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LCD Design Basics --- Interactive LCD Design Information

Edit: Blaze Display Technology Co.,Ltd      Date: Oct 28, 2015

【R&D Department of Blaze Display】Now that you can contact our sales for getting either the Custom LCD Glass Design Worksheet or the Custom LCD Module Design Worksheet, you can use these design notes as a guide to complete those sheets.  When you´re finished, you can Submit Request, or get additional information from our Technical Information and Application Notes.  This page is rather long, but hang in there, it´s very informative.

 


Type of Display - MODULE or PANEL (GLASS)
The most fundamental decision to make is if the display will have the drive circuitry attached (module), or not (glass only). There are advantages and disadvantages to each approach.

When purchasing a Custom LCD Module, the basic electronic design work will be done by Blaze Display. This obviously saves design time and reduces the manpower needed to bring a product to market. You can benefit from the experience we have gained during our previous designs to shorten the design cycle and deliver an optimized product. The major decisions you need to make are the interface type, with standard serial or parallel interfaces being the most common, and the type of integrated backlighting desired. The temperature range, viewing angle, viewing mode, and contrast will need to be considered, but those decisions are common to both approaches. 

The main reasons to buy just the LCD glass are to reduce costs, and provide design flexibility. The total cost of the components necessary to build the drive circuitry is typically less than the cost of a pre-built module, and as long as you have space on your existing PC board, you won´t have to pay for an extra PC board on which to mount the display, and have access to cost effective assembly and test . By doing a little homework, your design will work just as well as a module, and will allow the flexibility most designers need to adapt their design to ever changing demands.

 


LCD Image
The LCD can contain character icons, segments, graphic dot arrays, or any combination.
Icons:  By making a custom LCD, you can place images on the glass that specifically complement your produce, these are called "Icons".  These silhouettes can take the shape of any image you may need and count as one pixel or dot on the LCD.  Examples would be a "°C", "+/-", or a cat silhouette (if you make a product that interacted with cats).
Segments:  Segments on an LCD display make up a larger character, such as a segment in a seven segment numeric character (displays 0-9), or a segment in a 14 segment alpha-numeric character (displays 0-9 & A-Z).
Dot Arrays:  These dot arrays can be made in almost any size and dot count.  Examples would be character displays that use a series of 5x7 dot arrays to create a string of alpha numeric characters, or the larger 320 x 240 graphic arrays that make images along with variable size alpha-numeric characters.

 


Technology  - TN  -  STN  -  FSTN  -  DSTN  - Negative Image
The type of technology used is determined by the specific performance requirements you set for the display that you are designing.  Since several variations will do a fine job, the ultimate consideration is cost.  So here is a quick breakdown of the technologies we offer:

TN:  Low production and NRE costs, poor viewing angle, average contrast.  Coloration:  Black on Gray.  Static preferred, but operates well up to a 32:1 Mux rate. LCD Glass favorite.
STN:  Medium production and NRE costs, average viewing angle, average contrast.  Coloration Black on Green, or Dark Blue on Gray. Works well at high Multiplex rates. LCD Module favorite, high end LCD glass choice.
FSTN: Medium high production and NRE costs. Good viewing angle, excellent contrast.  Coloration Black on White. Works well at high Multiplex rates. Higher end LCD Module favorite.
DSTN:  Very high production and NRE costs.  Excellent viewing angle and contrast.  Coloration Black on White. Works well at high Multiplex rates. High end / wide temperature LCD Module favorite.
Negative Image: This is an effect that reverses the image on the display and is only offered with transmissive displays.  The visual effect is to allow the backlight to define the pixels turned "on" (transparent), while the "off" pixels remain opaque.  This configuration works best in moderate to low light conditions.

 


Viewing Mode & Polarizers
The viewing mode is controlled by the rear polarizer, and how much it does or does not reflect light.  We offer three modes which are:

Reflective: This type of polarizer gives the display the brightest appearance in high to moderate ambient light conditions, with the highest contrast ratio possible. Unfortunately, it will be difficult to read at night or under changing lightning conditions (think Game Boy). 
Transflective: (Favorite solution) If your display must be readable under a wide range of lighting conditions, you will generally want a transflective display so that it will look very good in the bright sunlight, but will also be back-lightable at twilight and at night.  The tradeoff with a transflective display is that it will not look as good as a reflective display during the day, and it will not look as good as a transmissive display at night. It will however enable you to have an acceptable compromise between the two, and provides a very acceptable appearance.
Transmissive: This display needs to have a working backlight, unless it gets its light from being a "window" type of device where the area behind the LCD has a light source room ambient light. Where it is in a contained display, the backlight may have a problem overpowering high ambient light. This type of display looks great for an indoor application, but is not very good in applications where power consumption is a problem and it needs a strong backlight.  Remember, this choice of polarizer will not operate unless there is an active backlight.

The polarizers are also affected by temperature, and a quality grade must be specified when choosing a polarizer.  After you have determined your environmental temperature requirements, you then need to choose the polarizer grade.  Here are your typical choices
Commercial Grade Polarizers:  Displays which will be indoors, or mostly indoors, can use commercial grade polarizers. These polarizers will hold up very well when used in most instrumentation, office and home products, and other applications where the products will be protected from high temperature, sunlight, and humidity.
Industrial Grade Polarizers:  For harsh environments, a polarizer specifically designed for outdoor, extremely humid conditions should be used. We call these polarizers industrial grade.

