Fig. 3 shows the schematic of the bootstrapped switch for the passive CB. In a switch-based passive CB to eliminate residual charge by shorting the working and reference electrodes, the time constant, directly related to the required discharging time, is related with the electrode-tissue impedance and the resistance of the switch. Therefore, a bootstrapped switch is employed instead of a conventional transmission gate switch for a faster passive CB. The voltage at the gate node of M6 is boosted to two times of the supply voltage while M2 and M7 are cascoded to prevent M1 and M8 from having gate-drain and gate-source voltages greater than VDD. The on-resistance of the bootstrapped switch is reduced to 19.56Ω from 43.48Ω of the conventional switch. Assuming discharging period of 8.9µs with 100µA current stimulation, the proposed passive CB scheme guarantees a residual charge of less than 1nC within a safe limit for the electrical current stimulation.
Measurement Results: The proposed electrical stimulator IC is fabricated in a 0.18µm standard CMOS process. The chip micrograph of the stimulator IC is shown in Fig. 4. The active area of the 1-channel stimulator and digital controller is 987.6µm × 815.7µm and 1.4mm × 1.4mm, respectively.
Fig. 5 shows the compliance voltage with respect to the output current amplitude from 10µA to 500µA when the reference voltages, Vrefp and Vrefn in Fig. 2 are applied with the 4.8/0.2V, 4.9V/0.1V, 4.95V/0.05V, and 4.98V/0.02V. The maximum compliance voltage is 4.9V with Vrefp and Vrefn of 4.98V and 0.02V at the output current of 10µA. At the output current of 50mA utilized in the in-vivo test, the stimulator satisfies the high compliance voltage of 4.75V.