## 2 System Overview  
### System Functionality  
The TcES system is shown in Figure 2a [18]. It consists of an external control unit and an internal implant unit. Except for the secondary coil, the implant unit is fully integrated on-chip and is shown in the dash box in Figure 2a. The internal implant unit consists of the power supply module, the data telemetry module, the digital controller and the stimulation unit. The inductively coupled AC power on the secondary coil is converted into DC power by the rectifier and stored in the on-chip  
Figure 1: The transcorneal electrical stimulation (TcES) implant in the eye. filtering capacitor at VDC. A 1.2-V low dropout regulator (LDO) and a 3-V LDO powered by VDC are used to supply power to the digital controller and the stimulation unit, respectively. The voltage detector monitors VDC for controlling the power. Peripheral circuits include the voltage protection circuit and the power-on reset for over-voltage protection and initialization during startup, respectively. The bandgap voltage reference generates the reference voltages for the 1.2-V LDO and the 3-V LDO, and also the bias currents for the stimulation unit. The data telemetry circuitry consists of a transmitter that includes an encoder and a data modulator, and a receiver that includes a demodulator and a decoder. The clock for the digital block is recovered from the power coupling frequency by a clock recovery circuit. The external control unit consists of the power transmission circuitry and the data telemetry circuitry. The DC-DC buck converter supplies power to the Class-E power amplifier that drives the series-resonant primary coil. An 8-bit digital-to-analog converter (DAC) is used to adjust the DC-DC output voltage to control the transmission power of the primary coil. A detection coil is used to receive the back-scattered data transmitted from the internal implant unit, and a digital controller is used to encode and decode the data. Figure 2: **(a)** The whole TeES system diagram (**b**) Downlink data package (**c**) Downlink data coding (**d**) Retina stimulation current (**e**) Uplink data package (**f**) Uplink data coding (**g**) Time multiplexing of the uplink and the downlink. For this TcES system, the power link and the data link share the same pair of coils. The output voltage of the rectifier V\({}_{\text{DC}}\) changes when the loading changes, as well as when the distance of the coils or their relative alignment changes. Adaptive power control is needed to dynamically adjust the emitted power to achieve a stable rectified DC voltage. It is divided into power regulation and power boosting. For power regulation, the voltage detector senses V\({}_{\text{DC}}\) periodically and the value is transmitted back to the external control unit through backscattering. By comparing with a reference voltage, the output of the DC-DC converter is adjusted to change the transmission power accordingly. Power boosting is activated to increase the transmission power prior to the scheduled task of electrical stimulation. With these proposed power control strategies, the rectified DC output is well regulated and achieves better immunity to coupling and loading variations. The implant unit generates a bi-polar current waveform for retina stimulation. For different patients or different therapy stages of the same patient, the stimulation profiles (current amplitude, stimulation frequency and duty ratio of current pulse) are different. The stimulation profile is programmed by the external control unit and sent through the data link. The digital controller reads the 13-bit configuration data, codes it, modulates it with amplitude shift keying (ASK) and then transmits it to the internal implant unit. The implant circuitry demodulates and decodes the 13-bit data, and stores it in a register that sets the DAC output with the stimulation profile. The current amplitude can be programmed to range from 5 \(\upmu\)A to 320 \(\upmu\)A, the stimulation frequency from 10 Hz to 160 Hz, and the current pulse duty ratio from 2.5% to 20%.