LCD - Know How

Fundamentals and design

LCD is the abbreviation for liquid crystal display. An LCD basically consists of two glass plates with a special liquid between them. The special attribute of this liquid is that it rotates or “twists” the plane of polarized light. This effect is influenced by the creation of an electrical field. The glass plates are thus each coated with a very thin metallic film. To obtain polarized light, you apply a polarization foil, the polarizer, to the bottom glass plate. Another foil must be applied to the bottom glass plate, but this time with a plane of polarization twisted by 90°. This is referred to as the analyzer.

In the idle state, the liquid twists the plane of polarization of the incoming light by 90° so that it can pass the analyzer unhindered. The LCD is thus transparent. If a specific voltage is applied to the metallic film coating, the crystals rotate in the liquid. This twists the plane of polarization of the light by another 90°, for example: The analyzer prevents the light getting through, and the LCD thus becomes opaque.

TN, STN, FSTN, blue mode, yellow-green mode

Liquids that twist the plane of polarized light by 90° are referred to as TN (Twisted Nematic). STN (Super Twisted Nematic) liquids twist the plane of polarized light by at least 180°. This gives the display improved contrast. However, this technology does color the display to a certain extent. The most common colors are referred to as yellow-green and blue mode. There is also a gray mode, which in practice is more blue than gray, however.

In order to counteract the undesired color effect, the FSTN technology uses an additional foil on the outer side, but this causes a loss of light and means that this technology is only effective with lit displays.

However, the different colors occur only in displays that are either not lit or that are lit with white light. If there is any color in the lighting (e.g. yellow-green LED lighting), it overrides the color of the display. A blue-mode LCD with yellow-green LED lighting will always appear yellow-green.

Static or multiplex driving method

Small displays with a small viewing area are generally statically driven. Static displays have the best contrast and the largest possible angle of view. The TN technology fulfills its purpose to the full here (black and white display, reasonably priced). The bigger displays get, however, the more lines become necessary in static operation (e.g. graphics 128x64=8192 segments =8192 lines). Since there is not enough space on either the display or a driver IC for so many lines, multiplexing is used. The display is thus divided up into rows and columns, and there is a segment at each intersection (128+64=192 lines). Scanning takes place row by row (64x, in other words a multiplex rate of 1:64). Because only 1 row is ever active at any one time, however, the contrast and the angle of view suffer the higher the multiplex rate becomes. This makes it essential to use STN.

Angle of view 6°°/12°°

Every LCD has a preferred angle of view at which the contrast of the display is at its optimum. Most displays are produced for the 6°° angle of view, which is also known as the bottom view (BV). This angle corresponds to that of a pocket calculator that is lying flat on a desktop.

12°° displays (top view, TV) are best built into a table-top unit. All displays can be read vertically from the front.

Reflective, transflective, transmissive

Reflective (unlit) displays have a 100% reflector on the rear side. Backlighting is thus not possible. Transflective displays have a semi-transparent reflector on the rear side. They can be read with or without lighting. When they are not lit, however, they are somewhat duller than a reflective version. Nevertheless, this is the best compromise for lit LCDs. Transmissive displays have no reflector at all. They can only be read with lighting, but they are very bright.

Reflective LCDTransflective LCDTransmissive LCD

Positive/negative displays

Most displays are produced in positive mode. They can be recognized by their black characters on a light background. They are available with or without lighting. Negative displays have a dark background and illuminated characters. They can only be used effectively with lighting. Without lighting they cannot be read.

Positive modeNegative mode
  Normal displayNormal display
  Inverted by software Inverted by software

Lighting

LCDs without lighting are hard to imagine these days. However, since there are basically four different types of lighting, the type selected depends very much on the application. Here is a brief overview to clarify the situation:

 LED
yellow/green
LED
white
ELCFL
 Advantages- 5V= supply
- Lifetime 100,000
  hours
- Very bright (light box)
- 5V= supply
- Lifetime 5,000~20,000
  hours*)
- White light
- Very bright
- Low-power
- Very flat
- Different
  colors available
- Extremely bright
- White light
 Disadvantages- Green-yellow color
- Uneven and not
  very bright as a
  light pipe
- Higher price- EL inverter
  required
- Lifetime
  5,000-10,000 hours
- Not very bright
- CFL inverter   required
- Lifetime
  10,000-20,000 hours
Overall verdict  Straightforward Ideal Cost-effective  Powerful



*) life time depends on ambient temperature and LED current

However, the lighting also determines the optical impression made by the display, and the display mode; blue or yellow-green – does not always have an influence. Below you can see the EAP162-3N display with different types of lighting by way of example:

 Lighting
LED yellow/greenEL blueEL/LED white
STN blue
STN yellow/green 

Temperature range, limits and destruction

Standard LCDs have a temperature range of 0 to +50°C. High-temperature displays are designed for operation in the range from -20 to +70°C. In this case, however, additional supply voltage is generally required. Since the contrast of any LCD is dependent on the temperature, a special temperature-compensation circuit is needed in order to use the entire temperature range, and this is particularly true for high-temperature displays (-20 to +70°C). Manual adjustment is possible but rather impractical for the user.

However, the storage temperature of a display should never be exceeded under any circumstances. An excessively high temperature can destroy the display very quickly. Direct exposure to the sun, for example, can destroy an LCD: This is because an LCD becomes darker (in positive mode) as it gets hotter. As it gets darker, it absorbs more light and converts it to heat. As a result, the display becomes even hotter and darker... In this way, temperatures of over 100°C can quickly be reached.

Dot-matrix, graphics and 7-segment displays

The first LCDs were 7-segment displays, and they are still found today in simple pocket calculators and digital watches. 7 segments allow all of the digits from 0 to 9 to be displayed.

Text displays require what is known as a dot matrix, an area consisting of 5x7=35 dots, in order to display all of the letters in the alphabet as well as various special characters. Graphics displays have a similar structure to text displays. In this case, however, there are no spaces between the lines and characters.

Display drivers and controllers

The semiconductor industry now offers a very large range of LCD drivers. We generally distinguish between pure display drivers without intelligence of their own, controllers with a display memory and possibly a character set, and micro-controllers with integrated LC drivers.

Pure display drivers work in a similar way to a shift register. They generally have a serial input. They require an external pulse, and in multiplex operation with high frequency they require new display data continuously in order to achieve a refresh frequency that is as high as possible (MSM5219, UPD7225, HD44100, LC7942, etc.). An example of a genuine controller is theHD44780 for dot-matrix displays: Once it has received the ASCII code, the controller manages its character set, memory and multiplexing entirely on its own. The following controllers are widely used for graphics displays: HD61202/3, HD61830, SED1520, SED1330, T6963.

All of the well-known uC manufacturers now offer one or more versions with integrated display drivers. They have their own display memory that can be accessed by command.