A Low-Power Fully Implantable 15-Channel Retinal Stimulator Chip

Retinal prostheses are being developed around the world in hopes of restoring useful vision for patients suffering from certain types of diseases like age-related macular degeneration (AMD) and retinitis pigmentosa. The central component of an electrical retinal prosthesis is a wirelessly powered and driven stimulator chip. The chip receives commands from the outside and outputs biphasic current pulses to an electrode array placed in the retina that stimulate the remaining retinal neurons. The chip contains 30 000 transistors in a 0.5 mu m technology (two-poly three-metal, 2P3M), occupies an area of 2.3 mm x 2.3 mm, and excluding the current sources consumes less than 2 mW of power. The chip is powered inductively via a 125 kHz power signal which is rectified to generate a +/- 2.5 V supply. The data signal is transmitted as an amplitude shift keyed (ASK) signal on a 13.56 MHz carrier. The data rate can be varied from 25 to 714 kHz and the symbol (0 or 1) is encoded as the pulse width of the data signal. A self-biased feedback-loop-based single-to-differential converter restores the signal to full rail levels. Clock and data recovery is performed by a self-biased low-power inverter-based delay-locked loop (DLL). The chip can receive four commands, and each command is 16 bits long. The current amplitude, pulse duration, and inter-pulse duration can be programmed by using the four commands.