Optimizing Backlight Efficiency Beyond Converters

　　Long battery life is a key metric in the portable electronics market. The LED backlight driver for LCD displays accounts for 25% to 40% of the total active system power consumption. In the past, designers ’ tools to minimize backlight display power consumption were limited to reducing the LED drive current while improving converter efficiency. Today, power savings of up to 50% are achieved by using optimized converters utilizing LED drivers, ambient light sensors , and content-adjusted backlight control (CABC) methods. These techniques can improve driver efficiency without significantly degrading the visual quality of displayed information (websites, videos, pictures, etc.).

　　Traditional Power Optimization

Traditionally, the main energy-saving technology　　around backlight drivers has been to choose a boost architecture. Two main types of boost topologies dominate the backlight driver 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.

　　Figure 1 LM3535—Switched Capacitor Boost

　　The switched capacitor boost relies on charging and discharging capacitors 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 switches. By selectively charging the series/shunt capacitors in the first phase input to ground, and then reconfiguring the shunt/series capacitors between the second phase input and output, the converter is able to provide an output voltage higher than the input voltage. (Switched capacitor boost converters are typically limited to a fixed voltage gain (1x, 3/2x, and sometimes 2x) to help improve solution efficiency while minimizing external component count.) In addition, the size of the switches used to configure the flying capacitors is critical to maximizing efficiency. Minimizing the output impedance of the gain allows the charge pump to remain at the lowest gain for a considerable period of time, helping to improve solution efficiency.

　　The amount of gain in a switched capacitor is very limited, while an inductive boost converter has 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 the "overboost" that can occur on the right side of a switched capacitor boost after a fixed gain transition occurs.

　　To optimize an 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 (larger inductors with the same inductance value will generally have higher impedance than smaller inductors). Increasing the boost switching frequency can reduce the physical size of the inductor by using an inductor with a lower inductance value, 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 usually larger than those with higher voltages. In addition, the high duty cycle (80%) associated with series backlight drivers can minimize the effects of low Vf diodes, as the device is only on for a short period of the switching cycle.

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

　　Figure 2 LM3530 - Inductive Boost

　　Ambient light detection

　　In addition to power converter optimization, other power saving features can be implemented to create an efficient backlight system. Many modern mobile phones use an ambient light sensor (ALS) to monitor the ambient lighting conditions and adjust the backlight intensity accordingly (more ambient light means the backlight must be driven at a higher current, while the backlight current can be reduced in low light conditions). In bright outdoor environments, very high levels of display backlighting are required to make the display visible. Conversely, in very dark environments, the backlight can be dimmed and still provide enough light to keep the display readable.

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

　　Figure 3 LM3535 controls 6 LEDs at 25mA

　　By adjusting the backlight to the appropriate level, the power drawn from the battery can be significantly reduced. Figure 3 shows a use case that highlights the potential power savings from using the system in five brightness zones: Sunlight, Cloudy Outdoors, Bright Office, Dark Room, and Night/Cinema. Brightness values ​​were set to 100%, 85%, 70%, 60%, and 50% (25mA, 21.3mA, 17.5mA, 15mA, and 12.5mA). As the ALS voltage rises (or ambient light is increased via a sensor), the driver IC samples the ALS voltage for a predetermined time before the driver forces a change in LED current. Sampling the ALS voltage for a specified (or averaged) time helps prevent LED flicker in rapidly changing lighting conditions.

　　Dynamic backlight control or content-adapted backlight control

　　Traditional mobile phone users are able to manually adjust the system's display brightness based on their preferences. Some users set the brightness to maximum all the time, while others adjust the brightness to a lower level to extend battery life. The manual adjustment scheme forces users to make compromises. Recent advances in LCD display drivers have provided system designers with a mechanism to adjust the backlight based on the information displayed on the screen. This concept is called dynamic or content-adjusted backlight control (DBC or CABC). By analyzing the display information, the display driver can directly communicate the desired backlight level to the backlight driver.

　　For example, if we use an inductive boost LED driver ( LM3530 from Texas Instruments ) to drive a string of six LEDs at 19mA to backlight a 3.5" screen for watching a TV show (about 20 minutes), the driver will constantly draw 137mA (assuming VBATTERY = 3.6V) from the battery. Using a DBC with the same backlight driver, the average current draw drops to 78mA if the brightness is set to 100%, 75%, 50%, and 33% of full scale. The backlight driver with DBC will draw 45% less average input current than a design without DBC.

In addition, DBC makes it possible to achieve higher contrast levels　　in LCD displays due to less light leakage from pixels due to backlight brightness changes . While DBC is effective in saving power, it also has a slight side effect on image quality. When DBC is running at lower brightness levels, white screen content will not look as bright and may sometimes have a slight gray tone. However, by choosing the right brightness level, designers can optimize power consumption and maintain image quality.

　　Figure 4 Power savings using LM3530 dynamic backlight control

　　Summarize

　　In summary, traditional efficiency improvements can be achieved by choosing a backlight driver boost architecture, and many new additional features can further reduce battery consumption. By using ambient light sensors, modern backlight drivers can provide appropriate backlighting based on the ambient light environment of the display. In addition, by using dynamic backlight control, the display driver can also adaptively adjust the backlight intensity based on the image content. Although these two new backlight adjustment technologies do not necessarily improve the efficiency of the LED driver, they can ultimately reduce input power consumption and increase the battery life of the mobile phone. These methods can be used without significantly reducing the visual quality displayed on the screen.