The popularity of LCD monitors developed rapidly over the past 5 years. This makes the technology growth in the notebook market has taken over a number of desktop applications, in the form factor they provide, without taking into account the text of the copy desk is very clear and large format CRT display. Now, with the merging of personal computers and multimedia experience, the video reproduction is becoming a key requirement of this technology. The history of the drawbacks of technology, and some have weakness in color performance in the motion picture and can not be copied, especially in very bright or very dark picture of the regional details of stains.
In addition to technical problems, the shortcomings of the other liquid crystal display has been the historical cost. If space is not a problem, the current cathode ray tube provides a cost-effective solutions. Panel prices have declined rapidly, especially where supply has outstripped demand in the second half of this year. This has led many manufacturers to lower prices, although some people also cut production to bring supply / demand curve back.
In this article, we will take a better video reproduction, the use of TFT - LCD's some new technology, and how the pressure to take measures to reduce the cost of violations of this drive.
lcd display module assembly
Figure 1 shows a typical circuit, liquid crystal display module . Both notebook panel, desktop monitor or television applications and design, all of these features are required, but some of the performance of each product are different.
Timing Controller is a digital IC is a core member of the panel. It is used to control the liquid crystal display for controlling data writing mechanism is responsible for regular scans. The device reset row and column driver began to write at the top of the monitor and the scan data to a line in the bottom of the screen. The line driver was used to choose which line is at a specific time to write its data, the power transmission. Column driver into digital video data in the entire analog voltage input to the display each pixel unit.
Each LCD panel clearly need a power supply. The group four main power voltage. Main power supply (AVDD) provide a high voltage for driving LCD display in analog integrated circuits, including the column used to drive to the display itself, the image of the contents of the driver a lot. Although many people in the TFT - LCD monitor as a digital display, the brightness of each pixel is actually stored by an analog voltage at the pixel level. Pixel RGB filters in front to make color reproduction. Because the nature of the display of the simulation, AVDD power supply rails have to provide good load regulation, and supply sufficient current to charge and discharge pixel within the display capacity very quickly. Other items in the display, including products for the digital logic IC supply and a high voltage and negative voltage Von VOFF This is used to display the internal power line driver. For high-performance video display, which is very important to use high-performance TFT - LCD regulator to reduce the image distortion and smearing. Devices like the EL7585 (Figure 2) includes all these items in one IC 4, and also includes power sequencing it is important to avoid damage to the LCD display
Fig. 2. EL7585 LCD supply voltage generated four
The VCOM amplifier is used to provide a display of pixels are very stable reference voltage. About half of this supply is a typical value of AVDD and the brightness of each pixel by a column driver that drives VCOM voltage between the voltage difference determined.
Between pixels from one and applied to the light it emits is a nonlinear relationship between voltage relationship. The so-called gamma curve '(Figure 3) is actually an S - curve nature, which can be positive or negative reference to VCOM. In fact, most of the alternate uses of a panel switch polarity, and other pixels. This gives an average DC voltage across the display 0V, burns and other effects to avoid the offset related. As each group has a different gamma curve response, the column drivers need a reference curve, so that they can correct voltage drive each pixel to obtain the necessary brightness. These curves are typical of the supply and use gamma buffer and a resistor string can be used to simulate the curve.
Figure 3. S-shaped gamma curve, a pixel from among the displayed and applied to the voltage of the light it emits is not linear.
In our block diagram of the final device is a LCD backlight lcd driver. Today, almost all of the LCD panel using CCFL (cold cathode fluorescent lamp) backlight. These are the LCD panel itself does not need light, but in fact on or off, from the light source behind the gate. The CCFL device needs very high voltage AC waveform to drive them. General notebook computers and monitors can use only one CCFL controller for each panel. However, a large panel, TV monitor that needs a brighter backlight, the need for more cold cathode fluorescent lamps, so more drivers. From each of these CCFL light also must match or the brightness of the image on the screen will be different.
Reduce motion blur
Watch video on the LCD screen with one of the biggest is motion blur. This effect comes from the LCD display is very slow to respond, leading to the back of the image of any appearance of a smear campaign. Today, LCD technology is 12 ms to 16 ms produced within the response time. Although much faster than 20ms is 30 milliseconds for a few years ago, this is still fast enough to completely eliminate this artifact. Of course, this has been the development of key areas and the LCD is now used to eliminate a number of technical problems.
LCD panel from the front of the most obvious solution is to speed up the response of liquid crystal material. The current materials can improve the response over drive each pixel. However, this still does not produce rapid response is required. Although some companies display and 5 ms (gray to gray) response time of the new materials and gray, black, about 1 millisecond time. Fast gray-black, they also make very effective use of the black insertion (see Figure 4). This technology is into an image of a black frame between each frame. This will create a rapid pulse, similar to the CRT monitor. The human brain filters out the flash and automatically generates intermediate images. The technology demonstration at the recent exhibition proved very effective.
Figure 4. Insert an image into a black frame between each black box. This will create a rapid pulse, similar to the CRT display, the human brain integration, to effect the motion.
If the LCD is not available quickly, this effect can also be simulated pulse backlighting. This technique also proved to be very effective. From the top in the end by the Ministry of scanning backlight, and further elimination of motion artifacts, can move in very sharp images.
Offset by the LCD panel, the required voltage VCOM is slightly different from the ideal one and a half AVDD. In turn, this may lead to the appearance of the display flashes. To eliminate this effect, VCOM is usually adjusted on the panel by the panel of blinking, until the is removed. This pot is being replaced by machinery of these offsets, the automatic adjustment, eliminating the possibility of human error and bias, through the digital potentiometer. Figure 5a shows the driving EL5111 Intersil's ISL45041 VCOM amplifiers, high power applications.
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