Patent Publication Number: US-7904171-B2

Title: Voltage converter for implantable microstimulator using RF-powering coil

Description:
The present application is a divisional of U.S. application Ser. No. 11/047,052, filed Jan. 31, 2005, which was a continuation application of U.S. patent application Ser. No. 10/679,621, filed Oct. 6, 2003 (now U.S. Pat. No. 6,856,838), which was a continuation of U.S. application Ser. No. 09/799,467, filed Mar. 5, 2001 (now U.S. Pat. No. 6,631,296), which application claimed the benefit of U.S. Provisional Application Ser. No. 60/189,992, filed Mar. 17, 2000. Priority is claimed to each of these applications, and each is incorporated by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to implantable medical devices, and more particularly to a voltage converter for use within an implantable microstimulator, or similar implantable device, that uses an RF-powering coil instead of capacitors to provide a voltage step-up and step-down function. 
     Many implantable medical devices, such as neural stimulators, sensors, and the like, utilize a battery as a primary source of operating power. Other types of implantable devices, such as cochlear stimulators, rely on the presence of an alternating magnetic field to induce an ac voltage into the implantable device, where the induced voltage is thereafter rectified and filtered in order to provide the primary operating power for the device. In both types of devices-a battery-powered device or an RF-powered device—there is a frequent need to derive other operating voltages within the device from the primary power source. That is, there is a frequent need to step up the voltage of the primary power source to a higher voltage in order to, e.g., generate a high stimulation current or for some other purpose. Similarly, in some devices, there is also a frequent need to step down the voltage of the primary power source to a lower voltage for use in certain types of circuits in order to, e.g., conserve power. 
     In order to perform the voltage step-up or step-down function, it is known in the art to use a charge-pump voltage converter circuit. Charge pump circuits typically rely on a network of capacitors and switches in order to step up and step down a primary voltage source. For example, in order to step up a primary voltage source, a network of, e.g., four capacitors, may be connected in parallel through a switching network and maintained in the parallel connection configuration until each capacitor charges to the voltage of the primary power source. The voltage of the primary power source is, e.g., the battery voltage (where a battery is used as the primary power source). Once thus charged, the capacitors are switched so that they are connected in series, thereby effectively creating a voltage across the series connection that is four times the voltage of the primary voltage source. The charge associated with this higher voltage may then be transferred to another capacitor, e.g., a holding capacitor, and this process (or charging parallel-connected capacitors, switching them in series, and then transferring the charge from the series connection to a holding capacitor) is repeated as many times as is necessary in order to pump up the charge on the holding capacitor to a voltage that is four times as great as the voltage of the primary power source. 
     While charge-pump circuits have proven effective for performing step up and step down functions, such circuits require a large number of capacitors, which capacitors may be quite large and bulky. Charge pump circuits that use large numbers of bulky capacitors are not well suited for implantable medical devices that must remain very small. Moreover, charge pump circuits tend to be relatively slow and inefficient in operation. What is needed, therefore, is a voltage converter circuit that is able to perform the step up or step down function, efficiently, quickly, and without having to rely on the use of a large number of bulky capacitor/s. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above and other needs by providing a voltage converter for use within small implantable electrical devices, such as a microstimulator, that uses a coil, instead of capacitors, to provide the voltage step up and step down function. The output voltage of such converter is controlled, or adjusted, through duty-cycle and/or ON/OFF modulation. Hence, good efficiencies are achieved for virtually any voltage within the compliance range of the converter. 
     In accordance with one aspect of the invention, applicable to implantable devices having an existing RF coil through which primary or charging power is provided, the existing RF coil is used in a time-multiplexing scheme to provide both the receipt of the RF signal and the voltage conversion function. This minimizes the number of components needed within the device, and thus allows the device to be packaged in a smaller housing, or frees up additional space within an existing housing for other circuit components. The result is an implantable device having a voltage converter that may be much smaller and/or more densely packed than prior implantable devices. 
     In accordance with another aspect of the invention, the voltage up/down converter circuit is controlled by a pulse width modulation (PWM) and/or ON/OFF modulation (OOM) low power control circuit. Such operation advantageously allows high efficiencies over a wide range of output voltages and current loads. 
