A timing controlled AC-DC converter for biomedical implants

Abstract

Many biomedical implants are powered from an external magnetic source [1, 2]. The magnetic source is inductively coupled to a coil inside the implant to induce an AC voltage, which is then further rectified to a DC voltage [1]. In general, higher supply voltages are often required for analog circuits, such as stimulation circuits in neuroprosthetic applications [2, 3], and lower supply voltages are usually needed for the digital blocks [4]. Hence, the rectified DC voltage is typically kept at a higher value for the analog circuits. A linear regulator is then used to convert the rectified DC voltage to a lower supply voltage for the digital circuits, which typically have power dissipation in the range of 2 – 5mW for neuroprosthetic applications [3]. However, this approach is not very power efficient and a more efficient approach is desired due to the limited power received from the weak magnetic coupling [5]. Buck converters [6, 7] and switched-capacitor (SC) converters have higher power efficiency, especially for high load conditions. However, they may not be suitable for biomedical implants due to the limited space inside the implants, which can only accommodate a few small discrete components. Although buck converters that use a bond-wire inductor [8] or an on-chip inductor [9] are possible solutions, an alternative approach based on a direct conversion of the induced AC voltage to a regulated DC voltage is proposed. High conversion efficiency can be achieved using a single small 220nF off-chip capacitor.