This paper presents a capacitive pressure sensor interface circuit design in 180 nm XH018 CMOS technology for an implantable capacitive pressure sensor, which has a wireless power supply and wireless data transfer function. It integrates full-bridge rectifiers, shorting control switches, low-dropout regulators, bandgap references, analog front end, single slope analog to digital converter (ADC), I2C, and an RC oscillator. The low-dropout regulators regulate the wireless power supply coming from the rectifier and provide a stable and accurate 1.8 V DC voltage to other blocks. The capacitance of the pressure sensor is sampled to a discrete voltage by the analog front end. The single slope ADC converts the discrete voltage into 11 bits of digital data, which is then converted into 1 kbps serial data out by the I2C block. The “1” of serial data is modulated to a 500 kHz digital signal that is used to control the shorting switch for wireless data transfer via inductive back scatter. This capacitive pressure sensor interface IC has a resolution of 0.98 mmHg (1.4 fF), average total power consumption of 7.8 mW, and ±3.2% accuracy at the worst case under a −20 to 80 °C temperature range, which improves to ±0.86% when operated between 20 and 60 °C.

An integrated active-rectifier including overvoltage protection for wireless power transfer has been fabricated in 180nm HV-CMOS. The rectifier operating at 600kHz can provide up to 600mW output power and achieves greater than 90% efficiency with output powers from 40mW to 600mW. This power range is suitable for a lifetime implantable pressure monitoring telemeter for patients with hydrocephalus. It also enables the use of small piezoelectric pumps or actuators for active shunt control. As reliability is a key concern for a lifetime implantable medical device, shorting-control has been implemented on chip for over-voltage protection and voltage regulation to maintain the output voltage within safe operating limits despite changes in load or coil coupling. Minimal additional power is dissipated while shorting-control is active.

In this work, a shorting control (SC) scheme is integrated into a complementary metal-oxide-semiconductor (CMOS) synchronous rectifier for the output voltage regulation of a wireless power supply. The rectifier is designed to operate in a parallel tuned pickup with a 500 mW output power capability for biomedical implants. Without any additional components, the proposed SC method enables the power pickup to operate with high efficiency under variable coupling conditions while maintaining the required load power to keep the output voltage constant. Desired operating conditions are achieved with increased power transfer capability at weak magnetic coupling conditions and higher power efficiency at strong coupling. A novel derivation describes the change in efficiency with SC duty ratio. Experimental validation is completed with an original custom CMOS integrated rectifier with embedded SC. It is demonstrated that the proposed SC method can increase the overall secondary pickup power transfer efficiency by 14% as the magnetic coupling increases to the stronger end.

Powering active implantable devices is challenging. Implants must be small and have a long lifetime. Wireless power offers energy over the life of the device. Using integrated circuits for managing wireless power transfer allows for a small implant. Often when implemented in CMOS, a synchronous rectifier is used to achieve AC-DC conversion. Current spikes can occur in the rectifier and must be considered to avoid electromigration. This paper presents the analysis of the peak currents in the synchronous rectifier. Reducing the peak current can be achieved by decreasing the size of the rectifying transistors. We have developed and fabricated the control circuitry for a synchronous rectifier in XFABs XH018 180nm high voltage CMOS process. A finalised synchronous rectifier is being designed which will be used to confirm our analysis.