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LED Back Lighting Solution using NXP UBA3070

 2009-04-15
Since its first introduction, the Thin Film Transistor (TFT) Liquid Crystal Display (LCD) has made much in road to our life. Its major advantage in light weight, low power and its capacity in miniaturization make it triumphant over Cathode Ray Tube (CRT) and Plasma in producing quality visual effects. A visit to the electrical appliance shop and you will notice that LCD is every where and it is here to stay.

One important aspect of the LCD is its backlight, a blank screen is what you will only see without the backlight. To ensure a proper display of colors, white is the ideal requirement of the backlight. Early LCDs adopted Cold Compact Fluorescent Light (CCFL) due to its white light effect. 

However, this comes with a cost, Fluorescent requires ballast to start up and maintain its continous lighting. The inclusion of this design makes the LCD bulky. For instance, a 26 inch LCD panel alone may requires up to 8 light tubes, thus 8 electronic ballast system including transformers. The design increases the weight of the product. The use of transformers also reduces the efficiency of energy conversion. Further more, the high voltage requirement to driving CCFL also makes it a technology which requires care to handle. In addition, CCFL may cause brightness issue if it is placed vertical. This is due to the sinking of the gas to the lower part of the fluorescent tube. The effect of this is darker picture on the LCD.

The introduction of LED to LCD solves the problem. The main properties of LED are its low power consumption, low voltage operation and simple design of driver circuit. However, instead of a single white LEDs most LCD manufacturers adopt a 3 colors LED for white simulation. With individual pixels containing the light filter of Red, Green and Blue (RGB). The use of RGB LED improves the brightness efficiency as individual light spectrum can pass through the narrow band pass filter on each pixel, should white be displayed, little light is block with this methology.

LEDs are driven by electrical current. As such current source is required to produce a consistent light intensity, thus the requirement of a back light driver IC. NXP produces just such a device, UBA3070.

The UBA3070 can drive up to a maximum of 600V power efficiently. It drives an external device therefore allowing it to control a wide range of LEDs from high to low power with the right Power FET chosen.

 
 
Figure 1: Simple Application Circuit



Figure 1 shows the LED driving application using UBA3070. The diagram suggest a simple design with few crucial components, namely the Sense Resistor and Power FET. In addition, UBA3070 comes with various features such as:
  • Direct Pulse Width Modulation (PWM) for dimming features.
  • Fast Transient response through cycle-by-cycle current control, thus prevent over or undershoots in LED current and enable efficient PWM dimming.
  • Over voltage lockout, leading edge blanking, over current and over temperature protection.
  • Low component count in design
Although the reference design uses a 12V DC source, its supply current requirement varies and can be lowered to a level of 9V DC depending on the system power.

Of course, an LED solution does come with its limitation. For one it does not provide an even spread of brightness to the LCD panel. However, this is solved with the use of a light diffuser to spread the light intensity. Another limitation is most probable the heat dissipation. As brighter LEDs are produced so does the increment of its heat. The LEDs would have to be properly designed as to allow heat dissipation while maintaining an acceptable intensity spread.

With continual rising demand of LCD more manufacturers are adopting to LED as the choice for their back lighting solution. The simplistic design of its power circuit is a plus point. It makes implementation possible in many terrain, ranging from small and compact design such as mobile phone to large scale application like a 19 inch LCD. It would not be a wonder if one day this solution is fitted into a larger screen LCD TV.



Written by Siew C.H.