Current leakage detection
An improved arrangement for detecting DC current leakage in implanted devices is disclosed. Currents are monitored, so that any unexpected imbalance indicates that a fault condition has occurred. The principle of the system is that if everything working correctly, the same current should be flowing into the tissue from VDDH-ELEC as is passing out of the tissue into VSS_ELEC. Any difference is indicative of current flowing into or out of the tissue from unknown paths, which is indicative of some fault. This replaces routine monitoring for specific faults.
1. A method of detecting DC current leakage from an electrical circuit of a device implantable in a recipient to tissue of the recipient, the device being configured to conduct current through at least one conductive path having multiple segments including a segment formed by tissue of the recipient, said method including at least the steps of:
mirroring, for each conductive path, a pre-tissue current and a post-tissue current representing currents at opposite ends of the tissue segment;
combining the pre-tissue and post-tissue currents to represent a total current;
determining whether the total current is within a desired tolerance of zero;
wherein if the total current is not within said desired tolerance of zero, determining that leakage current is present.
2. The method according to claim 1 , wherein the mirroring uses current mirroring devices.
3. The method according to claim 2 , wherein each current mirroring device uses one of a bipolar current mirror and a MOS current mirror.
4. The method according to claim 1 , wherein the implantable device is an implantable hearing device.
5. The method according to claim 4 , wherein the implanted device is a cochlear implant and the segments include electrodes of the cochlear implantable.
6. The method according to claim 1 , wherein if it is determined that current leakage is present, the method further includes the step of triggering a software diagnostic process.
7. The method according to claim 1 , wherein if it is determined that current leakage is present, the method further includes the step of providing an indication of the current leakage.
8. A method of detecting DC current leakage from a device implantable in a recipient to tissue of the recipient, the device being configured to conduct current through at least one conductive path having multiple segments including segments formed by a power rail, a ground rail, and a segment therebetween formed by tissue of the recipient, said method including at least the steps of:
setting a pre-tissue DAC-code for a first current-output digital-to-analog converter (current DAC) that indirectly represents a current at one end of the tissue segment;
setting a post-tissue DAC-code for a second current DAC that indirectly represents a current at the other end of the tissue segment;
using the first current DAC to adjust current at the one end of the tissue segment based on the pre-tissue DAC-code;
using the second current DAC to adjust current at the other one end of the tissue segment based on the post-tissue DAC-code;
comparing the pre-tissue DAC-code and the post-tissue DAC-code; and
if there is an absolute difference greater than a desired tolerance between the pre-tissue code and the post-tissue code, determining that current leakage is present.
9. The method according to claim 8 , wherein the implantable device is an implantable hearing device.
10. The method according to claim 9 , wherein the implantable device is a cochlear implant.
11. The method according to claim 8 , wherein:
the implantable device includes a power rail and a counterpart ground rail;
the setting a pre-tissue code includes:
measuring voltage on the power rail;
comparing the power rail voltage against a first desired value to produce a first adjustment signal; and
generating the pre-tissue code based on the first adjustment signal; and
the setting a post-tissue code includes:
measuring voltage on the ground rail;
comparing the ground rail voltage against a second desired value to produce a second adjustment signal; and
generating the post-tissue code based on the second adjustment signal.
12. The method according to claim 8 , wherein if it is determined that current leakage is present, the method further includes at least one of the following steps:
triggering a software diagnostic process; and
providing an indication of the current leakage.
13. A device, implantable in a recipient, comprising:
electrical output driving circuits powered by a respective power and ground rail, each driving circuit being configured to conduct current through a conductive path having multiple segments including segments formed by the power and ground rails, and a segment therebetween formed by tissue of the recipient;
a first current mirror, connected to a proxy node, for tracking a mirrored pre-tissue current, representing a first current passing through the power rail and at least indirectly representing a current at one end of the tissue segment;
a second current mirror, connected to the proxy node, for tracking a mirrored post-tissue current, representing a second current passing through the ground rail and at least indirectly representing a current at the other end of the tissue segment;
a resistor having a first side connected to the proxy node; and
a comparator for comparing the voltages on the first side and a second side of the resistor, such that if the comparator detects a difference greater than a desired tolerance, an indication is provided that current leakage is occurring.
14. The implantable device according to claim 13 , wherein the implantable device is an implantable hearing device.
15. The implantable device according to claim 14 , wherein the implantable device is a cochlear implant.
16. The implantable device according to claim 13 , wherein: each current mirror is one of a bipolar current mirror and a MOS current mirror.
17. A device, implantable in a recipient, comprising:
a first DAC-code generator configured to generate a pre-tissue DAC-code that indirectly represents a current at one end of the tissue segment;
a second DAC-code generator configured to generate a post-tissue DAC-code that indirectly represents a current at the other end of the tissue segment;
a first current DAC configured to adjust current at the one end of the tissue segment based on the pre-tissue DAC-code;
a second current DAC configured to adjust current at the other end of the tissue segment based on the post-tissue DAC-code; and
a digital comparator for comparing the pre-tissue DAC-code and the post-tissue DAC code, such that if the comparator detects an absolute difference greater than a desired tolerance, an indication is provided that current leakage is occurring.
18. The implantable device according to claim 17 , wherein the implantable device is an implantable hearing device.
19. The implantable device according to claim 18 , wherein the implantable device is a cochlear implant.
20. The implantable device according to claim 17 , further comprising:
a stimulation-power rail; and
a counterpart stimulation-ground rail;
a first analog comparator configured to compare the stimulation-power rail voltage against a first desired value and thereby produce a first adjustment signal; and
a second analog comparator configured to compare the stimulation-ground rail voltage against a second desired value and thereby produce a second adjustment signal;
wherein the first DAC-code generator is further configured to generate the pre-tissue DAC-code based on the first adjustment signal; and
wherein the second DAC-code generator is further configured to generate the post-tissue DAC-code based on the second adjustment signal.