System and method for measuring battery current

• US 6,584,355 B2
• Filed: 04/10/2001
• Issued: 06/24/2003
• Est. Priority Date: 04/10/2001
• Status: Expired due to Term

• Current Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Original Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Inventors: Stessman, Nicholas J.
• Primary Examiner(s): LAYNO, CARL HERNANDZ
• Application Number:
  US09/829,825
• Publication Number:
  US 20020163991A1
• Time in Patent Office: 805 Days
• Field of Search: 607/7, 607/9, 607/29
• US Class Current: 607/29
• CPC Class Codes: A61N 1/3708 for power depletion
• CPC Class Codes: G01D 3/08 with provision for safeguarding the apparatus, e.g. against abnormal operation, against breakdown

Abstract

Systems, devices and methods are provided for measuring battery current. According to one aspect, a medical device is provided that comprises a battery, a pulse generator, and a current measuring device. The pulse generator draws a pulse generator current from the power source, and the current measuring device determines the pulse generator current or tracks charge depletion from the battery. The current measuring device comprises an oscillator and a counter. The oscillator produces an oscillating output with a frequency of oscillation dependent on the pulse generator current, and the counter provides an oscillation count for the oscillating output. The current measuring device is capable of being calibrated while continuously determining the pulse generator current. In one embodiment, the current measuring device includes at least two current sources, each including an operational amplifier that has an autozeroing feature.

Claims

1. A medical device, comprising:

a battery;

a pulse generator, wherein the pulse generator draws a pulse generator current from the battery;

an electrode system in electrical communication with the pulse generator, wherein the electrode system applies electrical therapy to a patient; and

a current measuring device for determining the pulse generator current, wherein the current measurement device is adapted for being calibrated while determining the pulse generator current.

2. The medical device of claim 1, wherein the pulse generator comprises:

a processor;

a memory portion operably connected to the processor;

a pulse and sense portion operably connected to the processor and to the electrode system; and

a communication portion operably connected to the processor for communicating with a programmer.

3. The medical device of claim 1, wherein the current measuring device produces an oscillating output with a frequency of oscillation that is dependent on the pulse generator current and provides an oscillation count from which the pulse generator current is determined.

4. The medical device of claim 1, wherein the current measuring device determines the average pulse generator current over a period of time.

5. The medical device of claim 1, wherein the current measuring device determines a total charge depleted from the battery.

6. The medical device of claim 1, wherein the current measurement device detects potential faults in the medical device by comparing a measured current drain against an expected current drain.

7. The medical device of claim 1, wherein:

the current measurement device communicates with a programmer to report a measured pulse generator current;

the current measurement device is connected to a communication circuit during inactive communication periods; and

the current measurement device is disconnected from the communication circuit during active communication periods.

8. The medical device of claim 1, wherein the current measurement device includes:

an oscillator for producing the oscillating output with a frequency of oscillation that is dependent on the pulse generator current; and

a counter for providing an oscillator count from which the pulse generator current is determined.

9. The medical device of claim 8, wherein the oscillator comprises:

a capacitor charged by an oscillator input current that is dependent on the pulse generator current; and

a reset switch for discharging the capacitor when a voltage across the capacitor increases to a level equal to a reference voltage, wherein a resulting comparator output oscillates.

10. The medical device of claim 8, wherein the current measurement device further comprises a comparator for comparing a voltage across the capacitor with a reference voltage and providing a resulting comparator output to the counter.

11. The medical device of claim 8, wherein the counter is periodically reset using a known, stable time base.

12. The medical device of claim 8, wherein:

the current measurement device further comprises at least two current sources that provide an oscillator input current;

each of the current sources operates in a calibration state and a current measurement state; and

one of the at least two current sources operates in the calibration state as the other of the at least two current sources operates in the current measurement state.

13. The medical device of claim 12, wherein each of the at least two current sources includes an operational amplifier having a feature for autozeroing an offset voltage.

