An Ultra High-Frequency 8-Channel Neurostimulator Circuit With $\text{68}\%$ Peak Power Efficiency

In order to recruit neurons in excitable tissue, constant current neural stimulators are commonly used. Recently, ultra high-frequency (UHF) stimulation has been proposed and proven to have the same efficacy as constant-current stimulation. UHF stimulation uses a fundamentally different way of activating the tissue: each stimulation phase is made of a burst of current pulses with adjustable amplitude injected into the tissue at a high (e.g., <inline-formula><tex-math notation="LaTeX">$\text{1 MHz}$</tex-math></inline-formula>) frequency. This paper presents the design, integrated circuit (IC) implementation, and measurement results of a power efficient multichannel UHF neural stimulator. The core of the neurostimulator is based on our previously proposed architecture of an inductor-based buck–boost dc–dc converter without the external output capacitor. The ultimate goal of this work is to increase the power efficiency of the UHF stimulator for multiple-channel operation, while keeping the number of external components minimal. To this end, a number of novel approaches were employed in the integrated circuit design domain. More specifically, a novel zero-current detection scheme is proposed. It allows to remove the freewheel diode typically used in dc–dc converters to prevent current to flow back from the load to the inductor. Furthermore, a gate-driver circuit is implemented which allows the use of thin gate-oxide transistors as high-voltage switches. By doing so, and exploiting the fundamental working principle of the proposed current-controlled UHF stimulator, the need for a high-voltage supply is eliminated and the stimulator is powered up from a <inline-formula><tex-math notation="LaTeX">$\text{3.5 V}$</tex-math></inline-formula> input voltage. Both the current detection technique and the gate driving circuit of the current implementation allow to boost the power efficiency up to <inline-formula><tex-math notation="LaTeX">$\text{300}\%$</tex-math></inline-formula> when compared to previous UHF stimulator works. A peak power efficiency of <inline-formula><tex-math notation="LaTeX">$\text{68}\%$</tex-math></inline-formula> is achieved, while 8 independent channels with 16 fully configurable electrodes are used. The circuit is implemented in a <inline-formula><tex-math notation="LaTeX">$\text{0.18}\,\mu\text{m}$</tex-math></inline-formula> HV process, and the total chip area is <inline-formula><tex-math notation="LaTeX">$\text{3.65 mm}^2$</tex-math></inline-formula>.