The Drive Circuit Design For A Constant Current White LED

Now, more and more electronic equipments are portable and multifunctional, which make the DC/DC converters must have special properties, such as high efficiency, low quiescent current, small board space, low weight and low cost. The inductive DC/DC converter is the traditional approach for power management. But with the development of submicron technologies and the use of high value multi layer ceramic capacitors, the efficiency of charge pump is improved which make charge pumps are still widely used not only in light load application but also in higher output currents condition.Charge pump operation principle and a fixed current Charge Pump Converter with wide input supply voltage and multi output versions of current, which uses 0.6μm standard Bi-CMOS technology, are presented in this paper. The input supply voltage range is 2.7V to 5.5V and the output current versions include 1.2A, 400mA and 240mA. The quiescent current is 5mA with no load, and less than 1μA in shutdown mode.In this paper, the linear charge pump voltage regulator chip is mainly used in low voltage power supply for 5.1V. Its main structure is composed of the two times charge pump circuit and the LDO voltage regulator circuit. When the battery capacity is insufficient in electricity, which is below the fixed voltage at the output, Charge Pump will be pump the input voltage, the maximum output voltage will be two times of the power supply. And this output voltage which is divided by the resistances will be feedback to the nagative input of the error amplifier(AMP). After compared with REF voltage, the error value of this two input signal was magnified. This value was used to control the charge pump voltage conduction resistance (also known as cont-rolled resistance).then get a stable output voltage.No matter what the input supply voltage above or below the output voltage, the output will remain in regulation by automatically selecting step-up or step-down operation mode. It works well for step-up or step-down without the need for inductor, providing low EMI DC/DC conversion. The high switching frequency allows the use of small surface-mount capacitors, saving board space and reducing cost. It also includes thermal protection and current limit, protecting the load and the regulator during fault conditions. The LED current was regulated by the overload protection comparator for a fixed value. And the chip was used 24mV comparator to select voltage regulation mode or current regulation mode.First of all, the whole chip function diagram is designed after analyzing the principle of charge pump and the system required function. And then, the sub-block circuit is designed according to the whole chip function and structure. In the charge pump principle section, the principle of linear mode and skip mode are analyzed in detail. The principle of step-up/step-down and the main parameters are also given in there. In sub-block circuit section, the typical circuits including reference, operational amplifier, undervoltage protection and thermal protection are introduced. The basic principle and main parameters of every sub-block circuit are noted and the simulation results are given at the end of each segment.Based on the charge pump principle analysis and circuit de-sign in the beginning chapters, the whole chip simulation result done by HSPICE EDA to verified that the IC has achieved the expectations, both functions and electrical characteristics.