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A battery voltage measuring circuit for an implantable cardiac device is presented. Since the usable battery voltage for the device is limited to an upper range of voltages, the need for measuring lower voltages at which the battery is approaching end of life is of no use. The disclosed invention al

A battery voltage measuring circuit for an implantable cardiac device is presented. Since the usable battery voltage for the device is limited to an upper range of voltages, the need for measuring lower voltages at which the battery is approaching end of life is of no use. The disclosed invention allows for the measurement of a selectable upper range of battery levels that can be chosen without using a level shifting device such as a zener diode. Multiple voltage ranges with associated measurement resolutions can be achieved without using high current zener diode implementations. This allows for a trade-off between measurement range and resolution while resulting in a lower power and more accurate measurement circuit. Conventional zener diode implementations only allow for a single measurement range and are prone to non-linear error as the voltage measurement range increases.

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1. An implantable cardiac device having a hermetic container including a battery coupled to operational circuitry for providing functionality to the implantable medical device, the operational circuitry including a battery monitoring circuit comprising the following: a voltage divider to generate a

1. An implantable cardiac device having a hermetic container including a battery coupled to operational circuitry for providing functionality to the implantable medical device, the operational circuitry including a battery monitoring circuit comprising the following: a voltage divider to generate a scaled voltage related to a battery voltage;a resistor, Rdiff, having first and second nodes;an amplifier having an output current and taking the scaled voltage and applying the scaled voltage to the first node of the Rdiff resistor;a level shifting reference voltage input coupled to the second node of the Rdiff resistor, such that the current through the Rdiff resistor is related to the difference between the scaled voltage and the level shifting reference voltage at the level shifting reference voltage input;a current mirror coupled to the output of the amplifier driving an output resistance, coupled such that the current mirror uses the current through Rdiff as a reference; andan output generating a battery voltage indicator coupled to the current mirror. 2. The implantable cardiac device of claim 1 wherein the amplifier is a unity gain amplifier. 3. The implantable cardiac device of claim 1 wherein the voltage representative of the voltage output of the battery is generated using a resistive voltage divider circuit. 4. The implantable medical device of claim 1 wherein the level shifting reference voltage is a selectable input having at least first and second voltage values. 5. The implantable cardiac device of claim 4 wherein the output resistance is a selectable resistance having at least first and second resistance values. 6. The implantable cardiac device of claim 5 wherein the operational circuitry is configured to control each of the level shifting reference voltage and the output resistance to enable: a first battery voltage measurement range using the first voltage value for the level shifting reference voltage and the first resistance value for the output resistance; anda second battery voltage measurement range using the second voltage value for the level shifting reference voltage and the second resistance value for the output resistance. 7. The implantable cardiac device of claim 6 wherein the operational circuitry further comprises an analog-to-digital converter sub-circuit for converting the battery voltage indicator to a digital value, such that the operational circuitry can convert the digital value depending upon whether the first battery voltage measurement range is enabled or the second battery voltage measurement range is enabled. 8. The implantable cardiac device of claim 6 wherein the operational circuitry further comprises high power capacitors designed fix delivery of defibrillation therapy, in which the operational circuitry is configured to perform high current operations to charge the high power capacitors either as part of periodic reformation of the capacitors or for therapy delivery purposes, wherein the operational circuitry is configured to use the first battery voltage measurement range by default, and to use the second battery voltage measurement range following a high current operation using the high power capacitors for a predetermined period of time. 9. The implantable cardiac device of claim 6 wherein the operational circuitry further comprises high power capacitors designed for delivery of defibrillation therapy, in which the operational circuitry is configured to perform high current operations to charge the high power capacitors either as part of periodic reformation of the capacitors or for therapy delivery purposes, wherein the operational circuitry is configured to use the first battery voltage measurement range by default, and to use the second battery voltage measurement range following a high current operation using the high power capacitors until the battery voltage recovers after the high current operation. 10. The implantable cardiac device of claim 1 wherein the operational circuitry further comprises an analog-to-digital converter subcircuit fix converting the battery voltage indicator to a digital value. 11. The implantable cardiac device of claim 1 wherein the level shifting reference voltage is generated without the use of a zener diode. 12. A method of measuring battery voltage in an implantable cardiac device having a hermetic container including a battery coupled to operational circuitry for providing functionality to the implantable medical device, the operational circuitry including a battery monitoring circuit comprising the following: a voltage divider to generate a scaled voltage related to a battery voltage;a resistor, Rdiff, having first and second nodes;an amplifier having an output current and taking the scaled voltage and applying the scaled voltage to the first node of the Rdiff resistor;a level shifting reference voltage input coupled to the second node of the Rdiff resistor, such that the current through the Rdiff resistor is related to the difference between the scaled voltage and the level shifting reference voltage at the level shifting reference voltage input;a current mirror coupled to the output of the amplifier driving an output resistance, coupled such that the current mirror uses the current through Rdiff as a reference; andan output generating a battery voltage indicator coupled to the current mirror, the method comprising:selecting a coarse or fine measurement mode for the battery voltage measurement by: selecting a reference voltage for the level shifting reference voltage; andselecting an output resistance; andmeasuring the battery voltage by: allowing current through the voltage divider, taking a voltage from the voltage divider using a buffer;creating a reference current through Rdiff;creating a current related to the current through Rdiff using the current mirror; andpassing the related current through the output resistance to generate an output voltage providing a measure of the battery voltage. 13. The method of claim 12 further comprising passing the measure of battery voltage to an analog-to-digital converter to digitize the battery measurement. 14. The method of claim 12 wherein the level shifting reference voltage is a selectable input having at least first and second voltage values. 15. The method of claim 12 wherein: the coarse measurement mode is selected by choosing a first reference voltage and a first output resistance;the fine measurement mode is selected by choosing second reference voltage and a second output resistance;the first reference voltage is smaller than the second reference voltage; andthe first output resistance is smaller than the second output resistance. 16. The method of claim 12 wherein: the implantable medical device comprises high current circuitry capable of causing a high current event to take place; andthe step of selecting a coarse or fine measurement mode is performed by determining whether a recent high current event in the implantable device has occurred and: if a recent high current event has occurred, selecting the coarse measurement mode; orotherwise, selecting the fine measurement mode. 17. The method of claim 16 wherein it is determined that a recent high current event has occurred if a high current event has taken place within a fixed period of time. 18. The method of claim 16 wherein it is determined that a recent high current event has occurred if a high current event has taken place and, since the high current event, the battery voltage measured by the coarse mode has not been stable over consecutive measurements. 19. The method of claim 16 wherein the implantable cardiac device comprises high power capacitors designed for delivery of defibrillation therapy, in which a high current event includes charging of the high power capacitors for one of: periodic reformation of the capacitors orfor therapy delivery purposes. 20. The method of claim 12, wherein the battery measurement is performed without the use of a zener diode.