Research On High PSR LDO Applied In CMOS Image Sensor

With the increasing integrated function of CMOS image sensor chip,digital circuits,analog circuits and digital-analog hybrid circuits are integrated on one chip.How to efficiently and reasonably supply each circuit becomes a hot spot of CMOS image sensor chips.Low-dropout linear regulator(LDO)offer low cost,high integration,low noise,and simple peripheral circuit,making it ideal for powering analog and mixed circuits in CMOS image sensor chips.Based on the application requirements for noise sensitive circuits of CMOS image sensor chips,We have designed a high power supply rejection(PSR)with output-capacitor-free LDO.Firstly,since the noise on the power supply is transmitted to the output of the LDO through the reference voltage source,a subtractor circuit is introduced in the designed band-gap reference circuit to improve the noise rejection effect.Secondly,the power supply noise transfer function of operational amplifiers at all levels is analyzed,and them proceed power supply noise have compensation relationship,so we added to a PSR enhancement circuit in the error amplifier.The PSR enhancement circuit enhances the compensation effect of the noise at the output by adjusting the amplification factor of the PSR enhancement circuit level to the noise on the power supply.Thirdly,the stability of the output-capacitor-free LDO is analyzed.In this paper,the Q-reduction circuit is added on the basis of the Miller compensation,so that the LDO remains stable at a lower load current.Finally,the transient enhancement circuit is designed to improve transient response characteristics and over-temperature protection circuit and current-limiting protection circuit are designed to improve the robustness of the circuit.The designed LDO circuit was implemented in the UMC 110nm process.the design of principle and layout were completed,after that the post-simulation verification were carried out.Post-simulation results as follows:The output voltage is 1.8V;When the maximum load is 50mA,the dropout voltage is 183mV;Line regulation is better than 0.05%/V;The load regulation is better than 8.9%/A;When the load transient response and line transient response occur,overshoot and undershoot at the output are less than 200mV;When the load current is 50μA,the PSR is-88dB at 10KHz;When the load current is 50mA,the PSR is-80dB at 10KHz.