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LED is the technology choice in such battery-powered portable devices as mobile phones and PDAs thanks to its high luminous efficiency, durability, and small size. Low power white LEDs of approximately 0.1W (Watt) are used in backlighting LCD (Liquid Crystal Display) panels and keyboards. Multiple LEDs can be connected together to deliver higher brightness as with torch or flash light for illumination purpose. High-power LEDs of 1 W are used in camera phones with high resolution of 2M pixels or more to support photographing in dark environments.
Apart from the white LEDs, RGB (Red, Green, Blue) LEDs are also used to enhance the esthetic aspect of mobile phones. By mixing a precise amount of each of the three colors together, RGB LEDs allow the creation of a large palette of colors. Indicator light color LEDs blink whenever there is an incoming call or message, and color can be used to show the caller identity in terms of a user-defined group such as friends, family or business. This feature not only adds a personal touch in mobile phone, it is also very useful when the user is located in a noisy environment. To reinforce a user's audio and visual sensation, RGB LEDs are also used to create fun light effects. One example is to synchronize the RGB lighting to the rhythm of the ring-tone melody or MP3 music. Another interesting usage of RGB lighting is Panasonic's "Feel*Talk" function. With the RGB LEDs embedded beneath the cell phone chassis, it displays a color that corresponds to the user's mood.
Leading White LED suppliers
The white LED that is widely used for backlighting LCD and keyboard in portable devices was first developed by the Japanese manufacturer Nichia in 1996, by coating a GaN (gallium nitride) and InGaN (indium gallium nitride) blue LED with a yellowish phosphor compound. The development of blue and green LEDs has expanded the LED color palette. In the late 90s, multiple patent disputes between the major LED manufacturers from Japan, US and Europe made it difficult for new suppliers to enter the market. Fortunately, these lawsuits were gradually resolved by cross-licensing agreements and some manufacturers have since confirmed their leading technology position like Nichia, Toyoda Gosei, Cree, Philips Lumileds and OSRAM.
New manufacturers from Taiwan and Korea emerged several years ago and have started to enjoy high growth in revenues in the last two years. According to an article in Nikkei Electronics Asia (Dec 2006), Taiwan produces more than 40% of the worldwide blue LED production output, that are used to make white LEDs.
LED performance improvement
With high capital invested into LED development, white LED luminous efficacy has greatly improved since its invention. The top white LED in the market has a luminous efficacy of up to 100 lm/W (lumens per Watt), which is close to that of a fluorescent tube.
Leading developers have tried to use different coating compounds on blue LEDs and prototypes demonstrated improved lighting efficiency. It is expected that the number of LEDs required to backlight a panel will continuous to decrease. The number of LEDs required to backlight a standard LCD panel on mobile phone is ranged between 2 to 4, while 6-10 LEDs are required to backlight an LCD panel on a PDA or smartphone.
A high-power LED that can be driven at a continuous 1W was first developed by Philips Lumileds in the late 90s. Since then, other suppliers such as Nichia, Cree and Osram have released LEDs with high brightness. Due to the price erosion of low-power LEDs, major LED manufacturers are focusing on the development of the higher efficacy LED that can be used in large LCD panel backlighting and general lighting applications. This development effort will eventually decrease the unit price of high power LEDs that are only used in high-end camera phone models today.
Before discussing the driver topology and the new features in LED backlighting and flash, let us review the electrical characteristics of both LEDs and batteries commonly used in mobile phones and PDAs.
LED electrical characteristics
Depending on each supplier's technology, the forward voltage (Vf) varies between 2.7 to 4V. In general, high power LEDs have higher forward voltage up to 4.9V. LED drivers should therefore provide sufficient positive voltage to forward-bias the LED to emit light.
When using multiple LEDs for backlighting purpose, the forward voltage variation must be taken into account for the driver circuit design. In order obtain the same luminous intensity, thus same color from several LEDs, designers must ensure the forward current through each LED be the same. Low power LEDs are typically driven with a forward current of 20mA and a maximum value of 25mA, while high power LEDs can be driven with pulsed current as high as 1.5 A.
Battery electrical characteristics
The most common type of battery for mobile phone and PDA today is the cobalt-based Lithium-ion or Lithium-polymer rechargeable battery. 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. For example, a 1000 mAh (milli-ampere hour) battery should be discharged at a maximum current of 1A. Typical battery capacity used in mobile phones ranges between 650 to 1000 mAh. New types of lithium-ion battery with different cathode material are being developed to improve battery performance. When using such battery packs, designers should respect electrical limits and adjust the driving circuitry accordingly.
Now let us study the usage of LED in LCD backlight, fun light and a camera flash light.
LCD backlighting
When using LEDs with maximum forward voltage of 3.4 to 4 V, the input voltage from the battery will be either equivalent or above the necessary driving voltage. Hence, a current regulated step-up converter will be needed to drive LED either connected in parallel or in series.
Charge pump/switched capacitor converter
A charge pump converter is widely used to drive LEDs for backlighting. Compared to the inductive based step-up converter solutions, a charge pump driver is appreciated for its low solution cost, low profile and low noise characteristics. New integrated circuit design has gradually improved the efficiency in charge pump drivers with peak efficiency over 93% and an average efficiency of about 80%.
Charge pump drivers commonly operate in 1x and 2x modes, and in some devices the additional 1.33x and 1.5x mode are applied to improve efficiency. In this kind of solution, LEDs are connected in parallel and the output current to each LED is supplied by individually matched current sources. Top driver ICs offer typically 0.2% matching between any 2 LED current from the same circuit.
In portable devices, maximum LED current is applied whenever a keypad or touch screen is activated. After a some seconds of idle period, the LED current will then be decreased to minimize power consumption. A common way to control LED current is by applying a PWM pulse to the enable pin of the driver IC. By turning on and off the IC, the output current is average out by the duty cycle of the PWM signal. On new LED driver IC, digital control interface such as single wire (S-Wire) or IC are most commonly adopted for they require only 1 or 2 I/O port and their relative design simplicity.
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