Study On The Stability And Transient Response Characteristics Of LDO

The traditional single chip low dropout linear regulators(LDOs)often need an external micro-level output capacitor.The output capacitor and its equivalent series resistance(ESR)form a left half plane zero to counteract the influence of the secondary pole on the stability of the circuit.Therefore,the capacitance of the output capacitor and the impedance of its equivalent series resistance(ESR)plays a very important role in the stability of the circuit,resulting in a very small range of the impedance of the ESR.In some cases,the impedance of ESR of the capacitor itself may not reach the required value.An ohm-level resistor in series with the output capacitor is often required.The ohm-level resistor will seriously affect the LDO's transient response characteristics and power supply rejection ratio(PSRR).In addition,under the normal circumstances,the capacitance and the impedance of ESR of an actual capacitance is related to the voltage across the capacitor,the application temperature,the application frequency and so on.In particular,the capacitance and the impedance of the ESR of an actual capacitor may changes significantly with the change of the application temperature and the voltage across the capacitor,resulting in the significant reduction of the operating temperature range.The main objective of this paper is to design a kind of LDO circuit with high stability.The stability of this circuit can be guaranteed when the capacitance of the output capacitor and the impedance of its ESR changes greatly.First of all,this paper introduced the basic working principle of LDO.Then,the definition of LDO parameters is expounded in detail.Then,the frequency compensation method proposed in this paper is derived from the Miller effect.Then,the transient response process is described in detail.Based on these,this paper designed a LDO with characteristics of high stability and fast response.The circuit design is based on a 32 V bipolar manufacture process.The circuit mainly includes bias module,reference module,error amplifier module,drive module,output module,shutdown module and overheat protection module.Considering the transient response performance,the output terminal is connected with micro-level output capacitor.Considering the stability,a compensation capacitor for pole-splitting is added between the output terminal and the output node of the error amplifier.Finally,a simple method to improve the transient response is proposed in the paper.Specter simulation results show that the output voltage is-5V(± 3%).The load current range is 1mA ~ 200 mA.The dropout voltage is 170 mV.The load regulation is 0.026 mV / mA.The linear regulation rate is 2.77 mV / V.The PSRR is about 50 dB(1 kHz).When the capacitance of the output capacitor changes in the range of 1uF ~ 10 uF,and the impedance of the ESR changes in the range of 10m? ~ 3?,the circuit can always be stable.When the capacitance of the output capacitor is 10 uF,and the impedance of the ESR is 10m?,and the output current flips from 1m A to 200 mA,the peak of the output voltage is less than 1%.The output voltage can be stable in 50 us.On this basis,adding an external 10 nF bypass capacitor can make the peak of the output voltage be less than 0.5%.The output voltage can be stable in 25 us.