Implantable stimulator device with magnetic field sensing circuit
An implantable pulse generator (IPG) for an implantable medical device is disclosed herein. The IPG is capable of sensing the presence of an external magnetic field, such as a magnetic field associated with magnetic resonance imaging (MRI). The IPG includes a circuit that contains a magnetic core inductor and that is configured to boost a first voltage to a second voltage and use the second voltage to drive a current through a load. In a strong magnetic field, the magnetic core of the inductor becomes magnetically saturated, causing the inductance of the inductor to sharply drop. The inductance drop can be detected, for example, by detecting an increase in the second voltage. The circuit may be a boost converter circuit used to provide a compliance voltage for operation of the IPG.
1. An implantable pulse generator (IPG) configured to provide therapy to a patient, the IPG comprising:
first circuitry configured to boost a first voltage to a second voltage, the first circuitry comprising a magnetic-core inductor having an inductance, and
second circuitry configured to detect a change in the second voltage, the change in the second voltage indicating a change in the inductance when the IPG is in the presence of an external magnetic field, wherein
the first circuitry is a boost converter circuit configured to provide a compliance voltage and wherein the second voltage is the compliance voltage.
2. The IPG of claim 1 , wherein the first circuitry further comprises:
a transistor configured to receive a gate voltage, the gate voltage having an oscillation frequency and being modulated by a pulse width modulator having a duty cycle, and
an output stage configured to use the second voltage to drive a known current through a load, wherein
detecting a change in the second voltage comprises:
calculating an expected second voltage value from known values of the current, the first voltage, the oscillation frequency, and the duty cycle, and an assumed value for the inductance,
measuring the second voltage, and
comparing the second voltage to the calculated expected second voltage.
3. The IPG of claim 2 , wherein detecting a change in the second voltage further comprises calculating the inductance based on the measured second voltage.
4. The IPG of claim 3 , further comprising a processor configured to determine an external magnetic field strength based on the calculated inductance.
5. The IPG of claim 1 , wherein the IPG is configured to change from a first operational mode to a second operational mode based on the detected change in the parameter of the first circuitry second voltage.
6. The IPG of claim 5 , wherein the first operational mode is a normal mode and the second operational mode is an MRI-safe mode.
7. The IPG of claim 6 , wherein the MM-safe mode comprises disabling the IPG from providing stimulation to the patient.
8. The IPG of claim 6 , wherein the MM-safe mode comprises increasing a compliance voltage.
9. The IPG of claim 6 , wherein the MM-safe mode comprises disabling passive charge recovery.
10. The IPG of claim 1 , wherein the first circuitry further comprises:
at least one diode,
at least one capacitor, and
at least one transistor configured to receive a gate voltage, the gate voltage having an oscillation frequency and being modulated by a pulse width modulator having a duty cycle, wherein
the first circuitry is configured so that when the at least one transistor is on, the inductor discharges to ground, and when the at least one transistor is off, the inductor discharges through the at least one diode to the at least one capacitor.
11. The IPG of claim 10 , wherein the duty cycle is adjustable to maintain the second voltage on the at least one capacitor.
12. The IPG of claim 11 , wherein the first circuitry is configured to use the second voltage to drive a known current through a load.
13. The IPG of claim 12 , wherein the load comprises digital-to-analog (DAC) circuitry configured to supply current to one or more electrodes.
14. The IPG of claim 11 , wherein detecting the change in the second voltage comprises:
calculating an expected second voltage value from known values of the current, the first voltage, the oscillation frequency, and the duty cycle, and an assumed value for the inductance,
measuring the second voltage, and
comparing the second voltage to the calculated expected second voltage.
15. The IPG of claim 14 , wherein the second circuitry comprises a comparator circuit configured to compare the second voltage to a reference voltage.
16. The IPG of claim 1 , wherein the magnetic-core inductor comprises a toroidal magnet.