     According to another aspect of the invention, an implantable device containing a coil is provided, wherein the coil is used for multiple purposes, e.g., for receiving power from an external source and also as part of a voltage conversion circuit. Alternatively, or conjunctively, the coil may be used for receiving command information from an external source and also as part of a voltage conversion circuit. 
     It is thus a feature of the present invention to provide a voltage converter circuit for use within an implantable device, e.g., such as an implantable microstimulator or similar type of neural stimulator, that is compact, efficient, and provides a wide range of output voltages and currents. 
     It is a further feature of the invention to provide a voltage converter circuit that avoids the use of a network of capacitors switched between parallel and series, or other, configurations in order to provide the step up and step down voltage conversion function. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
         FIG. 1  is a block diagram of an implantable stimulator system; 
         FIG. 2  is a sectional schematic diagram that illustrates one type of implantable microstimulator within which the present invention may be used; 
         FIG. 3  is a functional block diagram of a typical implantable stimulator; 
         FIG. 4  illustrates a type of fly back converter circuit that may be used to step up the voltage of a power source without the use of a switched capacitor network; 
         FIG. 5  is a waveform diagram that defines what is meant by Aduty cycle@ for purposes of the present application; 
         FIGS. 6A-6E  illustrate simplified schematic diagrams of circuits that may be used in accordance with the present invention to respectively achieve the following implantable-device functions: voltage step up ( FIG. 6A ); voltage step down ( FIG. 6B ); energy reception ( FIG. 6C ); data reception ( FIG. 6D ); and data transmission ( FIG. 6E ); 
         FIG. 7  is a simplified schematic diagram that illustrates a voltage converter circuit made in accordance with the present invention that selectively performs the five implantable-device functions illustrated in  FIGS. 6A-6E ; and 
         FIG. 8  is a table that defines the operating state of the various switches M 1 ′, M 2 , M 3 , M 4  and M 5  utilized in the circuit of  FIG. 7  in order to select a desired operating mode for the circuit shown in  FIG. 7 . 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
     The present invention relates to a particular type of voltage converter that may be used within an implantable medical device, such as an implantable stimulator, sensor, pump or other type of medical device providing a desired medical function. The invention will be described below in terms of an implantable stimulator, but it is to be understood that the invention may be used within many different types of implantable devices. 
     To better understand the environment in which the invention is intended to be used, it will first be helpful to review a typical implantable stimulation system. Hence, with reference to  FIG. 1 , a block diagram of a representative implantable stimulator system  10  is illustrated. The system  10  includes an implant device  20 , implanted under the skin  18 , coupled to an external control unit  12  through implanted coil  22  and external coil  15 . The external coil  15  is typically carried in a housing  14  connected to the external control unit  12  via flexible cable  13 . An external power source  16 , which may be, e.g., a rechargeable or replaceable battery, provides operating power for the external control unit. The external power source  16  may also provide operating power for the implant device  20  through the link provided through the coils  15  and  22 , either continuously or on an intermittent basis. Intermittent power is provided, e.g., such as when the implant device includes a replenishable power source, such as a rechargeable battery, and the battery is intermittently recharged. 
     The implant device  20 , when functioning as a stimulator, includes a plurality of electrodes  24   a  and  24   b  connected to the implant device  20  via conductive leads or wires  23   a  and  23   b , respectively. The electrodes  24   a  and  24   b  are typically implanted near body tissue or nerves  26  that are to be stimulated. 
     In operation, the system  10  functions as follows: The implant device  20  and electrodes  24   a  and  24   b  are implanted in the desired location under the patient&#39;s skin  18 . It should be noted that while the implant coil  22  is shown separate from the implant device  20  in  FIG. 1 , the coil  22  is typically mounted to or housed within the same hermetically-sealed case used to house the electronic circuitry associated with the implant device  20 . Once implanted, power and/or control data, e.g., programming data, is transferred to the implant device from the external control unit  12  via electromagnetic coupling between the implant coil  22  and the external coil  15 . Once thus controlled or programmed, the implant device  20  operates as directed by the control signals received, or as steered by the program data stored therein, to generate electrical stimulation pulses for delivery to the tissue  26  via the electrodes  24   a  and  24   b.    
