Mobile Devices: Components boost display options

Cell phone design is hot. The buying public is fascinated by the latest and greatest designs that offer as many functions and features as possible, all packed into a palm-sized device. While high-end business users will get first crack at functional changes like the Wireless Application Protocol (WAP) phones for e-commerce, organizer functions and data connectivity, it's the new low- to mid-priced phones that are undergoing transformations.

What matters? At first it's size, shape and color, but the next impression is likely to be the display and display-keypad lighting. Lighting a phone's display, keypad and network status indicator is becoming a significant differentiator in the "coolness factor" wars.

Take a look at Japan, where cell phone adoption is second only to Western Europe's, for indicators of coming trends. The success of NTT's Docomo iMode services, where over 15 million of Japan's 52 million cell phone users have adopted the service in the last 22 months, has accelerated the adoption of phones with color displays. Even in phones with standard black and white STN displays, there is often a choice of backlight color. Creating all these colors, and doing it cost effectively with small size and high efficiency, has created a new set of challenges for cell phone, handheld PC and organizer devices.

Lighting needs
There are three major areas where lighting is needed in modern mobile handsets: the display, the keypad and the network status indicator. Two other areas have little or no functional purpose: antenna lights use some of the RF energy emitted by the transmitter to light an LED and indicate a ring. Flashing case lights can be added as an aftermarket product where they are typically found on the battery packs from third-party manufacturers. These are most popular in Asian Pacific markets.

Display lighting has received the most attention from the phone manufacturers because users can perceive small brightness differences or unevenness that may lead to quality judgments about the phone. Keypad lighting has used a large number of LEDs (up to 12) in the past, but color purity and matching is less important here. The network status LED is not found on all phones, but those that have it typically use green only or red and green combination LEDs to represent various network conditions (connected to cell site, out of range, on foreign network and ring, and so on).

With black and white LCD displays, the lighting color can be completely dictated by style, as found in the red, orange, green, blue, white and other choices offered in Japan. This is achieved by a combination of LED and electroluminescent technology, each with its strengths and weaknesses. Adding a color LCD display means answering the question of how to provide a white light. In a transmissive LCD, subtracting all the other colors from each of a red, green and blue subpixel produces color. Therefore, a white "color" source is necessary to begin with.

After a controlled amount of red, green and blue are generated for each pixel, the light is added beyond the display (by your eye) to create essentially any color. White LEDs and fluorescent tubes are the most common ways to generate this white light. Cold cathode fluorescent tubes are common only in displays that are larger than about four inches diagonally because of the difficulty in making very thin, short tubes. They deliver high brightness and are relatively efficient, but they have not been adapted to typical cell phone display sizes of 1.5 to 2 inches diagonally. Today, these small displays are the exclusive domain of the white LED.

The white LED represents a special challenge. Actually, a white LED starts life as a blue LED based on an indium gallium nitride junction. A yellow phosphor is applied over the top of the junction where the blue light energy causes emission from the phosphor and mixes with the original blue light. This is how a broad spectrum of wavelengths can be generated from the relatively narrow spectral content of the blue LED. Nichia Corp. invented this method, based on a high-efficiency blue LED, quite recently. The light emitted is not a collection of narrow bandgaprelated frequencies, but is fairly broad and even, except for an overabundance of blue. As might be expected, driving a white LED is identical to driving a blue LED based on the same elements.

Constant supply
The forward voltage required for highest efficiency from these new LEDs typically falls between 3.5 V and 4 V, higher than that for red and green LEDs. This is just high enough so that boost power converters are necessary to drive them from common battery-based supplies. All LEDs add another complication: they want to be driven from a constant current supply rather than a constant voltage. Manufacturing variations in the forward voltage can produce much more brightness variation when constant voltage is applied. To minimize that variation, as well as color shift in W-LED, a constant current supply is better. This has the added benefit of being more efficient in most cases, since the power dissipation of the current limiting resistor is eliminated in constant current designs.

As white LEDs became available in volume, the lowest cost approaches-first-generation solutions-to driving them typically used off-the-shelf boost and voltage regulation feedback. Those solutions re-used common architectures that had been tapped for development of power supplies for other 5-V needs within cell phones. Producing voltages around 5 V allowed enough headroom for the worst-case LED forward voltage, plus a small drop across a current-limiting resistor. Switched-capacitor techniques were used for designs with one to about four LEDs, while inductive boost chips were used when four or more LEDs were required.

One option, a switched-capacitor charge pump booster followed by a step-down LDO, can accommodate the full lithium ion input range and produces a voltage-regulated output. Setting the output at 4.1 V to 4.3 V minimizes the dissipation in the current-limiting resistor in series with the LED.

More options
Another option, putting the switched capacitor booster after the LDO, sacrifices a little input voltage range but is more efficient than the first option, and the LDO helps to filter conducted noise from the switched-capacitor charge pump. But the voltage regulation is not as good.

A third approach is used when more than four LEDs are driven, since the total power needed is more cost-effectively supplied when inductive architectures are used. This technique provides constant current by taking the feedback voltage from above the current-limiting resistor.

Because of the high cost of W-LEDs, the highest-volume applications continue to use four or fewer LEDs. That emphasizes the use of nonoptimal voltage-regulation solutions.

Keypad backlights and network-status LEDs are likely to continue to use mainly first-generation power solutions for the next year or two. Color LCD displays, on the other hand, will migrate quickly to switched-capacitor boost products with constant current regulation-second-generation solutions.

Adoption of color displays is driven by low-speed data applications such as i-mode, WAP, Symbian and Palm operating systems that add organizer and browser functionality.

Cell phone makers aren't waiting for 3G infrastructure build-out before adding color. In fact, the largest barrier to adding color displays is cost, followed by increased power consumption.

Second-generation W-LED power solutions for small color LCD backlights and frontlights, will be adopted for several reasons. Foremost is brightness matching between LEDs. This is most important when using only two LEDs, where users should perceive equal brightness from each side of the display. Due to the process variation in LED forward voltage, supplying a constant current means various voltages may be presented to each LED. Although power will still vary (I x V), the best brightness matching is achieved.

Since a current mirror architecture is used, LED current for all LEDs is set by one resistor using a large amplification ratio. By using a 20:1 or greater ratio, power consumption of the system is minimized.

Brightness can be controlled with a low-noise analog input, or with a digital output driven by a pulse-width-modulation algorithm. These inputs, along with active control of the RLIMIT resistor, can also be used for temperature compensation.

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