Research On Key Technology Of High Precision And Fast Positioning Voice Coil Motor Driver Chip

With the rapid development of portable electronic products,camera module which serves as a link between such products and human vision,has become one of the most widely used components.At present,most mobile phones with autofocus cameras are using voice coil motor to drive the lens to move to achieve auto-focusing.Voice coil motor is a special linear dc motor,and it can convert electrical energy into linear or circular motion mechanical energy without any intermediate conversion mechanism.Because of its simple structure,small size,high speed,high response and other advantages,it has become the first choice of camera module of mobile phone with auto focusing function.The voice coil motor is a typical representative of the second-order load with small damping coefficient.Direct drive will cause a long displacement stabilization time and dumping oscillation near the stable point,which will seriously affect the visual experience.In this dissertation,by designing a high precision and fast-positioning voice coil motor chip,innovative research has been done on a model,a method and two techniques.A unified electrical and mechanical model of voice coil motor displacement is established.A fast compensation driving method for second-order small damping coefficient load,a high precision curvature compensation reference source circuit and a new high precision hysteresis comparator are presented.The main innovations are as follows:(1)A unified electrical and mechanical model covering the spatial displacement variation of voice coil motors is presented.Through the analysis of mechanical balance equation and electrical balance equation of voice coil motor,RLC circuit and current control voltage source circuit are introduced;the displacement change of voice coil motor is equivalent to the capacitance voltage change of RLC circuit.The mathematical transformation of voltage and displacement is realized,and the unified electrical and mechanical model of spatial displacement variation is established,which lays the foundation for displacement and stability analysis of the system.(2)This dissertation presents a fast compensation driving method of the second order load with small damping coefficient.Based on the above model,and according to the transfer function of the second order system,the driving signal of the second order load with small damping coefficient is transformed.And the positive and negative step response with the same damping oscillation period are added with different time to compensate the damping oscillation amplitude of a single step response,so as to achieve the fast and stable driving response of the second order load with small damping coefficient.With this compensation driving method,an open loop controlled voice coil motor driver chip is designed and tape-out with 0.18?m standard CMOS(Complementary Metal Oxide Semiconductor)technology.Under the condition of a 10 ms resonant period,the displacement amplitude of voice coil motor corresponding to the optimal driving current is only 0.925% and the establishment time is only 7.8ms.The time of establishment is about 50% shorter than that of foreign products of the same type.(3)A new precise bandgap voltage reference with TlnT compensation of non-linear temperature is proposed.By using the voltage difference of two transistors working in different current states,the nonlinear compensation term of TlnT is generated.At the same time,the optimal high-precision reference voltage source is obtained by using the method of piecewise compensation.This design can achieve an ultra-low temperature coefficient(TC)of 0.918 ppm/°C from-40 °C to 125 °C.The minimum required supply voltage is 1.8V and the current consumption is 23 ?A at 25 °C.And the line regulation performance is 0.0055%/V at 25 °C with a supply voltage range of 1.8V to 3.6V.Compared with other reference circuits,the temperature coefficient and linear regulation of power supply have obvious advantages.(4)A high precision hysteresis comparator with Compensation Current Source(CCS)is proposed in this dissertation.A compensation current source is generated by the current difference between two MOS transistors operating in the linear region,and the CCS is applied in the traditional internal hysteresis comparator to compensate the process and temperature correlation in the hysteresis voltage,so as to obtain the high precision hysteresis voltage of low temperature drift.This design is based on 0.18?m CMOS process,and the simulation results show that the hysteresis voltage has a very low temperature coefficient.When the hysteresis voltage in typical design is 149.94 mV,the hysteresis voltage only changes by 1.3mV from-40 °C to 125 °C,the variation range is only 0.87%,and the temperature coefficient is 0.0079mV/°C,which is only 7.18% of the same type comparator circuit in China.It can be seen that the hysteresis voltage comparator proposed in this dissertation has very good linear regulation of power supply and temperature coefficient,especially suitable for high-precision applications.