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Battery management circuitry for an implantable medical device such as an implantable neurostimulator is described. The circuitry has a T-shape with respect to the battery terminal, with charging circuitry coupled between rectifier circuitry and the battery terminal on one side of the T, and load is

Battery management circuitry for an implantable medical device such as an implantable neurostimulator is described. The circuitry has a T-shape with respect to the battery terminal, with charging circuitry coupled between rectifier circuitry and the battery terminal on one side of the T, and load isolation circuitry coupled between the load and the battery terminal on the other side. The load isolation circuitry can comprise two switches wired in parallel. An undervoltage fault condition opens both switches to isolate the battery terminal from the load to prevent further dissipation of the battery. Other fault conditions will open only one the switches leaving the other closed to allow for reduced power to the load to continue implant operations albeit at safer low-power levels. The battery management circuitry can be fixed in a particular location on an integrated circuit which also includes for example the stimulation circuitry for the electrodes.

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1. Circuitry for an implantable medical device, comprising: a first charging path comprising a source powered by an input voltage and configured to produce power to charge a battery at a battery terminal node, andan overvoltage switch configured to gate the produced power to the battery terminal nod

1. Circuitry for an implantable medical device, comprising: a first charging path comprising a source powered by an input voltage and configured to produce power to charge a battery at a battery terminal node, andan overvoltage switch configured to gate the produced power to the battery terminal node, wherein the overvoltage switch is opened if a battery overvoltage signal is asserted; andat least one load switch comprising a first terminal directly connected to the battery terminal node and a second terminal coupled to a load within the implantable medical device, wherein the at least one load switch is configured to gate a battery voltage from the battery terminal voltage to the load, wherein the at least one load switch is opened at least if a battery undervoltage signal is asserted. 2. The circuitry of claim 1, wherein the circuitry is integrated on an integrated circuit. 3. The circuitry of claim 1, wherein the power producible by the source is a constant current, or a constant voltage, or both. 4. The circuitry of claim 1, wherein there are first and second load switches in parallel between the battery terminal node and the load. 5. The circuitry of claim 4, wherein the first load switch is opened if the battery undervoltage signal is asserted, and wherein the second load switch is opened if either the battery undervoltage signal or an overcurrent signal is asserted. 6. The circuitry of claim 5, further comprising an overcurrent detector for asserting the overcurrent signal if a current through the at least one load switch exceeds a threshold. 7. The circuit of claim 5, wherein the resistance of the first load switch is larger than the resistance of the second load switch. 8. The circuitry of claim 5, wherein the second load switch is additionally opened if a magnetic field detected signal is asserted. 9. The circuitry of claim 1, further comprising a coil and rectifier circuit for producing the input voltage. 10. The circuitry of claim 9, further comprising a diode between the rectifier circuit and the input voltage. 11. The circuitry of claim 1, further comprising an undervoltage detector for receiving the battery terminal node and for asserting the battery undervoltage signal when the voltage at the battery terminal node falls below a threshold. 12. The circuitry of claim 11, wherein the undervoltage detector is passive and doesn't require any control signals to assert the battery undervoltage signal. 13. The circuitry of claim 1, further comprising a discharge circuit for receiving the battery terminal node, the battery overvoltage signal, and the battery undervoltage signal, wherein the discharge circuitry couples the battery terminal node to ground when the battery overvoltage signal is asserted, but wherein the discharge circuitry decouples the battery terminal node from ground when the battery undervoltage signal is asserted. 14. The circuitry of claim 1, further comprising: a second charging path for passively charging the battery using the input voltage, wherein the second charging path couples to the battery terminal node by at least one first one diode. 15. The circuitry of claim 14, further comprising at least one second diode between the first and second charging paths. 16. The circuitry of claim 15, wherein the number of the at least one first diodes and the at least one second diodes is the same. 17. The circuitry of claim 15, wherein the at least one first diode and the at least one second diode are connected at their positive terminal, and wherein their negative terminals are coupled to either side of the overvoltage switch.