A Programmable Discharge Circuitry With Current Limiting Capability for a Retinal Prosthesis

Biphasic stimulation is the most commonly used electrical pattern in FES (Functional Electrical Stimulation). In such cases, a charge balanced waveform is essential to prevent any charge accumulation in the biological tissue. In a retinal prosthesis, a charge cancellation circuitry is used to discharge the stimulation sites on the retina periodically to ensure tissue safety. But discharging a node that has high voltage (due to charge accumulation) will induce large currents that might lead to unintended stimulation of the retina. This paper presents a new discharge circuitry which can act as a simple resistive discharge path for smaller voltages and a current limiter for higher voltages. In stimulation circuits which use a dual voltage scheme (positive and negative), the circuit limits the current at both polarities. The discharge profile is programmable digitally. In addition, a method to characterize inter-pixel leakage in a retinal prosthesis using this circuit is proposed