     Some implant devices  20  do not contain an implanted power source, and such devices must thus receive their operating power continuously from the external control unit. Other implant devices  20  do contain an implanted power source, e.g., a rechargeable battery, and such devices thus receive their operating power from the implanted power source. However, on a regular or periodic basis, such devices must have the implanted power source replenished, e.g., have the battery recharged. Such recharging occurs via a link with the external control unit  12 , or equivalent device, through the coils  22  and  15 . 
       FIG. 2  shows a sectional schematic diagram of one type of implantable microstimulator  30  within which the present invention may be used. The microstimulator device  30  includes electrical circuitry  32  housed within a hermetically-sealed case  34 . At each end of the case  34  are electrodes  36   a  and  36   b . These electrodes  36   a  and  36   b  are electrically connected to the electrical circuitry  32  via conductors, e.g., wires,  37   a  and  37   b , respectively, and appropriate feed-through conductors  38   a  and  38   b  that pass through the wall of the hermetically-sealed case  34 . 
     The advantage of the microstimulator device  30  is that it is very small, and can typically be easily implanted at the desired implant location through the lumen of a hypodermic needle, or other cannula. One embodiment of a microstimulator is disclosed, e.g., in U.S. Pat. No. 5,324,316, incorporated herein by reference. One method of making such a microstimulator is disclosed, e.g., in U.S. Pat. No. 5,405,367, also incorporated herein by reference. 
     To better appreciate the advantages offered by the present invention, reference is next made to  FIG. 3  where there is shown a functional block diagram of a typical implantable stimulator  40 . As seen in  FIG. 3 , the stimulator  40  includes electronic circuitry that performs the following functions: an energy receiver  42 , a data receiver  44 , a power source  46 , a control circuit  48 , a voltage converter  50 , a pulse generator  52 , and a back telemetry circuit  54 . An implanted coil  56  is connected to both the energy receiver  42  and the data receiver  44  and provides a means through which power and data signals may be received by the stimulator  40 . Another coil  58 , which in some embodiments may comprise the same, or a portion of, the coil  56 , is connected to the back telemetry circuit  54 , and provides a means through which back telemetry data may be sent to an external receiver. Such an external receiver may be included, for example, within the external control unit  12  ( FIG. 1 ). All of the above-described elements of the stimulator  40  are housed within an hermetically-sealed housing or case  60 , thereby allowing the stimulator  40  to be implanted within body tissue. 
     External to the housing  60 , but still adapted to be implanted within body tissue, is a plurality of electrodes  62   a ,  62   b . Electrical connection with the plurality of electrodes  62   a ,  62   b  is established through a plurality of wire conductors  63   a ,  63   b  (which may be included within a single implantable lead body, as is known in the art) which are respectively connected to a plurality of feed-through connectors  64   a ,  64   b  that pass through the hermetically-sealed wall of the case  60 . The pulse generator  52  is electrically coupled to the plurality of feed-through connectors  64   a ,  64   b  on the inside of the case  60 . 
     In operation, an RF signal (represented in  FIG. 3  by the wavy arrow  66 ) is received through coil  56 . Typically, the RF signal comprises a modulated carrier signal. The carrier signal is rectified in the energy receiver  42  and provides charging power for the power source  46 . The carrier signal is demodulated in the data receiver  44  and the data thus recovered provides control and/or programming data to the control circuit  48 . The control circuit  48 , typically a microprocessor, includes memory circuitry (not shown) wherein programming and/or control data may be stored. Based on this programming and/or control data, the control circuit  48  drives the pulse generator circuit  52  so that it generates and delivers electrical stimulation pulses to the patient through selected groupings of the plurality of electrodes  62   a ,  62   b.    
