Design Of An Integrated Bipolar Pulse Generator For Nerve Stimulation Based On A SMIC 0.18μm CMOS Technology

With the deepening trend of the world’s aging population,nervous system diseases have caused great pressure on society and medical system.Compared with traditional drug therapy and brain surgery,implantable nerve stimulation therapy has better curative effect for refractory nervous system diseases and brings better comfort to patients.Therefore,implantable electrical stimulation therapy has gradually become an important medical means to replace drug therapy and brain surgery.This thesis presents a bipolar nerve stimulation pulse generator,which is the core part of implantable nerve stimulator.It mainly includes bipolar stimulation pulse generator and active charge balancer to generate bipolar current stimulation pulse with adjustable amplitude,frequency and width,and to eliminate the residual charge of stimulation site.Bipolar stimulation pulse generator is mainly composed of pulse width modulation(PWM)signal controlled by microprocessor,reference current source,dual mirror current source and H-bridge.Firstly,PWM signal is used to control H-bridge to output bipolar stimulation pulse with adjustable frequency and pulse width.Secondly,a bandgap reference voltage source and a V-I converter are used to construct a reference current source by combining the bandgap reference voltage with an approximate zero temperature coefficient resistor,and the reference current is mirrored to a combined micro current source composed of a micro current source and a micro current well.The temperature drift coefficient of the reference current source is about 52.1ppm/℃.Then,the micro current source is duplicated twice to generate two groups of mirror current source.By adjusting the channel width of the transistor and using multiple transistors controlled by the switch in parallel,the current of a single group of mirror current source in the two groups of mirror current source is an integral multiple of the micro current source,and the multiple is programmable.Finally,the two mirror current sources are driven to the two arms of the H-bridge respectively,so that the pulse amplitude can be adjusted.The active charge balancer is mainly composed of a latch comparator based on pre amplification and a compensation current source.Firstly,in order to improve the speed and reduce the power consumption of the comparator,a high-speed comparator composed of a first-order preamplifier and a latch comparator is adopted.Secondly,the structure of the compensation current source is the same as that of the single mirror current source.The single mirror current source is connected to both ends of the load as the compensation current source and controlled by the switch.In order to prevent the accumulation of charge in the stimulation area,the active charge balancer detects the residual charge in the stimulation area immediately after each cycle of stimulation pulse.Then it compares the charge voltage at both ends of the stimulation area by using the latch comparator based on pre amplification,and compensates the charge at the stimulation area by controlling the on-off of the switches at both ends of the compensation current source through the output result of the comparator to eliminate residual charge.Because the charge voltage at both ends of the stimulation site is directly compared,the safety threshold range is small and the accumulation of residual charge is reduced.Considering the offset voltage of the comparator,the threshold range of charge voltage is about ±10m V.Based on SMIC 0.18μm CMOS process,the circuit design,pre simulation,layout design and post simulation of the integrated bipolar nerve stimulation pulse generator are completed.The overall layout area is about 840μm *696μm.The post simulation results show that the amplitude,frequency and width of the generated pulses are adjustable from0.01 m A to 5m A,10 Hz to 2000 Hz and 27μs to 20.39 ms.Under the influence of parasitic parameters,the frequency and width of the generated bipolar current stimulation pulses are similar to the ideal situation,but the maximum amplitude difference between the positive and negative biphasic pulses is 0.1m A,which deviates from the ideal situation.Finally,the integrated bipolar stimulation pulse generator module is taped out,and the chip area is 14mm*14mm after packaging.The results of chip test show that when the working voltage is 3.3V,the bipolar current stimulation pulse generator can produce bipolar current stimulation pulse with amplitude of 0.01 m A ～ 5.1m A,frequency of 9.8Hz ～ 1960 Hz,pulse width of 24μs ～ 20.2ms,and the maximum amplitude difference between two phases is 0.19 m A.The overall power consumption is 35.3mW.