Design Of Low Dropout Voltage Regulator

With the fast development of portable electronic products such as cell phone, camera andIPAD, DC voltage regulators become more and more important. As an important member of PMU(power management unit), low dropout (LDO) voltage regulator has been applied widely due to itsgood output stability and small voltage-ripples. However the conventional LDO needs an extracapacitor to ensure the output stability. Therefore, a capacitor-free LDO is designed, which is basedon other’s work and could be used in the digital GMR current sensor the author designed before.Firstly, the working principle and the factors affecting the performance of LDO are illustrated.Then, the compensation method of conventional LDO structure is analyzed using the frequencycompensation theory of amplifiers.Secondly, three primary frequency compensation methods of Nested Miller Compensation,Dynamic Frequency Compensation as well as Damping Factor Control Compensation ofcapacitor-free LDO, are analyzed and compared with each other using small signal models. Then amodified DFC module is designed basing on the Leung’s research.Thirdly, the circuit modules, including Bandgap Voltage Reference, Error amplifier, ProtectionCircuit, are designed and simulated, and the circuits are fully laid out. In order to obtainhigh-precision voltage reference, a curvature-compensated structure is adopted in the Bandgapvoltage reference to improve the temperature stability. The whole circuit is simulated using CSMC0.5μm processes. The simulation results show 76dB@10 KHz of the PSRR of the bandgapreference, 60dB@1 KHz of the PSRR of the LDO, 2.6ppm/℃and 7ppm/℃respectively of thetemperature coefficient of the bandgap reference and LDO.At last, the capacitor-free LDO is fabricated with CSMC 0.5μm CMOS technology. The chipwas tested and analyzed using DC power supply, function generator, digital oscilloscope. The testresults show with a change of power from 3.5V to 9V, the variation of the bandgap reference is 7mV,the variation of the LDO is 18.9mV, the typical value (under a 5V voltage supply) of the referencevoltage and LDO is 773mV and 3.516V respectively, and the dropout of the LDO is 190mV.