     In the process of generating the electrical stimulation pulses, which typically vary in amplitude as a function of the control and/or programming data, and in order to conserve power, it is necessary to provide a high level supply voltage to the pulse generator circuit  52 . For example, if the impedance between electrodes  62   a  and  62   b  is 1000 ohms, and if a stimulation current pulse having a magnitude of 10 ma is desired, a voltage of 10 volts must be present at the electrodes  62   a  and  62   b  (Ohms law: voltage=current x impedance). This means that an output voltage VO of at least 10 volts must be present at the output of the pulse generator circuit  52 . In turn, this means that a supply voltage VC, provided to the pulse generator circuit by the voltage converter  50 , must be greater than 10 volts, e.g., 12 volts or more due to losses within the pulse generation circuit. Hence, the voltage converter circuit  50  is typically used in a stimulator  40  to step up the power source voltage VS, e.g., the battery voltage, to a level suitable for use by the pulse generator circuit  52 . The power source voltage VS is typically a low value, e.g., 2 or 3 volts. Hence, in a typical stimulator device  40 , such as the one shown in  FIG. 3 , the voltage converter circuit  50  is needed to boost, or step up, the source voltage VS from its relatively low value to a higher level VC as needed by the pulse generator circuit  52 . Unfortunately, in order to provide such a step-up function, bulky and numerous circuit components, such as the capacitors used in a switched capacitor network, and/or transformers, must be employed. 
     The difference between the supply voltage VC and the output voltage VO may be referred to as the compliance voltage. In an ideal pulse generator circuit  52 , the compliance voltage is kept as low as possible because the power dissipated in the pulse generator circuit (which is generally considered as wasted or lost power because it does not represent power delivered to the tissue) is proportional to the square of the compliance voltage. In practice, the compliance voltage cannot always be minimized because the current delivered through the electrodes  62   a  and  62   b  to the body tissue varies over a wide range; and hence the compliance voltage must also vary over a wide range. 
     In some implantable stimulators  40 , in order to conserve the amount of power dissipated by the stimulator, the voltage converter circuit  50  is used to adjust the supply voltage VC, typically to provide a small number of discrete levels of supply voltage, as a function of the current to be delivered in the stimulation pulse. For example, a typical voltage converter circuit  50  may provide one of four different supply voltages VC to the pulse generator circuit  52 , e.g., a VC of 2.5, 5.0, 7.5 or 10 volts, as a function of the programmed amplitude of the stimulation pulse that is to be delivered to the tissue. An implantable stimulator having such a feature is described, e.g., in U.S. Pat. No. 5,522,865, incorporated herein by reference. 
     It is thus seen that the voltage converter circuit  50  performs a very important function within the implantable stimulator  40 . Unfortunately, however, the voltage converter circuit  50  represents additional circuitry that requires bulky circuit components, which takes up needed and valuable space within the case  60 , and much of which also consumes additional power. Further, most voltage converter circuits  50  tend to be very inefficient. That is, a capacitor charge pump circuit, for example, typically may operate at efficiencies that may be less than 50%. Thus, for most stimulators, e.g., of the type shown in  FIG. 2 , space and power considerations are paramount to the design of the stimulator. 
     The present invention advantageously provides circuitry for use within an implantable stimulator device that performs the voltage conversion function using fewer and less bulky components. This frees up valuable space within the case of the stimulator that may be used for other functions (or allows the case to be smaller), and consumes less power than has heretofore been achievable. Additionally, the present invention provides a circuit that performs multiple functions, thus allowing fewer circuit components to be used within the stimulator design, thereby permitting the overall stimulator design to be smaller or more compact. 
     Turning next to  FIG. 4 , a type of fly back converter circuit is illustrated that may be used to step up the voltage of a power source without the need for a switched capacitor network . The fly back circuit shown in  FIG. 4  includes an inductor or coil L 1  having one end connected to a power source  70 . The other end of the coil L 1  is connected to a first circuit node  72 . A switching transistor M 1  is connected between the first node  72  and ground. The transistor M 1  has a gate terminal  73  connected to a duty cycle control circuit  74 . When the transistor M 1  is turned ON, through application of a signal to its gate terminal  73 , node  72  is effectively switched to ground potential through a very low impedance path. When transistor M 1  is turned OFF, through absence of a signal applied to its gate terminal  73 , it represents a very high impedance path, and thus effectively maintains node  72  disconnected from ground. 
     Also connected to node  72  of the fly back circuit shown in  FIG. 4  is the cathode side of diode D 1 . The anode side of diode DI is connected to an output node  75 . An output capacitor C 1  is connected between the output node  75  and ground. A load, represented in  FIG. 4  by phantom resistor RL, is also connected between the output node  75  and ground. 