14. A medical device, comprising:

a battery;

a pulse generator, wherein the pulse generator draws a pulse generator current from the battery;

an electrode system in electrical communication with the pulse generator, wherein the electrode system applies electrical therapy to a patient; and

a current measuring device for determining the pulse generator current, the current measurement device including;

an oscillator for producing an oscillating output with a frequency of oscillation that is dependent on the pulse generator current;

a counter for providing an oscillator count from which the pulse generator current is determined; and

a current source for providing an oscillator input current to the oscillator that is dependent on the pulse generator current, wherein the current source includes a current divider for dividing current drawn from the battery into both the pulse generator current and an attenuated current.

15. The medical device of claim 14, wherein the current divider includes:

a sense resistor through which the pulse generator current flows;

an attenuation resistor through which the attenuated current flows; and

an operational amplifier for providing substantially equal voltage drops across the sense resistor and across the attenuation resistor.

16. The medical device of claim 15, wherein:

the current measurement device further comprises a second current source including a second operational amplifier;

the operational amplifiers have a feature for autozeroing an offset voltage; and

the current measuring device continuously measures current as one current source is autozeroed while the other current source continues to operate.

17. The medical device of claim 14, wherein the current source further includes at least one current mirror stage.

18. The medical device of claim 17, wherein the at least one current mirror stage includes:

a first current mirror stage for receiving the attenuated current; and

a second current mirror stage for providing the oscillator input current.

19. The medical device of claim 14, wherein the current source is adapted for providing the input current while being calibrated.

20. The medical device of claim 14, further comprising a pair of switches for selectively connecting a telemetry circuit to the current measurement device, wherein:

the pulse generator current includes a quiescent current drawn by the telemetry circuit during a non-telemetry session, and

the pulse generator current does not include a larger current drawn by the telemetry circuit during a telemetry session.

21. A device for determining a pulse generator current drawn from a battery of an implantable medical device, comprising:

a current source that provides an input current that is dependent on the pulse generator current, wherein the current source is adapted for providing the input current while being calibrated;

an oscillator for receiving the input current and producing an oscillating output, wherein the oscillating output has a frequency of oscillation dependent on the pulse generator current; and

a counter for providing an oscillation count for the oscillating output, wherein the pulse generator current is determined from the oscillation count.

22. The device of claim 21, wherein the current source includes a current divider for dividing current drawn from the battery into the pulse generator current and an attenuated current, the current divider including:

a sense resistor through which the pulse generator current flows;

an attenuation resistor through which the attenuated current flows; and

an operational amplifier for providing substantially equal voltage drops across the sense resistor and across the attenuation resistor.

23. The device of claim 22, wherein the operational amplifier further provides negative feedback.

24. The device of claim 22, further comprising a second current source including a second operational amplifier, wherein the operational amplifiers have a feature for autozeroing an offset voltage, and wherein one current source is autozeroing while the other current source continues to operate.

25. The device of claim 21, wherein the current source further includes at least one current mirror stage for performing a current reduction function.

26. A medical device, comprising:

a battery;

a pulse generator, wherein the pulse generator draws a pulse generator current from the battery;

an electrode system in electrical communication with the pulse generator, wherein the electrode system applies electrical therapy to a patient; and

a current measuring device for determining the pulse generator current, the current measurement device including;

an oscillator for producing an oscillating output with a frequency of oscillation that is dependent on the pulse generator current;

a counter for providing an oscillator count from which the pulse generator current is determined; and

a first and second current source for providing an oscillator input current to the oscillator that is dependent on the pulse generator current, wherein each current source includes a current divider for dividing current drawn from the battery into both the pulse generator current and an attenuated current, wherein the current divider includes;

a sense resistor through which the pulse generator current flows;

an attenuation resistor through which the attenuated current flows; and

an operational amplifier for providing substantially equal voltage drops across the sense resistor and across the attenuation resistor, wherein;

the operational amplifier further provides negative feedback; and

each operational amplifier has a feature for autozeroing an offset voltage such that the current measuring device continuously measures current as one current source is autozeroed while the other current source continues to operate.

