About LCD Displays

What is LCD, History, How it's work...


WHAT IS LCD?

As its name implies, Liquid Crystal is a material composed of dense particles in solid state that appear chemically in liquid form. It can be polarized according to the applied electrical energy. The use of this material, which was found by chance in 1888, in the image transmission technology began in 1962 with the first tests in RCA Laboratories. However, the tests performed in these years failed to achieve the desired results at room temperature, but only in 1966 and finally in 1972, the first active matrix LCD Display was produced by the United States..


George Heilmeier First LCD Test 1965

Ronald Friel First LCD Demo Setup 1968

Robert Lohman First LCD Clock 1968



HOW IT'S WORK

LCD Displays work according to the principle of passive display technology. Therefore, since they cannot emit light alone, they give the image of the natural light in the environment or with the help of the backlight placed on their backwards. They are preferred in applications where energy resources are limited (measuring instruments, clocks, meters, thermostats, etc.).

Liquid crystal material Although it is in liquid form, it is a crystal material in a solid form. These material particles, whose ends are similar to oval cylindrical tubes, are normally aligned in a spiral parallel to each other on top of each other. Molecules that are stationary under normal conditions can be controlled by electrical voltage. Today, TN(Twisted Nematic) mode is the most widely used type in this display technology.

LCD Displays consist essentially of liquid crystal material placed between two transparent glass sheets. The inner surface of the glasses has channels coated with conductive polymer to apply voltage. According to the electrical energy applied to the liquid crystal in these channels, they are aligned in a polarized linear order, or they are aligned from one spiral surface to the other surface. This spiral structure bends the light so that light can be transported between the polarizing filters positioned perpendicular to each other. When electrical energy is applied to them, they curtain the light and create the image (dark black segment) on the LCD by lining them linearly behind each other.





LCD VIEW MODE


    LCD Displays are grouped into Twisted Nematic (TN), Super Twisted Nematic (STN), Film Compensated Super Twisted Nematic (FSTN) and Color Super Twisted Nematic (CSTN).





Twisted Nematic (TN)

    They are preferred in terms of price performance because they provide a serious cost advantage. Liquid crystals are arranged in a spiral pattern of maximum 90 degrees and below. Although they are very economical, their viewing angles and contrasts are low. They are preferred in low cost applications (Digital Clocks, Energy Meters, Metering Controllers etc.).




Super Twister Nematic (STN)

    Unlike TN LCD displays, STN LCD displays are arranged in a helical arranged with a minimum angle of 90 and a maximum of 360 degrees. Due to this spiral arrangement, they enabled the production of high resolution active matrix LCD displays. The STN LCD Displays, which can typically be flexed from 180 degrees to 270 degrees, have higher viewing angles and contrasts than TN LCD Displays.




Film Compensated Super Twisted Nematic (FSTN)

    Contrary to STN Indicators, Retardation film added to polarizers reduces the Contrast and Angle loss on the STN LCD indicators to a minimum. Especially with backlight FSTN LCD Display image quality, much better than STN LCD Displays. The segments are darker (almost pure black) and the viewing angle is wider.




Vertical Alignment Twisted Nematic (VATN)

    These LCD Material have a similar structure like TN materials. The extra films added to these crystals allow the leakage to be below 5% in negative polarization. In this way VATN LCD Indicators provide high contrast and viewing angle. Image quality are very close to high contrast OLED displays.





LCD DRIVER TYPE


LCD Displays are divided into 3 sections according to the type of driver IC they contain. COB(Chip On Board), COG(Chip On Glass) ve Glass Panel(without driver IC) LCD Displays.


COB LCD (Chip On Board)

Driver IC's are located on the PCB, mounted together with the integrated LCD Glass. The connection between the LCD and the PCB can be in the form of a Pin Terminal or Elastomers Silicon Strips. This technology is commonly used in standard Character LCD and Graphic LCD Displays.