     Still with reference to  FIG. 4 , the duty cycle control circuit  74  applies a pulsed signal to the gate of transistor M 1 , thereby effectively turning transistor M 1  ON and OFF as controlled by the pulsed signal. For example, a high voltage applied to the gate of M 1  may turn M 1  ON (provide a low impedance path between node  72  and ground), and a low voltage applied to the gate of M 1  may turn M 1  OFF (provide a high impedance path between node  72  and ground). A sequence of high and low voltages may be applied to the gate  73  of transistor M 1  through application of a pulsed signal  81  generated by the duty cycle control circuit  74 . When a pulse is present, the voltage is high, and the transistor M 1  is turned ON. When a pulse is not present, the voltage is low, and the transistor M 1  is turned OFF. 
     The ratio of time when the pulse is high to the total cycle time is known as the “duty cycle”. The duty cycle is defined as shown in  FIG. 5 . As seen in  FIG. 5 , a pulsed signal  81  comprises a train of pulses  80 . Each pulse  80  comprises a high voltage for a period of time T 2  and a low voltage for a period of time T 3 . The total cycle time T 1  is equal to T 2  plus T 3 . Duty cycle is typically defined as a percentage and is computed as the ratio of T 2 /T 1  or T 2 /(T 2 +T 3 ). The duty cycle may thus vary from 0% when T 1 =0, to 100% when T 1 =T 2 . 
     The operation of the fly back circuit of  FIG. 4  is known in the art. Basically, when transistor M 1  is turned ON, during time period T 2 , circuit node  72  is connected to ground, which connects one side of the coil L 1  to ground. This connection of one side of the coil L 1  to ground causes an electrical current to start to flow from the power source  70  through the inductor coil L 1 . As soon as T 2  ends, however, and for the remaining time T 3  of the total cycle time T 1 , the node  72  floats (is not connected to ground), which causes the voltage at node  72  to step up to a high value (higher than the voltage of the power source VS, as electrical current continues to flow through coil L 1 , through diode D 1 , to charge capacitor C 1 . Thus, during time T 2 , current starts to flow through the coil L 1 , which causes electromagnetic energy to be stored in the coil. During time T 3 , this energy is transferred to capacitor C 1 , thus charging C 1 . Eventually, typically over several cycles, C 1  is charged up to a voltage that is higher than the power source voltage VS. Capacitor C 1  is blocked from discharging to ground through transistor M 1  by diode D 1  when M 1  is turned ON during time T 2 . The stored charge held on capacitor C 1  thus provides an output voltage VOUT (greater than VS) that causes an output current IO to flow through the load resistor RL. 
     The magnitude of the output voltage VOUT and output current IO may advantageously be controlled by adjusting the duty cycle of the signal  81 . A higher duty cycle causes both VOUT and IO to increase, whereas a lower duty cycle causes VOUT and IO to decrease. Because the duty cycle is adjusted by controlling the pulse width (T 2 ) of the pulses  80 , the duty cycle control circuit  74  may also be referred to as a pulse width modulator circuit. 
     Still with reference to  FIG. 4 , it should also be noted that feedback may optionally be employed to better control and regulate the output voltage VOUT. That is, a sensing circuit  76 A may be used to monitor the output voltage VOUT, and to compare the sensed output voltage to either a reference voltage VREF and/or a programmed reference signal PROG (which typically is presented to the sensing circuit  76 A as a digital signal). The sensing circuit  76 A generates a difference signal, on signal line  76 C, representing the difference between the sensed output voltage VOUT and the reference voltage VREF and/or PROG. This difference signal controls a gate control circuit  76 B, which modulates the gate of transistor M 1  so as to drive the difference signal to zero. 
     Turning next to  FIGS. 6A-6E , additional simplified schematic diagrams of circuits are illustrated that may be used in accordance with the present invention to achieve desired functions. More particularly, a voltage step up function may be achieved using the circuit shown in  FIG. 6A ; a voltage step down function may be achieved using the circuit of  FIG. 6B ; an energy reception function may be achieved using the circuit of  FIG. 6C ; a data reception function may be achieved using the circuit of  FIG. 6D ; and a data transmission function may be achieved using the circuit of  FIG. 6E . Advantageously, many of the components used in the circuits of  FIGS. 6A-6E  may be the same. Common reference numerals are used to denote the components that may be the same. A brief explanation of each of these functions will next be described. 
