Biphasic, energy-efficient, current-controlled stimulation back-end for retinal visual prosthesis

This paper reports an energy-efficient waveform generator, dedicated to implantable retinal microstimulators. The circuit features flexible current-mode stimuli such as rising and falling exponential pulses in addition to rectangular pulses. In order to apply the stimulation current to the electrode at the defined current levels (±96μA) and with sufficient voltage headroom (±3V), a class AB second generation current conveyor is designed as the output stage. To upconvert the 1.2-V supply voltage to ±3.3V, the output stage is equipped with an on-chip electrode-tissue driver. Duration of the generated current pulses is programmable within the range of 100μs to 3ms. Current-steering DACs are used to set the amplitudes of pulses. They exhibit DNL and INL of 0.04 and 0.17LSB, respectively. The amplitude, duration, and time constant of exponential pulses are independently programmable. Designed in the IBM in 130nm process, the circuit consumes 1.5×1.5 mm² of silicon area. Post-layout simulation results indicate that the stimuli generator meets expected requirements when connected to electrode-tissue impedance as high as 30kΩ. The proposed design consumes a maximum of 1.2mW in the rectangular pulse mode.