A micro-power precision switched-capacitor charge meter system for implantable medical devices

Estimating the remaining capacity of the battery for implantable medical devices such as pacemakers, defibrillators, and neuro-stimulators is critical information. This paper presents a precision system, which can be a part of an integrated circuit of an implantable medical device, to monitor the charge depletion. The system consists of a sensing resistor, a precision voltage controlled oscillator (VCO), and an accumulating counter. A jitter-free micro-power precision voltage controlled oscillator is designed for this particular system. The oscillator consists of a fully differential, double-sampling bilinear switched-capacitor integrator with a fully differential compactor/latch. When operating with a 256 Hz sampling frequency, the VCO consumes less than 100 nA. The circuit can operate with a single 1.8 V power supply. This micro-power operation makes the circuit ideal for implantable medical devices at the end of the life of their batteries. The VCO was fabricated in the MOSIS 2-/spl mu/m, double-polysilicon, double-metal P-well CMOS process. Finally, the experimental data of the implemented circuit is reported. The linearity of the VCO is examined, and it is shown the oscillator is capable of sensing input voltages as small as 50 /spl mu/Vs. The dynamic range of the VCO is 60 dB.