Voltage converter for implantable microstimulator using RF-powering coil

A voltage converter for use within small implantable electrical devices, such as a microstimulator, uses a coil, instead of capacitors, to provide a voltage step up and step down function. The output voltage is controlled, or adjusted, through duty-cycle modulation. 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. In accordance with another aspect of the invention, the voltage up/down converter circuit is controlled by a pulse width modulation (PWM) low power control circuit. Such operation allows high efficiencies over a wide range of output voltages and current loads. The invention thus provides a voltage converter circuit for use within an implantable device that is compact, efficient, and provides a wide range of output voltages and currents.

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.

DETAILED DESCRIPTION OF THE INVENTION

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'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. Number 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 receive 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 pow r 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 continuities 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 impedance). This means that an output voltage V o 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 V C , 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 V S , e.g., the battery voltage, to a level suitable for use by the pulse generator circuit 52 . The power source voltage V S 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 V S from its relatively low value to a higher level V C 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 V C and the output voltage V O 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 V C , 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 V C to the pulse generator circuit 52 , e.g., a V C 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 D 1 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 R L , 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 V S . 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 V S . 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 V OUT (greater than V S ) that causes an output current lo to flow through the load resistor R L .

The magnitude of the output voltage V OUT and output current I O may advantageously be controlled by adjusting the duty cycle of the signal 81 . A higher duty cycle causes both V OUT and I O to increase, whereas a lower duty cycle causes V OUT and I O 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 V OUT . That is, a sensing circuit 76 A may be used to monitor the output voltage V OUT , and to compare the sensed output voltage to either a reference voltage V REF 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 V OUT and the reference voltage V REF 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 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. 6 E. 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 well 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 R L 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 V OUT 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 V OUT 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. 6 A), voltage step down (FIG. 6 B), energy reception (FIG. 6 C), data reception (FIG. 6 D), and data transmission (FIG. 6 E). 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 V S . 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 Ni 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 (pH). 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 pH.

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 V S .

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 V OUT 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 V OUT 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. 6 B), 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'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. 6 B.

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. 6 D.

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. 6 E. Hence, in such configuration and mode, the circuit performs a data transmit function, as described previously in connection with FIG. 6 E.

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.