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Display and indicator lighting in mobile phones.

To support the larger display backlights required thanks to the popularity of phones with large touch screen displays, a high number of white LEDs are needed. In addition, illumination with color LEDs can increase the charm of a phone design.

Primary phone displays are getting larger, with mid--to high-end phone models increasingly featuring displays up to 3.5". To provide a bright and even backlight on these displays, up to 10 LEDs are needed, and ensuring even luminosity throughout the whole display area typically requires these LEDs to be connected in series. Assuming the maximum forward voltage of a white LED to be 3.5 V, driving 10 units will require at least 35 V from the driver output.

The Power to Illuminate

If we assume the current through the LEDs is 25 mA, the power required to drive the large LCD backlight is 25 mA x 35 V, or 875 mW. Compare this with smaller display backlighting, where only five LEDs may be needed, with a driving power of only 438 mW. To minimize the impact of the increasing LCD backlighting power consumption on battery operating time, ambient light sensing functionality may be required.

[FIGURE 1 OMITTED]

The ambient light of typical office lighting is 320 lux to 380 lux and that of a family living room or a building hallway is well below 100 lux. Figure 1 illustrates the relationship between the ambient light and the display luminosity required to ensure readability. The diagram shows that, when the user is in an office or dimmed light environment, the LCD backlight can be dimmed by at least 20 to 30 percent. Given that display luminosity is directly proportional to the driving current, ambient light sensing offers a saving in backlight power consumption of more than 20 percent.

The Human Eye and Light

It is interesting to note that the human eye's response to light is a logarithmic, rather than a purely linear relationship. Tenerating a linearly increasing backlight--a light effect sometimes referred to as "fade-in"--requires the driving current to be increased in a logarithmic profile. Using drivers in which this function is integrated eliminates the need for the processor to generate a real time varying frequency signal to pilot the backlight current.

Consider a backlighting scheme based around ON Semiconductor's NCP5021 1.3 MHz inductive boost LED driver, which can drive up to 10 LEDs in series with very high efficiencies of up to 90 percent. Compared to charge pump type drivers, the inductive boost structure offers perfect LED current matching and high efficiency. An integrated logarithmic current profile with 31 current levels compensates the eye's response to light. The integrated gradual dimming function is called up via [I.sup.2]C commands to automatically generate either a fade-in or fade-out effect without using host processor resources, leaving it free to concentrate on other tasks. Finally, a built-in ambient light sensing capability can optimize power consumption. Based on the input from an analog ambient light sensor, the driver can adjust the backlight current automatically to match display luminosity to the ambient lighting conditions. Thanks to the adjustable gain control through the [I.sub.2]C interface, the driver can operate with any type of photo sensor. When implementing ambient light functionality it is recommended that the RC filter is tuned to remove all short light pulses that may interfere with the ambient light detection.

RGB indicator light

Generating color with Red, Green, Blue (RGB) LEDs demands an LED driver with three individually controlled outputs. These outputs dictate the contribution of each color to generate the precise color needed. Due to the low power involved and the need to individually control LEDs connected in parallel, this type of driver usually employs a charge pump structure to step-up the battery voltage. At the same time, control via an [I.sup.2]C interface ensures precise timing control.

Figure 2 depicts the operation of both gradual dimming and ambient light sensing. At start up, the display can be lit up with a smooth fade-in effect to the preset maximum level (as defined by the user). As soon as the ambient light drops below the corresponding current ambient light level, backlight current will start to decrease to adapt to the new light conditions. If the ambient light is significantly below a preset minimum, the backlight should settle at the minimum LED current. When the ambient light increases again, the backlight current will increase in line with the corresponding ambient light level. The rate of change of the backlight current is specified by the phone designer and can be changed by the end user. Because the complete operation is automatic it requires no intervention from the main processor.

[FIGURE 2 OMITTED]

By Crystal Lam, On Semiconductor, www.onsemi.com
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Title Annotation:ECN[R] Focus on Lighting
Author:Lam, Crystal
Publication:ECN-Electronic Component News
Date:May 15, 2009
Words:796
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