Dissertation > Industrial Technology > Radio electronics, telecommunications technology > Radio equipment,telecommunications equipment > Power supply

Research&Design of a Self-Started Low Dropout Regulator for Buck Converters

Author ZuoJunJie
Tutor FengQuanYuan
School Southwest Jiaotong University
Course Circuits and Systems
Keywords DC-DC converter self-started LDO bandgap temperature compensation
Type Master's thesis
Year 2012
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With the development of information science and electronic technology, electronic equipment has in-depth impact of production and lifestyle of human beings, which has become an essential part of modern society. All electronic devices need energy. For their high efficiency, DC-DC switching power management chips become an integral part of electronic equipment, especially in pursuit of low-carbon lifestyle. LDO (Low Drop-out Regulator), as a solution to power the DC-DC switching power management IC themselves, become an important issue of the research on DC-DC systems.LDO inside the DC-DC swiching power management IC need a bandgap to provide a reference voltage, this bandgap is usually powered by the power supply. DC-DC switching power supply chip control systems also need a bandgap, for precision consideration, it is powerd by LDO output. For saving chip area and the reducing power consumption, this paper analyzes and designed a LDO architecture that the LDO is using a bandgap reference which will be shared with the control system. By using a start-up circuit, the LDO will act while the reference voltage do not established. While the reference is powered by the output of the LDO, it then begins to act and establish quickly, provides a stable reference voltage for the LDO, so as to stabilize the LDO output. Meanwhile the start-up circuit turns off.In the LDO control loop design, to reduce the dependence of external capacitor equivalent series resistance (ESR) and improve LDO’s transient response without increaseing too much static power, an enhanced AB follower as a buffer is introduced. It separates the pole produced by the LDO error amplifier output resistance and the parasitic capacitance of the power tube formation in the small-signal, so as to reduceing the dependence of the external capacitor ESR. When a large load changed comes, the buffer can increase or reduce the driving current to improve capacity of charge or discharge of the power MOSFET parasitic capacitance, thereby speed up the response of the LDO load.A transistor emitter connected structure is selected as a reference voltage generator, which will improve the loop gain. To meet the requirements of the accuracy of the bandgap reference, the thermal voltage is used on a transistor with negative feedback resistor to generate a compensation current for high-order compensation. This work makes the bandgap temperature drift lower.In order to verify the validity of the design, a LDO is designed under0.5μmBCD process model. The LDO works in the5.5-18V range, the maximum output current is10mA. Hspice simulation results show that the LDO starts up correctly, the static current consumption is56.6μA considering the start-up overshot, linear regulation rate is0.8mV/V, and the load regulation is2.2mV/mA. Power supply rejection ratio (PSRR) is greater than70dB. Reference voltage drift is2.17μV/°C, with a temperature ranging from-40°to125°. The entire system is working properly, can power the DC-DC chip with a stable voltage and provides a precision reference voltage.

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