Design Of Step-up DC/DC White LED Driver Chip

As one of the most promising light sources, White LED has many advantages, such as high luminous efficiency, high luminous intensity, long life period etc. The White LED driver's main function is providing constant current, and now it has become the star of power management field. Using step-up DC/DC converter to drive White LED is a major method for portable applications. The design of the step-up DC/DC converter Chip, with the current mode PWM control and synchronous rectification, is introduced in this thesis.In the main part of this paper, firstly, the whole chip's structure is constructed. And then, the subcircuits such as bandgap, current-sensing and slope compensation circuit, gate driver for power MOSFETs, soft-start circuit and IR comparator are introduced in details. Instead of the traditional series resistor technique, the lossless SENSEFET technique is utilized, which reduces the power dissipation. Without the external capacitor, the soft-start circuit can realize 700μs start-up delay by charging the small on-chip capacitor at long intervals. An improved gate-driver for the synchronous rectifier, which is not only reducing the power dissipation, but also protecting output stage, is proposed. Finally, the whole chip simulation results by HSPICE are given: the start-up delay attains 800μs, the output voltage ripple coefficient is only 1.64%, the efficiency is above 70% under moderate load, and the over-voltage potection and PWM luminance modulation are effective.