       FIG. 6A  depicts a circuit that performs a voltage step up function. This circuit is substantially the same as the circuit previously described in connection with  FIG. 4 , except that the load resistance RL is not shown. However, it is to be understood that a load resistance may be present. It should also be understood that whereas  FIG. 4  shows a duty cycle control circuit  74  controlling switch M 1 ,  FIG. 6A  shows a PWM (pulse width modulation) control circuit  74 ′ controlling switch M 1 . These circuits perform the same function (turning switch M 1  ON or OFF) and, for purposes of the present invention, are substantially the same. 
       FIG. 6B  depicts a circuit that performs a voltage step down function. As seen in  FIG. 6B , a coil L 1  is connected between circuit nodes  75  and  76 . Node  75  represents the output node of the circuit whereon the output voltage VOUT is present. Capacitor C 1  is connected between node  75  and ground. The anode side of a diode D 2  is connected to node  76 , while the cathode side of diode D 2  is connected to ground. One leg of a transistor switch M 2  is connected to node  76 , while the other leg of transistor switch M 2  is connected to the power source  70  at node  77 . A gate terminal  78  of transistor M 2  is connected to pulse-width modulation (PWM) control circuit  74 ″. 
       FIG. 6C  shows a circuit that receives energy from an external source. The energy receive circuit shown in  FIG. 6C  includes a coil L 1  having a capacitor C 1  connected in parallel with the coil L 1 , with one side of the parallel connection being grounded. The coil L 1  and capacitor C 1  comprise an “LC” circuit that is tuned to the frequency of an incoming RF signal  83  (represented in  FIG. 6C  by a wavy arrow). Diode D 1  is connected between output node  75  and the other side of the L 1 -C 2  parallel connection, with the cathode of D 1  being connected to node  75 . Capacitor C 1  is connected between output node  75  and ground. 
     In operation, the circuit shown in  FIG. 6C  receives the incoming RF signal  83  through coil L 1 , tuned to the frequency of the signal  83  by capacitor C 2 . Diode D 1  rectifies the signal, storing the positive half cycles of the received signal  83  on capacitor C 1 . The voltage thus developed on capacitor C 1  functions as an output voltage VOUT for use within the implant device. 
     Next, in  FIG. 6D , a data receiver circuit is illustrated. Such data receiver circuit includes coil L 1  connected in parallel with variable capacitor C 3 . A modulated RF signal  88 ′ is received through the coil L 1 . The value of C 3  is adjusted, as required, so that the L 1 -C 3  circuit is tuned to the frequency of modulation applied to the incoming RF signal  88 ′. Node  72 ′, which represents an output node of the L 1 -C 3  circuit, is connected to the input of an amplifier U 1 . The output signal provided by the amplifier U 1  comprises a Data Out signal that reflects the modulation applied to the incoming modulated RF signal  88 ′. 
     Turning to  FIG. 6E , a simple data transmitter circuit is depicted. The data transmitter circuit includes a coil L 1  connected in parallel with an adjustable variable capacitor C 3 . One side of the L 1 -C 3  parallel connection is connected to a power source  70 . The other side of the L 1 -C 3  parallel connection, identified as node  72 ′ in  FIG. 6E , is connected to the anode of diode D 3 . The cathode of diode D 3  is connected through a switch transistor M 3  to ground. The gate terminal of switch M 3  is driven by a “Data Mod” (data modulation) signal. Thus, in operation, when switch M 3  is closed, a current is drawn through the L 1 -C 3  parallel circuit. When switch M 3  is open, no current is drawn through the L 1 -C 3  parallel connection. The on-off current flow through the L 1 -C 3  parallel connection causes a varying current to flow through coil L 1  as controlled by the on-off pattern of the Data Mod signal. This current flow, as is known in the art, induces a varying magnetic field, which in turn causes an RF signal  89  to be radiated, or transmitted, from coil L 1 . 
