A 2.4µW 400nC/s constant charge injector for wirelessly-powered electro-acupuncture

An ultra-low-power constant charge injector (CCI) circuit is presented for wirelessly-powered electro-acupuncture (EA). The CCI adopts adaptive pulse-width and frequency-drift (APF) calibration loop that not only accommodates to body impedance variation (BIV) of 100-200kΩ but also is tolerable to frequency-drift. The proposed 5Hz current starving clock generator combined with sub-V_th reference circuit provides supply voltage dependency of 0.2Hz/V and temperature dependency of 0.018Hz/°C while consuming only 1μW. These variations are finely calibrated by the APF loop to ensure injectable charge intensity as stable as 399.33-400.45nC/s. The low-power low-voltage G_m circuit in CCI incorporates diode-limited and chopper-modulated inputs to prevent the damage from static electricity of the body while providing linear voltage-to-current conversion. The proposed CCI, simulated in 0.18μm CMOS technology with supply voltage of 1.0V, consumes 2.4μW of power.