threshold and below a constant charge voltage threshold and then in a Constant Voltage (CV)-mode until the charge current decreases to an end of charge limit, which may be, e.g., substantially zero. In CC-mode, the voltage at the gate of the regulating transistor MP 2   150  is close to ground, and the regulating transistor MP 2   150  is fully opened and the charging current goes to the battery  116 . In CV-mode, the voltage at the gate of the regulating transistor MP 2   150  is close to Vcoil minus the voltage drop across CM 2   136  and S 2   140 , and the charging current that goes to the battery  116  is gradually reduced and charging is stopped finally. In this example embodiment, the charging circuit  122  includes a delay cell DL  152  which helps smooth the transition from trickle charge mode to normal charge mode. 
     Referring to  FIG. 4 , in an example embodiment, the load voltage regulator  124  is implemented as a linear regulator which includes an amplifier A 3   154 , a PMOS pass transistor MP 3   156  and a voltage divider  158  (RV 1  and RV 2 ), configured as shown. In this example embodiment, the load voltage regulator  124  converts the input voltage Vcoil to a fixed optimum operating voltage VOUT (e.g., 3.6V) and supplies the operating voltage to power the stimulation electronic circuit  112  alone when the battery is disconnected or defective. In an example embodiment, the load voltage regulator  124  supplies voltage and current to the stimulation electronic circuit  112  based on the stimulation need. 
     Referring to  FIG. 5 , in an example embodiment, the load switch  126  is controlled at terminal  126   a  by the stimulation electronic circuit  112 . In this example embodiment, the stimulation electronic circuit  112  monitors the battery voltage and provides a control signal Load_OK to the load switch  126 . In an example embodiment, the load switch  126  is normally open between terminals  126   b  and  126   c , and is only closed when the battery voltage is close to the output voltage of the load voltage regulator  124  which prevents additional current from the load voltage regulator  124  from flowing into the battery  116 . For normal operation, the load switch  126  must be closed before the external power supply  118  is removed to connect the stimulation electronic circuit  112  to the battery  116 . In an example embodiment, if the battery voltage is lower than the level the stimulation electronic circuit  112  can operate at, the stimulation electronic circuit  112  shuts itself off before the external power supply  118  is applied. 
     In an example embodiment, the load switch  126  is implemented as a PMOS transistor MPL, and control circuitry  160  (for the load switch  126 ) is configured as shown in  FIG. 5 . When Load_OK is low, in this example embodiment, a NMOS transistor MNL  162  is off so the gate of PMOS transistor MPL  126  is pulled high and the load switch  126  is off. When Load_OK is high, NMOS transistor MNL  162  is on so the gate of PMOS transistor MPL  126  is pulled low and the load switch  126  is on. In an example embodiment, the substrate of the PMOS transistor MPL  126  is always tied to the highest voltage potential of the charger circuit so that the substrate diode is always reverse biased to avoid current leakage. In an example embodiment, the substrate of the PMOS transistor MPL  126  is hooked (electrically connected) to one diode drop below Vcoil if the external power supply  118  is present, or to the battery voltage if the external power supply  118  is not present. In this example embodiment, two diodes  164  and  166  (D 1  and D 2 ) are used to prevent the reverse current from the battery. Alternatively, the diodes D 1  and D 2  can be replaced with transistors configured to function substantially as diodes. In this example embodiment, three resistors  168 ,  170 , and  172  (RL 1 , RL 2  and RL 3 ) in Mega Ohms range are used as shown to prevent direct current flowing from Vcoil to the battery. 
     Although the present invention has been described in terms of the example embodiments above, numerous modifications and/or additions to the above-described embodiments would be readily apparent to one skilled in the art. It is intended that the scope of the present invention extends to all such modifications and/or additions.