aa alkaline battery.New backlight adjustment technology reduces input power consumption and improves battery life

　　Long battery life is a key indicator in the portable electronics market. LED backlight drivers for LCD displays account for 25% to 40% of the total effective system power consumption. In the past, designers' tools for minimizing power consumption in backlit displays were limited to reducing LED drive current while increasing converter efficiency. Today, power savings of up to 50% are achieved by using optimized converters utilizing LED drivers, ambient light sensors, and content-adapted backlight control (CABC) methods. These technologies increase drive efficiency without seriously degrading the visual quality of displayed information (websites, videos, pictures, etc.).

　　Traditional power optimization

　　Traditionally the main energy-saving technology surrounding backlight drivers has been the choice of boost architecture. Two main types of boost topologies dominate backlight drive architecture: inductive boost and switched capacitor boost. Inductive boost is usually used in series LED driver applications, while switched capacitor boost is usually used in parallel LED driver architectures.

　　Switched capacitor boost relies on the charging and discharging of a capacitor to create a boosted output voltage. The gain number of the switched capacitor boost is determined by the number of flying capacitors and internal MOSFET switching tubes. By selectively charging the combined series/parallel capacitor between the input and ground of the first phase, and then reconfiguring the parallel/series capacitance between the input and output of the second phase, the converter is able to provide a voltage ratio higher than the input voltage. higher output voltage. (Switched capacitor boost converters are often limited to a fixed voltage gain (1x, 3/2x, sometimes 2x) to help increase the efficiency of the solution while minimizing external component count.) Additionally, The size of the switching transistor used to configure the flying capacitor is critical to maximizing efficiency. Minimizing the gain output impedance allows the charge pump to remain at minimum gain for a significant period of time, helping to increase the efficiency of the solution.

　　Switched capacitors have a very limited amount of gain, and inductive boost converters have infinite gain. By adjusting the switching duty cycle of the inductive boost, the exact boost gain required to support the load (LED string) can be achieved. This optimization helps prevent excessive boost that can occur on the right side of a switched capacitor boost after a fixed gain transition.

　　In order to optimize the inductive boost converter, the on-resistance (RDSON) of the NMOS power switch and the series resistance of the inductor should be minimized. Unfortunately, reducing these two parameters usually results in an increase in physical size (generally a larger inductor with the same inductance value will have higher impedance than a smaller inductor.) Increasing the boost switching frequency can be achieved by using an inductor with a larger Inductors with low inductance values are used to reduce the physical size of the inductor, but increasing the switching frequency results in increased switching power losses. Selecting a Schottky diode with a low forward turn-on voltage will help improve conversion efficiency, and lower forward voltage Schottky diodes are typically larger in size than those with higher voltages. Additionally, the high duty cycle (80%) associated with the series backlight driver minimizes the impact of low Vf diodes since the device is only on for a short period of the switching cycle.

　　The series LED driver implementation method helps minimize the power losses associated with the current control element (usually the current sink). In the case of a series converter, a current sink is needed to control the current through the LED string, while in a parallel converter system each LED also needs a current sink. To further improve efficiency, the current source (currentsource) regulation voltage should be set at a level slightly higher than the sink's headroom (or dropout) voltage to prevent input voltage and/or output voltage changes due to output capacitor charge/discharge cycles. Current changes in the LED string caused by ripple sag.

　　Ambient light detection

　　In addition to power converter optimization, other power-saving features can be implemented to create an efficient backlight system. Many modern phones employ a power-saving mechanism that uses an ambient light sensor (ALS) to monitor ambient lighting conditions and adjust the backlight intensity accordingly (more ambient light means the backlight must be driven at a higher current, while at low backlight current can be reduced under lighting conditions). In bright outdoor environments, a very high level of monitor backlighting is required so that the monitor can be seen clearly. In contrast, in very dark environments, the backlight can be dimmed while still providing enough light to keep the display readable.

　　Ambient light detection requires a light sensor or photodiode in combination with detection circuitry. Most light sensors are current-based devices that provide an output current proportional to the amount of light entering the sensor. This environmental information can be used to determine environmental conditions (outdoors, office, movie theater, etc.) and then used to adjust the backlight to a predetermined brightness level.

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