27. A method for determining current drain for a battery-operated medical device, comprising:

providing an input current to an oscillator, wherein;

the input current is dependent on a pulse generator current; and

the oscillator responds to the input current by generating an oscillating output with a frequency of oscillation that is dependent on the pulse generator current;

providing an oscillation count for the oscillating output over a period of time; and

determining the pulse generator current based on the oscillation count over the period of time,

wherein providing an oscillating output with a frequency of oscillation that is dependent on a pulse generator current comprises;

charging a capacitor;

comparing a voltage formed across the capacitor with a reference voltage; and

discharging the capacitor when the voltage across the capacitor increases to a level equal to the reference voltage.

28. A method for determining current drain for a battery-operated medical device, comprising:

providing an input current to an oscillator, wherein;

the input current is dependent on a pulse generator current; and

the oscillator responds to the input current by generating an oscillating output with a frequency of oscillation that is dependent on the pulse generator current;

providing an oscillation count for the oscillating output over a period of time; and

determining the pulse generator current based on the oscillation count over the period of time,

wherein providing an oscillator input current to the oscillator that is dependent on the pulse generator current comprises;

dividing current drawn from a battery into the pulse generator current and into an attenuated current that is dependent on the pulse generator current; and

reducing the attenuated current to the oscillating input current.

29. A method for determining current drain for a battery-operated medical device, comprising:

providing an input current to an oscillator, wherein;

the input current is dependent on a pulse generator current; and

the oscillator responds to the input current by generating an oscillating output with a frequency of oscillation that is dependent on the pulse generator current;

providing an oscillation count for the oscillating output over a period of time; and

determining the pulse generator current based on the oscillation count over the period of time,

wherein;

providing an input current to an oscillator includes providing at least two current sources;

each of the at least two current sources operate in a current measurement state in which the current source is providing the input current that allows the pulse generator current to be determined, and further operate in a calibration state in which the current source is being calibrated; and

one of the at least two current sources is operating in the current measurement state while the other of the at least two current sources is operating in the calibration state.

30. The method of claim 29, wherein:

each of the at least two current sources includes an operational amplifier having a feature for autozeroing an offset voltage; and

the current sources are calibrated by autozeroing the offset voltage.

31. A method for tracking charge depletion of a battery in an implantable medical device, comprising:

providing an input current to an oscillator, wherein;

the input current is dependent on a pulse generator current drawn by the implantable medical device; and

the oscillator responds to the input current by generating an oscillating output with a frequency of oscillation that is dependent on the pulse generator current;

providing an oscillation count for the oscillating output over a period of time; and

determining the charge depletion of a battery by continuously summing the oscillation count over successive periods of time,

wherein providing an oscillating output with a frequency of oscillation that is dependent on a pulse generator current comprises;

charging a capacitor;

comparing a voltage formed across the capacitor with a reference voltage; and

discharging the capacitor when the voltage across the capacitor increases to a level equal to the reference voltage.

32. A method for tracking charge depletion of a battery in an implantable medical device, comprising:

providing an input current to an oscillator, wherein;

the input current is dependent on a pulse generator current drawn by the implantable medical device; and

the oscillator responds to the input current by generating an oscillating output with a frequency of oscillation that is dependent on the pulse generator current;

providing an oscillation count for the oscillating output over a period of time; and

determining the charge depletion of a battery by continuously summing the oscillation count over successive periods of time,

wherein providing an oscillator input current to the oscillator that is dependent on the pulse generator current comprises;

dividing current drawn from a battery into the pulse generator current and into an attenuated current that is dependent on the pulse generator current; and

reducing the attenuated current to the oscillating input current.

33. A method for tracking charge depletion of a battery in an implantable medical device, comprising:

providing an input current to an oscillator, wherein;

the input current is dependent on a pulse generator current drawn by the implantable medical device; and

the oscillator responds to the input current by generating an oscillating output with a frequency of oscillation that is dependent on the pulse generator current;

providing an oscillation count for the oscillating output over a period of time; and

determining the charge depletion of a battery by continuously summing the oscillation count over successive periods of time,

wherein;

providing an input current to an oscillator includes providing at least two current sources;

each of the at least two current sources operate in a current measurement state in which the current source is generating the input current that allows the pulse generator current to be determined, and further operate in a calibration state in which the current source is being calibrated; and

one of the at least two current sources is operating in the current measurement state while the other of the at least two current sources is operating in the calibration state.

