Current Driver Circuits and Safety Features for a Retinal Prosthesis

Abstract

Purpose: : This work is related to the efforts of the Boston Retinal Implant Project to develop a sub-retinal prosthesis to restore vision to the blind. Specifically, this poster presents aspects of current driver circuit design for the prosthesis. The design includes methods of maintaining precision while decreasing area, as well as independent safety features monitoring the voltage of every electrode.

Methods: : An implant with more than 200 individually-addressable stimulation channels is being designed, and the high-voltage current driver circuits represent a large portion of the chip area. Novel feedback circuits allow us to implement these drivers within the area constraints while maintaining the required precision in the 7-bit current digital-to-analog converter. Also, each electrode driver has its own safety monitor circuit, which ensures that the electrode does not exceed safe voltage limits and is properly grounded between pulses.The drivers are integrated into a prosthesis chip including bidirectional telemetry, digital controls, and power management. The chip is sealed in a small hermetic package, connected to a conformal, biocompatible secondary coil for wireless power and image data. Circuits were designed and simulated in Cadence, and laid out for fabrication.

Results: : Current driver circuits are presented that satisfy the size, power, performance, and safety requirements for this implant. The prosthesis ASIC fits in a 5mm x 4.5mm pad frame, dissipates less than 10 mW of standby power, and has independent current amplitude and timing, and the ability to use electrodes as current returns. Current driver simulation circuits and results are presented, and account for variations in process and supply voltage. Safety feature are presented, along with simulated failure modes. Layouts are presented to show that the large number of channels fit on the small chip.

Conclusions: : A large number of current drivers have been designed for a retinal prosthesis, using novel circuits to maintain precision while reducing area. Safety features independently monitor each electrode for failures or overvoltage. The current drivers in this chip will be suitable for clinical trials of a chronically-implanted retinal prosthesis.

Keywords: retina • retinitis • retinal connections, networks, circuitry