Low-power system and methods for neuromodulation

1. An implantable or other ambulatory apparatus comprising:

an electrostimulation energy storage capacitor, communicatively coupled to a circuit path to provide quasi-constant current neural stimulation through a load from the electrostimulation energy storage capacitor;

a current measuring circuit communicatively coupled to the circuit path and configured to obtain a measure of quasi-constant current delivered to the load;

a control circuit communicatively coupled to the current measuring circuit, wherein the control circuit is configured to initiate a delivery of biphasic electrical neural therapy to the load including delivery of quasi-constant current neural stimulation during a first phase of the biphasic electrical neural therapy and determine, according to the measure of quasi-constant current delivered to the load, a voltage level sufficient to source the measured quasi-constant current, and initiate adjustment of the voltage level of the storage capacitor to the determined voltage level for a subsequent delivery of quasi-constant current; and

a recharge circuit communicatively coupled to a port configured for electrical connection to the load, wherein the recharge circuit includes a recharge capacitor, wherein the recharge capacitor is configured to:

store electrical energy from the quasi-constant current neural stimulation during the first phase of the biphasic electrical neural therapy; and

discharge the electrical energy to the load during a second phase of the biphasic electrical neural therapy.

2. The apparatus of claim 1 , wherein the control circuit is configured to recurrently initiate obtaining a measure of the delivered quasi-constant load current and adjusting the voltage level of the storage capacitor to minimize a difference between the measured quasi-constant current and the specified target load current.

3. The apparatus of claim 1 , including:

a voltage measuring circuit communicatively coupled to the storage capacitor and configured to measure a change in voltage of the storage capacitor resulting from delivery of the quasi-constant current neural stimulation, and

wherein the control circuit includes a current estimator circuit configured to estimate the quasi-constant current delivered to the load from the measured change in voltage of the storage capacitor.

4. The apparatus of claim 3 ,

wherein measuring a change in voltage of the storage capacitor includes measuring the change in voltage during the first phase of the biphasic electrical neural therapy.

5. The apparatus of claim 1 , wherein the circuit path includes a sense resistor arranged in series with a port configured for electrical connection to the load, and wherein the current monitoring circuit is configured to measure a voltage across the sense resistor and obtain the measure of quasi-constant current delivered to the load using the measured voltage.

6. The apparatus of claim 1 , including:

a resistance measuring circuit communicatively coupled to the circuit path to obtain a measure of resistance of the load, and

wherein the control circuit is configured to initiate adjustment of the voltage level of the storage capacitor for a delivery of quasi-constant current according to the measure of resistance and the target load current.

7. The apparatus of claim 1 , wherein the control circuit is configured to initiate a reduction in the voltage level of the storage capacitor to a minimum voltage level sufficient to operate the current measurement circuit and to provide the specified load current value.

8. The apparatus of claim 1 , including:

a voltage reference circuit configured to provide a changeable reference voltage value; and

a voltage converter circuit, communicatively coupled to the storage capacitor and the voltage reference circuit, configured to provide energy to the storage capacitor according to the reference voltage value,

wherein the control circuit is configured to initiate adjustment of the voltage level of the storage capacitor by changing the reference voltage value.

9. The apparatus of claim 1 , including:

a safety mechanism circuit communicatively coupled to the voltage measuring circuit and the control circuit,

wherein the safety mechanism circuit is configured to monitor delivery of electrical energy during the constant current stimulation, and

wherein the control circuit is configured to end the constant current stimulation when the safety mechanism circuit indicates that the electrical energy exceeds a specified energy level threshold.

10. A method comprising:

delivering quasi-constant current neural stimulation through a load from an electrostimulation energy storage capacitor using a medical device, including delivering quasi-constant current neural stimulation through the load in a first phase of energy delivery and delivering a voltage pulse to the load in a second phase of energy delivery;

obtaining, using the device, a measure of the quasi-constant current delivered to the load;

determining , according to the measure of quasi-constant current delivered to the load, a voltage level sufficient to source the measured quasi-constant current; and

adjusting, using the device, the voltage level of the storage capacitor to the determined voltage level for a subsequent quasi-constant current neural stimulation.

11. The method of claim 10 , including recurrently obtaining the measure of the delivered quasi-constant load current and adjusting the voltage level of the storage capacitor to minimize a difference between the measured quasi-constant current and the specified target load current.

12. The method of claim 10 , including:

measuring a change in voltage of the storage capacitor from delivery of the quasi-constant current neural stimulation; and

wherein obtaining a measure of current includes estimating the quasi-constant current delivered to the load from the measured change in voltage and duration of quasi-constant neural stimulation.

13. The method of claim 12 , including delivering biphasic electrical neural therapy to the load,

wherein delivering quasi-constant current neural stimulation includes delivering quasi-constant current neural stimulation during a first phase of the biphasic electrical neural therapy, and

wherein measuring a change in voltage of the storage capacitor includes measuring the change during the first phase of the biphasic electrical neural therapy.

14. The method of claim 10 , including measuring voltage across a sense resistor in series with the load, wherein obtaining a measure of current includes measuring the quasi-constant current delivered to the load using the measured voltage.

15. The method of claim 10 , including:

obtaining a measure of resistance of the load, and

wherein adjusting the voltage level of the storage capacitor includes adjusting the voltage level of the storage capacitor according to the determined resistance and the target load current.

16. The method of claim 10 , wherein adjusting the voltage level of the storage capacitor includes reducing the voltage level of the storage capacitor to a minimum voltage level required to operate a current measuring circuit and to provide the specified target load current.

17. The method of claim 10 , including:

charging a second capacitor using the constant current neural stimulation delivered through the load in the first phase; and

wherein delivering a voltage pulse includes delivering a voltage pulse to the load from the second capacitor in the second phase of energy delivery.

18. The method of claim 10 , including:

monitoring electrical energy delivered to the load during the quasi-constant current stimulation; and

ending the quasi-constant current stimulation if at least one of the electrical energy exceeds a specified energy level threshold and a rate of discharge of the electrical energy exceeds a specified threshold discharge rate.