34. The method of claim 33, wherein:

each of the at least two current sources includes an operational amplifier having a feature for autozeroing an offset voltage; and

the offset voltage of the operational amplifier is autozeroed to calibrate the current source.

35. A method of determining battery status of an implantable medical device, comprising:

determining a total charge capacity for a battery of an implantable medical device;

determining a degree of precision for tracking charge depleted from the battery;

determining a replacement percentage;

tracking the charge depleted from the battery, including;

providing an input current to an oscillator, wherein;

the input current is dependent on a pulse generator current; and

the oscillator responds to the input current by generating an oscillating output with a frequency of oscillation that is dependent on the pulse generator current;

providing an oscillation count for the oscillating output over a period of time; and

determining the pulse generator current based on the oscillation count over the period of time; and

providing an indication when remaining charge in the battery equals the replacement percentage.

36. The method of claim 35, wherein:

providing an input current to an oscillator includes providing at least two current sources;

each of the at least two current sources operate in a current measurement state in which the current source is providing the input current that allows the pulse generator current to be determined, and further operate in a calibration state in which the current source is being calibrated; and

one of the at least two current sources is operating in the current measurement state while the other of the at least two current sources is operating in the calibration state.

37. The method of claim 36, wherein:

each of the at least two current sources includes an operational amplifier having a feature for autozeroing an offset voltage; and

the offset voltage of the operational amplifier is autozeroed to calibrate the current source.

38. A method of detecting potential faults within an implantable medical device, comprising:

determining an upper limit for current from a battery of the implantable medical device; and

determining current drain from the battery, including;

providing an input current to an oscillator, wherein;

the input current is dependent on a pulse generator current; and

the oscillator responds to the input current by generating an oscillating output with a frequency of oscillation that is dependent on the pulse generator current;

providing an oscillation count for the oscillating output over a period of time; and

determining the pulse generator current based on the oscillation count over the period of time.

39. The method of claim 38, wherein determining an upper limit for current comprises determining an expected current drain and calculating the upper limit based on the expected current drain.

40. The method of claim 38, wherein:

providing an input current to an oscillator includes providing at least two current sources;

each of the at least two current sources operate in a current measurement state in which the current source is generating the input current that allows the pulse generator current to be determined, and further operate in a calibration state in which the current source is being calibrated; and

one of the at least two current sources is operating in the current measurement state while the other of the at least two current sources is operating in the calibration state.

41. The method of claim 40, wherein:

each of the at least two current sources includes an operational amplifier having a feature for autozeroing an offset voltage; and

the current sources are calibrated by autozeroing the offset voltage.

42. A method of reporting drain current in an implantable medical device, comprising:

determining current drain from the battery using a current measurement circuit, including;

providing an input current to an oscillator, wherein;

the input current is dependent on a pulse generator current; and

the oscillator responds to the input current by generating an oscillating output with a frequency of oscillation that is dependent on the pulse generator current;

providing an oscillation count for the oscillating output over a period of time; and

determining the pulse generator current based on the oscillation count over the period of time;

disconnecting a telemetry circuit from the current measuring circuit in preparation for a telemetry session;

interrogating the implantable medical device via telemetry; and

upon completing the telemetry session, connecting the telemetry circuit to the current measuring circuit.

43. The method of claim 42, wherein:

providing an input current to an oscillator includes providing at least two current sources;

each of the at least two current sources operate in a current measurement state in which the current source is providing the input current that allows the pulse generator current to be determined, and further operate in a calibration state in which the current source is being calibrated; and

one of the at least two current sources is operating in the current measurement state while the other of the at least two current sources is operating in the calibration state.

44. The method of claim 43, wherein:

each of the at least two current sources includes an operational amplifier having a feature for autozeroing an offset voltage; and

the offset voltage of the operational amplifier is autozeroed to calibrate the current source.