     Thus it is seen that the circuits illustrated in  FIGS. 6A-6E  provide the functions of voltage step up ( FIG. 6A ), voltage step down ( FIG. 6B ), energy reception ( FIG. 6C ), data reception ( FIG. 6D ), and data transmission ( FIG. 6E ). All of these functions are typically required within an implantable stimulator device ( FIG. 3 ). 
     In order to perform the functions provided by the circuits shown in  FIGS. 6A-6E , while at the same time reducing the number of components needed for each function, and thereby reduce the overall size (and hence volume, weight and power) of the circuitry that carries out such functions, the present invention advantageously combines all the functions performed by the individual circuits shown in  FIGS. 6A-6E  into one circuit as shown in  FIG. 7 . Such combined circuit may be referred to as a “voltage converter using an RF-powering coil”, and is particularly suited for use within an implantable medical device, such as an implantable neural stimulator. 
     Advantageously, the combined circuit provided by the present invention, and shown in  FIG. 7 , uses an RF-powering coil in combination with other circuit elements to perform the function of receiving RF power from an external source. The received RF power may be modulated in order to transmit control data into the circuit. Further, such RF-powering coil may be used to help transmit data out of the circuit. Significantly, the RF-coil used to receive power, data, and to transmit data, may also be used to selectively convert the received power (i.e., voltage) up or down in order to make operation of the circuit more efficient. 
     The circuit of  FIG. 7  (i.e., the voltage converter circuit using an RF-powering coil provided by the present invention) includes a receiving/transmitting coil L 1 ′. The coil L 1 ′ includes ends attached to circuit nodes  72 ′ and  85 , respectively. Node  85 , in turn, is connected through transistor switch M 1 ′ to source voltage VS. The coil L 1 ′ further includes a tap point  85 ′, where there are N 2  turns of the coil between tap point  85 ′ and node  85 , and N 1  turns between tap point  85 ′ and node  72 ′. The coil L 1 ′ thus has a total of N turns, where N=N 1 +N 2 . Representative values of N 1  are 10 to 100 turns, and for N 2  are also 10 to 100 turns, and wherein the inductance of coil L 1 ′ is between about 10 to 100 microhenries (μH). However, in some embodiments, N 1  and N 2  may vary from 1 to 1000 turns, and L 1 ′ may vary between 1 to 1000 μH. 
     Still with reference to  FIG. 7 , a series combination of a capacitor C 3 ′ and transistor switch M 4  is connected between circuit node  72 ′ and tap point  85 ′. Another transistor switch M 5  connects the tap point  85 ′ of coil L 1 ′ to ground (node  87 ). Yet another transistor switch M 2  connects the tap point  85 ′ to the source voltage VS. 
     The cathode end of a diode D 2  is also connected to the tap point  85 ′ of the coil L 1 ′; while the anode end of diode D 2  is connected to ground. 
     The cathode end of another diode D 3  is connected to node  72 ′. The anode end of diode D 3  is connected through transistor switch M 3  to ground (node  87 ). The anode end of diode D 3  is also connected to the input of signal amplifier U 1 . 
     The cathode end of yet another diode D 1  is also connected to node  72 ′. The anode end of diode D 1  is connected to circuit node  75 ′. A capacitor C 1  is connected between node  75 ′ and ground (node  87 ). Circuit node  75 ′ is the location where the output voltage VOUT is made available when the circuit operates in a voltage step up or step down mode. If needed, a suitable voltage clamp circuit  91  may be connected between node  75 ′ and ground in order to prevent the voltage at the output node  75 ′ from exceeding some predetermined value. 
     It is thus seen that the circuit of  FIG. 7  includes five transistor switches, M 1 ′, M 2 , M 3 , M 4  and M 5 . The state of these five switches, whether ON, OFF, or modulated with PWM data or signal data, determines which circuit function is performed as defined in the table presented in  FIG. 8 . That is, as seen in  FIG. 8 , in order for the circuit of  FIG. 7  to operate in a voltage step up mode, switch M 1 ′ is turned ON, M 2  is turned OFF, M 3  is modulated with a PWM signal from a suitable duty cycle control circuit (see  FIGS. 4 and 5 ), and both M 4  and M 5  are turned OFF. Under these conditions, the circuit of  FIG. 7  effectively reduces to the circuit shown in  FIG. 6A , with the only difference being diode D 3  being added in series with switch M 3  (which addition does not significantly alter the operation of the circuit). In such configuration and mode, the level of the output voltage VOUT is determined in large part by the duty cycle of the signal applied to the gate of transistor switch M 3 , as explained previously. 
