Hysteresis Current Control In The Constant Current Driving For LED

In the lighting field, The new-generation photosourcen White-Light LED has great market potentials. Compared with the traditional lighting sources, the advantages of LEDs are obvious, such as high luminous efficiency, high response speed,low power consumption, small size, long life, and so on. LEDs have been widely applied to back lighting, flashlamp, screen display, the signal direction and transportation means. Considering characteristics of white LEDs and necessity of uniform brightness, white LEDs need to be driven by special power management circuits.The boom of white LED greatly promotes the development of integrated circuits for driving LED, which begins to play an important role in the market of power management.In the paper, the development of LED driving is introduced at first. At present, there are two driving mode for LED-constant voltage driving and constant current driving. Luminance is mainly controlled by LED driving current, but the constant current driving can eliminate the change of current in the LEDs caused by the voltage change with the changes of temperature and process, so that we can achieve a constant brightness of the LED and the control mode of constant-current driving is a common way. LED-constant current driving is presented in this paper.As for the control mode of constant-current driving, the hysteresis-current control technique has advantages of simple structure, Self-stabilization and practicality. Three kinds of methods for driving LED(linear regulation mode, capacitor switching mode, inductor switching mode) are discussed in this paper. Though the analysis of several modes of constant-current control, it is concluded that the hysteresis-current control mode is very suitable for driving LED due to its self-stability and simplicity.A chip which is used in the buck application to drive white LED with a constant current is proposed in this paper. Using high-side current sensing topology, it controls the driving current for LED to be averagely constant through hysteresis-current control technique. The chip realizes constant-current driving without complicated circuit design, provides the accurate control of current and self stabilization. The chip is mainly composed of a voltage regulator, a bandgap reference, a two-stage folded operational amplifier, a comparator, an output driving circuit and a power MOSFET.All the sub-circuit are designed and simulated, at last the synthetic circuit is simulated, the key parameters of the synthetic circuit are verified by Hspice, and the results of simulation meet the function requirement. It is designed and fabricated with a 0.5um 5V/18V/40V CDMOS process with input voltage ranged from 4.5V to 28V, with working temperature ranged from-40℃to+125℃. The chip can provide a driving current of 350mA for LED, it can provide an adjustable driving current through adjusting the outside current-sensing resistor. The duty cycle of the signal DIM can be adjusted to realize lightness regulation of LED. Hspice shows that LED driving current is hysteresis, and the Precision of constant current can be controlled within the range of 6.3%.