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Pulse Position Modulation Telemetry Circuit for Implantable Microstimulator Using RF-Powering Coil

IP.com Disclosure Number: IPCOM000005362D
Publication Date: 2001-Aug-31
Document File: 14 page(s) / 68K

Publishing Venue

The IP.com Prior Art Database

Abstract

A Pulse Position Modulation (PPM) telemetry system for use within small implantable electrical devices, such as a microstimulator, uses an existing RF powering coil and capacitor, to transmit data from the implantable device to an external device. The coil is disconnected from the capacitor, and the capacitor is charged from a power source within the microstimulator. At the correct time, the capacitor is connected to the RF powering coil, thereby allowing the capacitor to discharge through the coil, causing a pulse to be transmitted from the coil. Data may be encoded using pulses created in this manner by the temporal position of the pulses. In accordance with one aspect of the PPM telemetry system, applicable to implantable devices having an existing RF-powering coil through which primary or charging power is received, the existing RF-powering coil is used in a time-multiplexing scheme to provide both the receipt of the RF signal and the PPM telemetry transmission. Such time-multiplexing 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 PPM telemetry system, a PPM system may encode signals in either an analog fashion by continuously varying the time between pulses, or in a digital fashion by assigning one pattern to a "1", and another pattern to a "0". The PPM telemetry system is thus suitable for use within an implantable device, and is compact, and efficient.

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Pulse Position Modulation Telemetry Circuit for Implantable Microstimulator Using RF-Powering Coil

Summary

Known 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. 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.

Fully implantable microstimulators may also be required to include circuits to transmit data to systems external to the body of the patient, or to other microstimulators, for reporting the internal status of the device, and for control purposes. However, transmission of such data with a conventional RF transmission systems (e.g., AM or FM) may consume a significant amount of energy and require relatively complex circuitry. Additional energy consumption results in a requirement for larger energy storage devices, but there is minimal space available inside an implantable microstimulator for such larger energy storage devices or for additional circuitry. Known data transmission systems incorporate an inductive link, but such inductive links require close proximity between the transmitting coil in the implanted microstimulator, and the receiving coil in the external device. Additionally, transmission systems incorporating inductive links may also consume significant energy.

What is therefore needed for implantable microstimulators, 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 capacitors, and a telemetry system, that does not require substantial circuitry or consume signi...