10 th Annual Conference of the International FES Society July 2005 – Montreal, Canada Design and Implementation of High Power Efficiency Modules for a Cortical Visual Stimulator Sawan M , Coulombe J, Hu Y École Polytechnique de Montréal, 5255 Decelles, Montreal, Quebec, Canada, H3T 2B1 Mohamad.Sawan@Polymtl.ca Abstract We present in this paper custom building blocks for a visual cortical implantable stimulator. Voltages are kept low throughout the power chain, from rectification to stimulation stages, which is possible via dynamic monopolar stimulation. To minimize power losses in regulators, a high efficiency 2/3 switched capacitors DC/DC converter is used, in conjunction with a feedback loop controlling the wirelessly transmitted power. The continuous feedback loop is possible through Phase Shift Keying downlink demodulation, performed by a differential hard-limited Costas loop. Experimental results from integrated circuits are presented. 1. INTRODUCTION Electrical microstimulation of the visual cortex is a promising approach for providing a number of totally blind subjects with functional vision. Although published results today cannot specify the exact requirements for such a prosthesis, hundreds of electrodes are expected to be required [1], each individually stimulating at rates at or above 100 Hz. High parallelism, and proper data encoding make this goal achievable with today’s technology and know-how, with respect primarily to data throughput at reasonable carrier frequencies (< 20 MHz) [2- 3]. On the other hand, no effort shall be minimized for reducing power consumption of such massively parallel stimulators. Prohibitively large power dissipation results in tissue heating and battery drain, to be particularly avoided for practically continuously used devices, as the targeted visual implant. Different key components of an implant have been designed with power consumption in mind. In this paper, we present the overall architecture of the targeted implant, and describe the main circuits having a direct effect on power efficiency of the device. The main experimental results are presented, before concluding. 2. METHODS The system being designed by our team consists of an external controller which drives a multi- module implant through an inductive link. The implant is comprised of an interface module for power recovery and data transmission, and several stimulation modules disposed on top of microelectrode arrays penetrating in the primary visual cortex. Because the location and size of the interface module is not critically constrained, discrete components are used and enable a high efficiency Switched Capacitor (SC) DC/DC converter to reduce power losses in the regulation stage. Efficiency is further enhanced in this stage by a feedback loop controlling the input voltage such that it stays low in varying conditions. For the loop to be stable, a full- duplex data link must be implemented. Constant amplitude downlink (external to internal) modulation is used to enable reliable Load Shift Keying (LSK) uplink (internal to external). Finally, the supply voltage can also be kept low by using dynamic monopolar stimulation, minimizing current path impedance while allowing each stimulation pulse to swing from one rail to the other. 2.1 Power recovery Power recovery and regulation play a crucial role in the efficiency of the complete implantable system. Although popular mainly for their small size and low noise, linear regulators suffer from poor efficiency when their drop-out voltage is large, such as for REG L in the dual voltage configuration of Figure 1(a), where V L < V H . Drop-out through regulators can be reduced by two approaches using high efficiency DC/DC converters. The first (step-up, b) is to target the rectified voltage, V REC , to be close to the lower regulated voltage, then convert it to a value slightly above