Current management system for a stimulation output stage of an implantable neurostimulation system
View Patent ↗A current management system for use in the stimulation output stage of a neurostimulation system can be programmed to steer different amounts of current through different stimulation electrodes to vary how strongly the tissue adjacent each electrode is stimulated during a particular programmed stimulation episode. An stimulation electrode drive circuit associated with each electrode that is available for stimulation allows independent control of the flow of current through that electrode. A reference electrode is provided in the circuit to source or sink current as necessary to balance the currents going into and out of the patient, so that no stimulation electrode is required to serve that purpose. More specifically, by configuring the circuit to maintain a constant potential at the reference electrode (e.g., a potential that is approximately half way between a top and bottom voltage rail), the reference electrode will source or sink currents as necessary to cause the net current flow into the patient to be equal to the net current flowing out of the patient, thus satisfying Kirchhoff's current law.
1. A stimulation output stage of a neurostimulation system that is at least partially implantable in a body of a human patient comprising:
a plurality of stimulation electrodes implantable in the patient so that each is adjacent or in the patient's tissue;
a plurality of stimulation electrode drive circuits, each of the stimulation drive circuits associated with one of the plurality of stimulation electrodes and including a current sink portion and a current source portion and having a voltage reservoir associated there with;
a reference electrode;
a reference electrode drive circuit associated with the reference electrode and having a constant reference voltage associated there with;
a controller configured to output digital control and timing signals to a bias circuit, the bias circuit configured to provide a digital output and an analog output for each of a plurality of the stimulation drive circuits, the digital output corresponding to one of a plurality of program instructions that control an operation of the stimulation drive circuit to which the digital output is provided, and the analog output determining an amplitude of any current a stimulation electrode drive circuit will cause to be sourced or sunk through its associated stimulation electrode;
wherein each of the plurality of stimulation electrode drive circuits is adapted to: in response to receipt of a first program instruction, enable the current source portion of the stimulation drive circuit to establish a reference current for the stimulation electrode associated with the stimulation drive circuit so that the stimulation electrode sources current into the patient; in response to receipt of a second program instruction, enable the current sink portion of the stimulation drive circuit to establish a reference current for the stimulation electrode associated with the stimulation drive circuit so that the stimulation electrode sinks current out of the patient; in response to receipt of a third program instruction, short the stimulation electrode associated with the stimulation drive circuit; and in response to receipt of a fourth program instruction, cause the stimulation electrode associated with the stimulation drive circuit to have a high impedance; and
wherein the reference electrode drive circuit is adapted to maintain the reference electrode at a constant voltage so that the reference electrode sinks or sources current to balance the current being sourced or sunk by the stimulation electrodes.
2. The stimulation output stage of claim 1 wherein the reference electrode comprises an electrically conductive surface area of a housing for an implantable neurostimulator.
3. The stimulation output stage of claim 1 wherein the voltage reservoir is at a potential between ground potential and a positive voltage.
4. The stimulation output stage of claim 1 wherein the voltage reservoir is at a potential between a negative voltage and a positive voltage.
5. The stimulation output stage of claim 1 wherein each stimulation electrode drive circuit is configured to generate a signal corresponding to a condition that occurs when a voltage reservoir associated with a stimulation electrode drive circuit and a stimulation electrode is not sufficient to permit a programmed amount of current to be sourced or sunk through the stimulation electrode.
6. The stimulation output stage of claim 1 further comprising a feature that will automatically adjust the reservoir voltage within a predetermined range of voltages whenever the reservoir voltage is insufficient to allow a stimulation electrode drive circuit to source or sink a programmed amount of current through its associated stimulation electrode.