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A fixed frequency external power source having an external coil is inductively coupled with an implanted coil of an implanted medical device. The implant device has an electronic impedance transformer as part of its load circuit. Such electronic impedance transformer sets a proper voltage and curren

A fixed frequency external power source having an external coil is inductively coupled with an implanted coil of an implanted medical device. The implant device has an electronic impedance transformer as part of its load circuit. Such electronic impedance transformer sets a proper voltage and current ratio (impedance) so that the coil set, i.e., the external coil and the implanted coil, are loaded with an optimal load. Such optimal loading, in turn, significantly minimizes any mismatch loss from the inductive link between the external coil and the implant coil, and allows wide ranges in the voltage and load resistance and coil separation, while at the same time maintaining an optimal load condition. The impedance transformer is especially applicable to fully implantable cochlear stimulation systems wherein, during one mode of operation, a relatively large power level must be transferred for charging the implanted power storage element, e.g., a rechargeable battery, but wherein another mode of operation, the implant is operated and powered from an external unit and a relatively small power level is transferred to the implant device. The ratio of these power levels may be large, e.g., about 30 to 1, and unless the coil set, i.e., the external coil and implanted coil, are altered between these different load conditions, a huge mismatch loss may occur, which mismatch greatly reduces the power transfer efficiency. The impedance transformer of the invention minimizes such a mismatch loss.

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1. An implantable medical device adapted to receive power through an inductive link, said implantable medical device comprising: an implantable coil adapted to receive an inductively coupled signal from a power source; a rectifier circuit connected to the implantable coil, wherein the rectifier

1. An implantable medical device adapted to receive power through an inductive link, said implantable medical device comprising: an implantable coil adapted to receive an inductively coupled signal from a power source; a rectifier circuit connected to the implantable coil, wherein the rectifier circuit rectifies the inductively coupled signal and produces an operating dc voltage therefrom; electronic circuitry powered from said operating dc voltage that performs a specified function, said electronic circuitry requiring differing amounts of power as a function of how the electronic circuitry is configured; and impedance transforming circuitry connected to the rectifier circuit that monitors changes in the amount of power used by the electronic circuitry and effectuates changes in load impedance as seen by the rectifier circuit so that the load impedance as seen by the rectifier circuit remains approximately constant. 2. The implantable medical device of claim 1 wherein the impedance transforming circuitry comprises a switching regular circuit.3. The implantable medical device of claim 2 wherein the switching regulator circuit comprises a stabilization regulator circuit selected from the group comprising: a step up, a step down, a buck/boost, and a SEPIC regulator circuit.4. The implantable medical device of claim 3 wherein the stabilization regulator circuit includes a voltage control loop and an input resistance RINerror foldback loop.5. The implantable medical device of claim 3 wherein the stabilization regulator circuit comprises: a switch that switches the operating dc voltage generated by the rectifier circuit ON or OFF as controlled by a switch control signal, the switch control signal having a duty cycle; a filter circuit that filters the switched operating dc voltage to produce an output voltage (V 0) having an amplitude that varies as a function of the duty cycle of the switch control signal; an impedance sensing circuit that senses the output voltage (V 0) and output current I0delivered to the electronic circuitry; and a control circuit that generates the switch control signal as a function of changes sensed in the output voltage V 0and output current I0delivered to the electronic circuitry.6. A time-varying impedance transformer for use within an implantable medical device, said implantable medical device being coupled to an external device for receiving power through an inductive link that includes an external coil and an implanted coil, wherein the external coil and the implanted coil comprise a coil set, and wherein the amount of power coupled to the implantable medical device is a function of how much mismatch loss occurs as power is coupled into the implantable medical device, said time-varying impedance transformer comprising a switching regulator that includes means for making the mismatch loss through the coil set constant. 7. The time-varying impedance transformer of claim 6 wherein the means for making the mismatch loss through the coil set constant includes means for operating the switching regulator so as to make the ratio of output voltage and output current as seen by the coil set constant, thereby rendering the mismatch loss constant, even though individual output voltages and output currents are not constant.8. An implantable device adapted to receive power through an inductive link, said implantable device comprising: implantable coil means for receiving an inductively coupled signal from a power source; rectifying means connected to the implantable coil for rectifying the inductively coupled signal and producing an operating dc voltage therefrom; electronic circuit means powered by said operating dc voltage for performing a plurality of specified functions, wherein differing amounts of power are required by said electronic circuit means depending upon the function being performed; and impedance transforming means connected to the rectifying means for monitoring change s in the amount of power used by the electronic circuit means and effectuating changes in load impedance as seen by the rectifying means so that load impedance as seen by the rectifying means remains approximately constant. 9. The implantable device of claim 8 wherein the impedance transforming means comprises a stabilized switching regulator circuit having a voltage control loop and an input resistance RINerror foldback loop.10. The implantable device of claim 9 wherein the stabilized switching regulator circuit further comprises: switching means for switching the operating dc voltage generated by the rectifying means ON or OFF as controlled by a switch control signal, the switch control signal having a duty cycle; filter means for filtering the switched operating dc voltage to produce an output voltage V 0having an amplitude that varies as a function of the duty cycle of the switch control signal; impedance sensing means for sensing the output voltage V 0and output current I0delivered to the electronic circuit means; and control circuit means for generating the switch control signal as a function of changes sensed in the output voltage V 0and output current I0delivered to the electronic circuitry.11. A method of receiving power within an implantable device through an inductive link, comprising: receiving an inductively coupled signal from an external power source through the inductive link; rectifying the inductively coupled signal and producing an operating dc voltage therefrom; applying the operating dc voltage to a multi-function electronic circuit contained within the implantable device, wherein the power consumed by the multi-function electronic circuit varies depending upon the function being performed by the electronic circuit; and monitoring changes in the amount of power used by the electronic circuit; and effectuating changes in load impedance as seen at an input of the inductive link so that load impedance as seen by the inductive link may be kept approximately constant. 12. The method of claim 11 wherein effectuating changes in load impedance to keep load impedance approximately constant, as seen at the input of the inductive link, comprises applying a voltage control loop and an input resistance error foldback loop to a stabilized switching regulator circuit.13. The method of claim 11 wherein monitoring changes in power and effectuating changes in load impedance comprises: switching the operating dc voltage ON or OFF as controlled by a switch control signal, the switch control signal having a duty cycle; filtering the switched operating dc voltage to produce an output voltage V 0having an amplitude that varies as a function of the duty cycle of the switch control signal; sensing the output voltage V 0and output current I0delivered to the multi-function electronic circuit, wherein the ratio of V0to I0comprises a measure of the load impedance; and generating the switch control signal as a function of changes sensed in the output voltage V 0and output current I0delivered to the multi-function electronic circuitry.