     Similarly, as defined in  FIG. 8 , for the circuit of  FIG. 7  to operate in a voltage step down mode, switch M 1 ′ is turned OFF, switch M 2  is modulated with a PWM signal from a suitable duty cycle control circuit  74 ″ ( FIG. 6B ), and switches M 3 , M 4  and M 5  are all turned OFF. Under these conditions, the circuit of  FIG. 7  effectively reduces to the circuit shown in  FIG. 6B , with the only difference being diode D 1  connected between nodes  72 ′ and  75 ′ (which diode does not significantly alter the circuit&#39;s operation), and only a portion of coil L 1 ′ being used (i.e., only the turns N 1  are used). In such configuration and mode, the circuit performs a voltage step down function, as described previously in connection with  FIG. 6B . 
     As defined in  FIG. 8 , the circuit of  FIG. 7  may also selectively operate in an energy receive mode and a data receive mode by turning switches M 1 ′, M 2  and M 3  OFF, and by turning switches M 4  and M 5  ON. With the switches in these positions, the circuit of  FIG. 7  effectively reduces to the circuit shown in  FIG. 6C , and to the circuit shown in  FIG. 6D , with the only difference being that just a portion (N 1  turns) of the coil L 1 ′ is used as part of the circuit. In this configuration and mode, the circuit of  FIG. 7  thus performs both an energy receive function as described previously in connection with  FIG. 6C , and a data receive function as described previously in connection with  FIG. 6D . 
     As further defined in  FIG. 8 , the circuit of  FIG. 7  may also selectively operate in a data transmit mode by turning switch M 2  OFF, by modulating switch M 3  with a data signal, and by turning switch M 1 ′ ON. Switch M 4  may be either OFF or ON depending upon whether capacitor C 3 ′ is deemed necessary to better tune coil L 1 ′ for efficient data transmission. For many data transmissions, capacitor C 3 ′ should not be needed. Under these conditions, the circuit of  FIG. 7  effectively reduces to the circuit shown in  FIG. 6E . Hence, in such configuration and mode, the circuit performs a data transmit function, as described previously in connection with  FIG. 6E . 
     Thus, it is seen that by selectively controlling the state of the switches M 1 ′, M 2 , M 3 , M 4  and M 5 , the circuit of  FIG. 7  may operate in any one of five different modes. Some of these modes, e.g., the energy receive mode and the data receive mode, may operate simultaneously. Others of the modes may be invoked in a time-multiplexed manner, e.g., with a first mode being followed by a second mode, and with the second mode being followed by a third mode, as required, depending upon the particular application at hand. Thus, for example, an energy and data receive mode may operate as a first mode to allow the device to receive operating power (e.g, to recharge a battery) and/or to receive initial programming control signals. This first mode may then be followed by a second mode, e.g., a voltage step up mode, initiated by changing the state of switches M 1 ′, M 2 , M 3 , M 4  and M 5  as defined in  FIG. 8 , during which the voltage of the primary power source is stepped up to a voltage needed by the device in order for it to perform its intended function. Subsequently, as required, a third mode, e.g., a data transmit mode, may be invoked in order to allow the implant device to transmit data to an external receiver. 
     The component values of the components, i.e., the transistor switches and capacitors and coil, used in the circuit of  FIG. 7  may be readily ascertained by those of skill in the art for a particular application and desired RF frequency. 
     It is thus seen that the invention described herein provides a voltage converter circuit for use within an implantable device, e.g., such as an implantable microstimulator or similar type of neural stimulator, that is compact, efficient, and provides a wide range of output voltages and currents. 
     It is further seen that the invention provides a voltage converter circuit that avoids the use of a network of capacitors switched between parallel and series, or other, configurations in order to provide the step up and step down voltage conversion function. 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.