The Evolution of Switched Capacitor Backlight Drivers in Mobile Phones

Today the majority of mobile phones use white LEDs to provide backlighting for the color display. Nearly every phone designer strives to drive the backlight with the most efficient, smallest size, and simplest solution available. Typically they decide to implement one of the two most common types of drivers used for LED backlighting: inductive boost converters or charge pumps. To ensure equal brightness of each LED, designers either put all LEDs in series and drive them with a constant current boost converter, or use a switched capacitor charge pump with individual current control for each LED (see Figure 1). The primary argument for using an inductive boost converter has been efficiency. While their performance varies from device to device depending upon switching frequency, resistance of the switches or MOSFETs inside the IC, and the resistance of the external inductor used, generally inductive boost converters offer efficiency levels ranging from 80 to 90 percent. To achieve that efficiency, however, designers must accept a number of liabilities. Inductive boost converters generally require more external components than alternative drivers, involve a more complex design process, and occupy more PCB space. In addition, converters are inherently less efficient in light load applications because their MOSFET gate charge losses and quiescent current losses outweigh MOSFET conduction losses.