A Constant-current White Led Driver Circuit Design

A white LED driver circuit is presented in this paper. The circuit drives up to four white LEDs with regulated constant current for uniform intensity. By utilizing proprietary adaptive 1x/1.5x modes and ultra-low-dropout current regulators, it maintains the highest possible efficiency over the full 1-cell Li+ battery input voltage range. The 1MHz fixed frequency switching allows for tiny external components and the regulation scheme is optimized to ensure low EMI and low input ripple. An external resistor sets the full-scale LED current, while two digital inputs control on/off and select amongst three levels of brightness.The circuit operates in 1x mode until just above dropout. Then the circuit switches to 1.5x charge pump mode to regulate the lowest cathode voltage of LED1–LED4 to 150mV and maintain constant LED brightness even at very low battery voltages. Using this topology, there is no LED brightness change during the 1x/1.5x switchover, which guarantees no flicker on the display. The 1x mode produces almost no ripple, while the 1.5x mode regulates the output voltage by controlling the rate at which the transfer capacitors are charged. In this way, the switching frequency remains constant for reduced input ripple and stable noise spectrum.According to the functions required by the system, the whole chip framework was designed firstly, under which all the sub circuits were accomplished in the circuit design chapter. In this chapter, the author introduced several typical sub circuits, CHARGE PUMP, MINIMUM SELECT, LED CURRENT REGULATOR, LDO CURRENT REGULATOR and separately specified the basic working principles and the calculation approach of major parameter of these sub circuit.The author simulated the circuit by applying EDA tools HSPICE and listed the details of the simulation result. The simulation result indicates that the IC has achieved the expectation, both function target and electrical characteristics.