COG LCD (Chip On Glass)

The driver IC is positioned on the integrated glass with a silicone sealant. The connections to the segments are directly connected on the chip as thin terminals. This technology is often used in thin Graphic LCD Displays. They are also preferred to reduce for the number of pins on the custom design LCD displays.


Glass LCD (No Driver)

This LCD type Display does not contain any drivers. Only includes separate or integrated backlight. Such LCD displays are preferred in applications where engineer use their own driver circuits. (with seperated Driver IC or MCU contains integrated "RAM Mapping LCD Driver" section) Glass LCD Indicators are preferred in low power consumption applications and systems where special segments and icons are needed economically. (such as clock, energymeters, personal medical equipments etc.)






LCD TERMINAL TYPE


LCD Displays may have different connection types with user circuits or driver boards. Pin Terminal, Zebra Connector (Elastomers Silicon Strips), Heat Seal and FPC (FLEX Cable) types are available.


Pin Terminal

It is used extensively for glass LCD displays and COG LCDs. The conductive metal pins are connected in such a way that they contact the polymer coated spots on the windscreen or rear window. The connection points of the pins are fixed with adhesive glue.



Elastomers Silicon Strips

This technology is extensively used in COB LCD displays. The layered conductive and insulating silicone laminated strip provides power transmission between the terminals and the PCB and the conductive polymer paths on the LCD glass. In this type of Displays, the glass LCD is mounted on the PCB with a metal or plastic frame.



Heat Seals

This terminal assembly is preferred by high-volume manufacturers since it requires thermal mounting expertise. Conductive carbon ribbon is connect to the surface of PCB and the conductive polymer terminals on LCD by heat treatment. Although the cost of production is cheaper, its use is reduced due to its high wastage.



FPC (FLEX Cable)

It is highly preferred for COG(Chip on Glass) LCDs. The film cable connected on Chip positioned on LCD Glass(COG). FPC cable connect to mainboard with ZIF connectors. It provides ease of use in production. However, due to high COG drive IC costs, they are not used widely. Nowadays this terminal type only used on graphic LCD Displays and TFT displays.






LCD POLARIZATION TYPE


Different polarization filters should be used in LCD Displays according to backlight options. Polarization filters are located on the top and bottom of the LCD Glass.


Reflective

This polarization type has no backlight option. LCD Display can be read by ambient light or external light source. Reflective film fully reflects the light in the environment. Reflective polarization type should be used in mobile applications and applications where energy consumption is important. (Ex: Energy meters, Calculators, Measuring instruments, etc.) This filter is only used for Positive LCD Indicators.



Transflective (Semi Transparent)

In this polarization type, the polarization filter on the back of the LCD Display is in Transflective(semi-transparent) mode. The LCD can be read with both the outdoor light source and the Backlight. Most of the positive LCD modules are Transflective. Because both natural light in the environment and dark areas should be read with their own backlighting. This filter is used only in Positive LCD Indicators as in Reflective Polarized LCDs.



Transmissive (w/o polarizer)

This LCD display does not have a Polarization filter. Negative LCDs are preferred this technology because negative LCD displays cannot be read with natural light outside. STN Blue / White, VATN (Vertical Alignment Twisted Nematic), FSTN Negative Black / White (Black / White), such as negative polarized LCD Displays Transmissive Polarization should be preferred.






LCD VIEW ANGLE



LCD Displays are classified according to duty cycle and flow direction of liquid crystal material. Based on these criteria, there are significant changes in the viewing angle of the LCD display. This difference in LCD viewing angle is expressed in relation to the Clock Position system, a military relative positioning system. Basically, the LCD is expressed at two different angles, 6:00 o'clock and 12:00 o'clock. According to this system, if the LCD position is below the user's head level (eg dishwasher, etc., white goods on the floor, floor scales, etc.), it should be preferred at 12:00 o'clock. This provides a clearer reading from the top of the indicator below the eye level. If the LCD is above eye level (elevators, on-door displays, refrigerators, etc.), the display direction should be selected at 6:00. This allows the LCD Display above the eye level to be read more clearly.






