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For many biomedical implantable systems, power is transferred into implants through an inductively coupled coil pair. When the receiver coil enters a strong magnetic field generated by the transmitter coil, the implanted device can receive more than needed power. This may lead to the generation of so high received voltages that the implant may be damaged (which may also have consequences for the patient). In addition, during the regulation of the rectified DC voltage, the low-dropout (LDO) voltage regulator exhibits poor power efficiency if the input voltage is much higher than the output voltage. Thus, the corresponding heating effect is increased. This study introduces a high-voltage CMOS dynamic power control circuit that can be integrated in the implantable power receiver front-end. The circuit is capable of auto-detuning the receiver inductive-capacitive (LC) tank to adjust the power reception level according to the actual magnetic field strength. The experimental results show that the power control circuit reduces the impact of the varied magnetic field on the rectified voltage. Also, the prototype measurements show that the power efficiency of the LDO regulator increases from 35 to 72%, when the power control circuit activates the detuning function.
Date of Publication: July 2011