Light Emitting Diodes is the pre-eminent choice of technology in essentially battery-powered devices and they emphasize the lighting esthetics and performance of cell phones, PDAs and other mobile devices. Over time, certain characteristics such as durability, high luminous efficiency and small size of cell phones and hand-held devices have become responsible for making LEDs popular. CRYSTAL LAM elaborates…

Next-generation cell phones, futuristic smartphones, all-in-one hand-held mobile communication devices will be loaded with high-end and "rich" multi-media applications, features and software in the next few years. Chip vendors, OEMs, software suppliers, developers, designers and other hardware vendors are working overtime to load these devices and phones and meet the key and marketdriven requirements of a huge mass of customers in the future. In the light of the fastpaced developments shaping the growth and acceptance of the mobile phones and other mobile communication devices, it becomesimportant to highlight the importance of Light Emitting Diodes (LEDs), their pre-eminence as the choice of technology in essentially battery-powered devices and how they emphasise the lighting esthetics and performance of cell phones, PDAs and other mobile devices.

Over time, certain characteristics such as durability, high luminous efficiency and small size of cell phones and hand-held devices have become responsible for making LEDs popular. Also, most of the end applications that are now more amenable for relatively high-end displays will also drive the demand for better luminous efficiency. Evolving socio-cultural contexts and the engaging characteristics that virtually all mobile users in the retail segment for cell phones exhibit have pushed RGB (Red, Green, Blue) LEDs to the forefront in recent times.

In the context of such fastchanging shifts and userdictated market dynamics, it must be pointed out that the development of blue and green LEDs in the nineties has expanded the LED color palette, so to speak. The color LEDs lend that personal and esthetic touch desired by vast masses of cell phone users and irrespective of market demographics in different regional markets. In fact, RGB LEDs in many mobile phones can also reflect moods and contextual feelings of users and given the user penchant for personalising their devices more and more will become highly popular in the future. Expectedly, LED performance improvement holds the key to defining the next level of acceptance and usage.

Massive investments have been the order of the day to enhance performance and reflect the characteristics, form factor and new-age designs and form factors of cell phones and mobile communication devices. Major players have been talking about the need for fast-paced developments in boosting the performance of LEDs to reflect the burgeoning growth of the mobile phone industry.

Many different coating compounds are being used on blue LED and leading industry players predict that the number of LEDs required to backlight a panel will continue to decrease. While the number of LEDs required for backlighting a standard LCD panel on a mobile phone is ranged between 2 to 4 LEDs, anywhere between 6- 10 LEDs are required to backlight the LCD panel on PDA or smartphones. Quite clearly, with the imminent flameout of low power LED, leading LED manufacturers are concentrating on developing higher efficacy LED. These can be used in large LCD panel backlighting and general lighting applications.

The crucial point is any decrease in the unit price of high-power LEDs- and better lighting efficiencywhich is prevalent in highend camera phone models will result in usage in the much more applications - happy and software-packed cell phones of tomorrow. Still many more new features in LED backlighting and flash light will undoubtedly enhance the very esthetics, look and feel of nextgeneration phones and mobile devices, and manufacturers are well-aware about spurring more capital investment into LED development to bring about such changes. Thus battery electrical characteristics become important in the context of investments in LED and LED development.

For long, the most common type of battery for mobile phones and PDAs has been the cobalt-based Lithium-ion or Lithium-polymer rechargeable battery. These Lithium-based rechargeable batteries have nominal voltage of 3.6 to 3.7 V, with operation voltage from 4.2 to 3.2 V. To guarantee safe operation, this type of Lithium-based battery should be charged or discharged within 1C only, where C is specified by the capacity rating of the battery.

Furthermore, in the light of changing industry demands and market shifts, new types of lithium-ion batteries with different cathode materials are being developed to improve battery performance. So when such a battery pack is being used, designers should work within the electrical limits and adjust the driving circuitry accordingly. LED flash in mobile phones has been regarded as a gimmick in many quarters. This observation is justified as many so-called flashlights are made up of several low power LEDs and they are driven by a relative low current from 20 to 60 mA.

This kind of lighting has been used as a portable torchlight but its weakluminosity is not sufficient to support photographing in dark environment. In fact, one or more high power LEDs will be needed in order to support photographing object at 1 m or more. The key barrier for designers and other developers to adopt the high power LED remains the cost issue. Mass volume, high power LEDs are supplied mainly by the top 5 global suppliers. However, unit prices are expected to fall soon and consequently the driving circuit cost will follow suit as production volume picks up.

Xenon flash is already available on mobile phones with 3M pixel or more to support a high quality camera function. To provide a comparable luminous intensity as xenon flash from LEDs, one can drive 2 or more high power LED as flash light. An inductive step-up converter with high power driving circuit of 4.5 W is capable to power 2 LEDs in series with a driving current of 500 mA. It is a moot point that time-out protection is necessary in this kind of driver to prevent the LEDs from being damaged by a prolonged operation.

A switch should also be built in the driver IC to allow the switching between the torch and flash current level. To be sure, in recent times, high-volume LED supply has pushed down the unit price of low power LED for backlighting LCD panels for mobile phones, PDAs and other communication devices. New backlight drivers have built-in esthetic features like gradual dimming, ICON mode and mood lighting that does not require any software design and do not consume any microprocessor resource.

Thus, these LED drivers can help portable manufacturers to reduce development time - so crucial in these faster time-to-market cycles. Most power flash consists of either several standard LEDs or only 1 high power LED. The limited use of high power LED in camera phone today is due to high unit cost of the LED. Already observed in some high-end mobile phones and smartphones is the use of 2 LEDs in order to provide higher brightness flash to enhance the camera phone image.

As camera phones gradually replace digital still cameras, higher power flash solution will become available in order to provide a real camera experience to the user. Thus LED and its drivers will continue to be widely used in a number of next-generation portable communication devices and mobile handsets.

As camera mobile handsets. phones gradually replace digital still cameras, higher power flash solution will become available in order to provide a real camera experience to the user