LCD VIEW MODE (Positive/Negative)


Monochrome LCD Displays have two different display modes. Polarization is achieved by changing the direction of the filters.


Positive LCD

In this type of display, the background is light coloured. The segments on the LCD screen obscure ambient light. Standard Green / Black or Black / White LCD displays are included in this group.

  • TN: Gray segments on Light Yellow/Green surface.
  • STN-Green: Dark Gray segments on Light Yellow/Green surface.
  • STN-Silver: Dark Gray/Black Segments on white surface.
  • FSTN: Black Segments on Light Gray/White Surface.


Negative LCD

In particular, the blue color indicators fall into this group. In negative mode, the background is Dark (Black or Dark Navy) and segments are light color (White or light white).

  • TN: Light Green or Gray Segments on dark gray surface (Low Contrast).
  • STN ("Blue-Negative"): White/Blue segments on dark Navy blue surface.
  • FSTN: White Segments on Dark Gray Surface (White Backlight).
  • VATN, EBN, IBN: Pure White segments on dark black surface (High Contrast/Wide View Angle/White Backlight).






LCD BACKLIGHT


The most common type ofbacklight used in LCD Displays is LED Backlight. The backlight color of the LCD Indicators is determined by the color of the LED used. They can be produced in Blue, Yellow, Green, White and RGB (Red / Green / Blue) colors.

Normally in driverless LCD Displays backlight is manufactured with transparent acrylic plates surrounded by a reflective tape and diffuser films placed on the surfaces of these plates. The Side Strip LED, which is selected according to the pin connections of the acrylic plate and LCD Display, is adhered to the appropriate edge of the acrylic plate, keeping the light in the transparent plate and illuminating the LCD surface. LCD backlight are manufactured in two different types.


Acrylic Backlight;

In this type of backlight, acrylic materials are cut manually. The edges of the cut acrylic material are covering with reflective tape. Diffuser films are adhered to the top and bottom surface of the acrylic material and Side LED fitting on acrylic. According to user request, glass LCD Display and Acrylic LED Backlight are glued together or packaged separately. This is preferred because it provides the advantage of molding at the initial production costs despite the high unit cost of backlight.



Molded Backlight

In this type of backlight, transparent plastic material is produced with a special injection mold. Due to the low raw material waste, the unit cost is more economical. However, initial production costs are slightly higher than manual cut acrylic backlight due to molding costs.






LCD DRIVING METHOD


The LCD displays are driven directly or in multiple segments with one common point and multiple segments as in LED displays. They can be directly driven by the microcontroller or controlled by external drive circuits.


The liquid crystal must be driven with alternating voltage. DC drive would destroy the liquid crystal. This AC drive requirement is the main factor for any power consumption. The electrical equivalence for the driving stage is a capacitor. Its electrodes are the backplane or common plane, controlled by signal COM and the segment driven by SEG. The frequency of the AC drive is low - in the range of 1000 Hz to 30 Hz. Therefore, users prefer external LCD drivers (eg HT1621B, PCF85176) or microcontrollers with integrated LCD driver (eg HT67Fxx, ATmega169A). Thanks to these drives, the microcontroller is not kept in continuous cycle and a faster solution is developed.

Source Code Example: HT1621_LCD_Drive






LCD TEMPERATURE RANGE


LCD Displays not suitable for operation at high operating temperatures such as LED indicators. However, thanks to technological advances, models capable of operating at high and low temperatures have been produced. The operating temperatures are listed below.


High temperature LCD Display

STN/FSTN Standard Temperature Range (-ST)

  • Operating Temperature: 0°C to +50°C
  • Storage Temperature: -10°C to +60°C

STN/FSTN Extended Temperature Range (-ET)

  • Operating Temperature: -20°C to +70°C
  • Storage Temperature: -30°C to +80°C

XFSTN Extended Temperature Range (-XET)

  • Operating Temperature: -30°C to +85°C
  • Storage Temperature: -40°C to +90°C






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