# High-Efficiency, Low-Power, Fully Integrated Neural Electrical Stimulation Circuit
## 1 Introduction  
At present, there are more and more patients with brain diseases, and traditional drug treatment can no longer meet the demand, so the research in the field of the brain and brain diseases has become a hot research direction. Brain-computer interface technology (BCI) is an advanced brain science research technology. By implanting electrodes at designated positions, the brain can transmit information to external devices, so as to stimulate nerves. Neural stimulation is an important part of a brain-computer interface. It injects a specific amount of charge into nerve tissue through electrodes and produces a certain physiological response, so as to achieve the purpose of treatment [1]. Neural stimulation technology has been widely used in cochlear implantation, neuromuscular stimulation and deep-brain stimulation [2, 3, 4]. It not only realizes the modulation and decoding of neural signals but also provides users with more accurate and efficient human-computer interaction through function recovery, behavior intervention and neural feedback. At present, there are threemain types of neural stimulation circuits: voltage-controlled, current-controlled and charge-controlled stimulation. Voltage-controlled stimulation is the earliest nerve stimulation mode applied in history [5]. It achieves the purpose of stimulation by applying a fixed voltage at both ends of the electrodes. Although this mode has the benefit of low power consumption, the stimulation current is uncontrollable, which is prone to cause irreversible damage to the tissue. On the other hand, charge-controlled stimulation injects the charge into the target tissue [6]. It is safer, but it needs to be realized using bulky capacitors, which limits the miniaturization of the neural stimulation circuit. Current-controlled stimulation is the most used stimulation method at present, as the stimulation current is controllable, which is therefore a safer option. When applying neural stimulation, the electrode-tissue interface generally requires a stimulation current as high as 1 mA [7]. Utilizing a well-designed electrode, the electrode-tissue impedance can be in the range of a few tens of kilo-Ohms to mega-Ohms [8]. Therefore, it would require the stimulation circuit to sustain more than 10 V voltage, making high-voltage generation an essential part of a current-controlled stimulation circuit. To achieve high efficiency remains a critical challenge for the design of the high-voltage-generation circuit. In 2012, [PERSON] et al. proposed a high-voltage-generation circuit that monitors and regulates the DC-DC converter current [9] using a current sensor, thus reducing energy waste. However, this method utilizes large inductance. In 2013, [PERSON] proposed a head-mounted deep-brain stimulator [10], which regulates the supply voltage and improves the efficiency, but this method increases the complexity of the circuit. In 2017, [PERSON] et al. proposed a neurostimulation [11], which combined digitally adjusting the supply voltage, achieving an efficiency of 54%. However, the supply voltage of each channel cannot be individually controlled. In 2024, Peking University proposed a high-energy-efficiency current-mode neurostimulator [12] based on the dynamic power supply voltage. It can achieve a significant increase in the efficiency of the system. However, this method also utilizes a large inductor. In this paper, we propose a neural stimulation circuit designed solely using low-voltage devices under the standard CMOS process. It eliminates the usage of bulky high-voltage devices and reduces the fabrication cost. More importantly, it allows a more compact integration with the recording and signal processing circuit using the standard CMOS process to form a complete closed-loop brain-computer interface system. A series-parallel charge pump with a negative-voltage assist module was utilized to generate high-voltage output, which reduces the number of charge pump stages at the same time to improve the efficiency of the circuit.