by Andrew Leone
What is an LCD?
Liquid crystal displays (LCDs) are a type of electronic display used in everything from televisions and computer monitors to digital watches and PDAs. An LCD is made up of two sheets of polarizing material, one on top and one on the bottom with a liquid crystal solution suspended between them. Each pixel or dot on the LCD screen has an electric circuit corresponding to it. This creates rows and columns making up the display. As an electric current passes through the liquid, the internal crystals align or untwist so that light cannot pass through to the front of the display. This process, which describes a monochrome display, may sound simple, but it is quite complex.
With the introduction of color come even more complex processes and complex circuitry. There are two basic types of liquid crystal displays on the market: Passive-matrix and active-matrix. The passivematrix type is less expensive and not as high quality as the thin film transistor or active-matrix display.
LCD panels cannot work by themselves. Other components need to be integrated-with the LCD. To create a working assembly, an interface controller, backlight system and inverter are also needed as shown in Figure 1.
The interface controller accepts external display information and sends it to the appropriate LCD drivers of the LCD screen. The interface controller receives information from the graphics controller of the computer or embedded system. Interface controllers can either be analog or digital.
The backlight system creates a viewable display in various environments. The light generated from the backlight system shines through the back of the LCD. The backlight system is the bulkiest and highest power consuming part of the LCD assembly.
The inverter boosts the voltage and current supplied to the circuit in order to power the backlight system. The inverter generates voltages around 500 Vac, sometimes exceeding 1 kVac on start-up. A typical system is shown in Figure 2.
Types of LCDs
Liquid crystal displays with a small number of segments,
such as those used in digital watches and pocket calculators have a single electrical contact for each segment. An external dedicated circuit supplies an electric charge to control each segment. This display structure is unwieldy for more than a few display elements.
The less expensive and lower quality passive-matrix LCD is used in digital watches, PDAs, and older notebook computers. The passive-matrix display employs twisted nematic technology. An electric current is applied to the crystals in order to make them untwist or twist to allow light through. Each pixel of the passive-matrix display has a row and column assignment with an electronic circuit at each intersection. The pixels are addressed individually by row and column addresses and must remain either twisted or untwisted between refreshes without the benefit of a steady electrical charge. This creates a very slow response time and poor contrast for passive-matrix LCDs.
Active-matrix displays are used in such products as notebook computers, computer monitors and flat-screen TVs. A matrix of thin-film transistors (TFTs) is added to the polarizing and color filters of the LCD panel. Each pixel has its own dedicated transistor. This technology produces a display that is much brighter and sharper than passive-matrix displays of the same size and generally has quicker response times and a greater viewing angle. An explanation of the different types of active-matrix displays follows.
The TN+Film (Twisted Nematic) display is the lowest cost active-matrix display and therefore the most common consumer display type. The pixel response time on latest crop of TN panels is much faster than their predecessors. With a faster response time, the shadow-trail artifacts are limited on the display. Disadvantages of the TN display are limited viewing angles and an inability to display full 24-bit true color. Color reproduction on TN panels is poor for the most part. As backlighting technology becomes more and more advanced, the color representation becomes better and better. LED backlights are proving to be a lot better than other implementations in this area.
IPS (In-Plane Switching) was introduced about 10 years ago to improve on the viewing angles and color reproduction of TN panels. But these improvements cause a slowing of response time. In addition, older IPS panels were more expensive than TN displays. S-IPS (Super-IPS) has all the benefits of IPS technology with improved pixel refresh timing. S-IPS technology is usually available in larger displays aimed at the professional market-not consumer. Tweaks to this technology continue to improve viewing angles and color contrasts.
Another type of active-matrix display is MVA (Multi-domain Vertical Alignment). MVA falls between the TN and IPS technologies. Fast pixel response, improved viewing angles, and high brightness contrast and color reproduction are features of the MVA technology. MVA is higher cost compared to IPS and TN. And even though MVA has fast pixel response, it isn't fast enough for most applications for the price.
These days more and more applications, including embedded applications, are using touch screen capabilities instead of traditional keyboards. A touch screen overlay on the LCD can now be used as a product specific "input" device.
Different types of touch screens exist in the market, each having different price points and specific applications. These are explained in the Step-by-Step Design article on Page 20.
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