| There are mainly two structures of white LED driver, based on charge pump and based on switching power supply. Both of them have advantages and disadvantages respectively.After analyzing the principle of charge pump,a white LED driver circuit based on unregulated charge pump is designed firstly, then its drawback is indicated and modified by using a linear mode charge pump. It regulates the lowest cathode voltage of LED1–LED4 to 150mV and maintains constant LED brightness when the circuit switches to 1.5x charge pump mode. Using this topology, there is no significant LED brightness change during the 1x/1.5x switchover, it also has low ripple compared to Skip mode. A optimized mode selector which can select mode switchover point with forward voltage of white LED is designed finally, it can keep the charge pump working in high efficient 1X mode as long as possible.In the section of sub-block circuits, typical circuits including reference, operational amplifier,oscillator and mode selector are introduced. The basic principles and main parameters of every sub-block circuit are noted and the simulation results are given at the end of each segment. A current mode bandgap reference is used to achieve more voltage references flexibly.The whole circuit is simulated by HSPICE and the details of the result are listed. The simulation results indicate that the circuit can drive four white LEDs with regulated constant current for uniform intensity. It can work in 1X or 1.5X mode automatically. By utilizing adaptive 1x/1.5x modes and ultra-low-dropout current regulators, it maintains the highest possible efficiency over the full input voltage range. The 1MHz fixed frequency switching allows for tiny external components. An external resistor sets the full-scale LED current, while two digital inputs control on/off and select amongst three levels of brightness.It has 2.7v to 5.5v input voltage range. It can be used as a backlight driver for mobile phone and other portable applications. |
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