 


Viewing Angle / Direction
The viewing angle of a LCD part is defined the angles above, below, left, and right of the point-of-view that is perpendicular to the center of the display.  These points are defined as:
      6:00 viewing angle has optimum contrast from below the perpendicular viewing plane (most popular).
     12:00 viewing angle has optimum contrast from above the perpendicular viewing plane.
      9:00 and 3:00 viewing angles have optimum contrast to the left and right side of the perpendicular viewing plane (uncommon).
Please keep in mind that viewing angle is less important for a direct drive display. As a general rule, the higher the multiplex rate, the more important the viewing angle becomes. Also, if your display is going to be viewed by a person wearing polarized sunglasses, you must specify this in the comment section to ensure that the display will not look blank to an observer wearing polarized sunglasses. However keep in mind that for quoting purposes, the price of the display will not change much (usually only a few cents) if you change your mind for the final viewing angle of the display.

 

 

Drive Method
The Drive Method specifies how each segment of the LCD display is connected to the LCD driving circuitry.  There are two methods offered, they are:

Static Drive:  LCD Glass or LCD Modules with a simple segment displays are the only parts that have an option of Static Drive.  The Static Drive configuration means that there is an individual control line to select each LCD segment and there is only a single common line that connects to them all.  This configuration produces the best display with the widest temperature range, but it requires more interconnections (>Cost in pins and controller) that a multiplexed display would require.
Multiplexed Drive:  The Multiplexed Drive configuration means that each control line selects several LCD segments and the final selection is made by selecting the correct backplane/common that also connects to several LCD segments.  This configuration uses less interconnections which is cost effective for smaller displays, and mandatory for high density dot array displays where there are more dots than LCD ledge space to provide interconnects.  This configuration degrades the temperature and image performance slightly.

 

 

Indoor / Outdoor
In general, the two things that destroy displays (other than hitting them with a rock) are high heat and humidity. The parts of a display which are most sensitive to heat and humidity are the polarizers. Under extreme conditions, the seals which hold the glass together can fail, however at Blaze Display we have developed materials which effectively eliminate this failure mode. When designing a display for an extremely rugged environment, it is important to make sure that seals will not be a problem. In general then, the environmental considerations comes down to the polarizers.

Displays which will be indoors, or mostly indoors, can use commercial grade polarizers, These polarizers will hold up very well when used in most instrumentation, office and home products, and other applications where the products will be protected from high temperature and humidity.

For harsh environments, a polarizer specifically designed for outdoor, extremely humid conditions should be used. We call these polarizers industrial grade. A wide variety of different outdoor materials is available, with varying contrast, coloration, and efficiency. Because of the many different options, we can supply samples of displays with different polarizers so that you can choose the one which best fits your application.

Another minor point is that you may have to specify that the display must be readable while wearing polarized sunglasses. If this is not specified, because of normal polarizer orientation, the display will appear blank to an observer wearing polarized sunglasses. This point becomes important when designing gas pumps, dashboards, cockpit instrumentation, or marine products.


Environmental Considerations - Temperature Range
The operating and storage temperature range of an LCD are important considerations, since operating outside of those ranges will result in a display that is not readable (outside operational range) or permanently damage the LCD (outside storage range).  The combination of the LCD fluid, polarizers, operational voltage, and multiplex rate determine the temperature range of the part.  So instead of going through the different combinations of these parameters, here are some general guidelines you can use when specifying the temperature range of your LCD, and Blaze Display can then assist you in the process of selecting the correct fluid, polarizers, and voltage for your application.  These values are typical operating temperature ranges:

LCD TN Glass / Static or Low Multiplex Rates:

    Std Temp Range: -10°C to +60°C, Wide Temp Range: -40°C to +80°C, Specialized Temp range: up to -55°C to +110°C
LCD TN Modules / Multiplexed:
    Std Temp Range: -0°C to +50°C, Wide Temp Range: -20°C to +70°C, Specialized Temp range: -40°C to +90°C
LCD STN & FSTN Modules / Multiplexed:
    Std Temp Range: 0°C to +50°C, Wide Temp Range: -20°C to +70°C, Specialized Temp range: -40°C to +80°C
LCD DSTN Modules / Multiplexed:
    Std Temp Range: 0°C to +50°C, Wide Temp Range: -20°C to +70°C, Specialized Temp range: -50°C to +88°C

Another note to remember is the LCD Glass and Polarizers are not the only limiting factors in the temperature range of the LCDs.  You also need to take into consideration the temperature limitations of the backlight and controller ICs that may be present along with the LCD.

 

The temperature range of a normal display is usually -40c to +85c when driven in a direct drive mode. Our standard everyday #3 fluid therefore has this temperature range. We also have many other standard fluids, as well as specialty LCD fluids which have operating temperature ranges as low as  -55c and as high as  +122c. Temperature ranges of our standard fluids can be found in our fluid table.

Under extreme conditions, it is possible for a fluid to reach its clearing point, i.e. the point where the display turns completely black because of the heat. While this does not hurt the display, it can be an extreme nuisance. By using our #6 fluid, the clearing point can be raised to +105c, which should be high enough for most   applications. Again, specialty fluids are available to extend the clearing point up to +122C.

Voltage threshold must also be considered when selecting a fluid. The threshold voltages can also be found in our fluid table. These values must be considered when working from low voltage drive circuits, i.e. use our #4 fluid when your power source is a 3V battery, and also at high multiplex rates, where the ratio of on to off voltage comes into play. An discussion of voltage thresholds at high mux rates can be found in our Application Notes section under "Multiplexing an LCD" 

 

LCD Panel (Glass) Interconnection Method (LCD Glass Only)
There are 3 ways to bring the conductive traces on the surface of the glass to your control circuitry, they are:
Solder Pins:  For reliability sake, pins are the most desirable connection method available.  These are metal pins crimped to the edge of the glass that allow the LCD to be soldered to PCB. In general, if you can keep the multiplex rate low, we recommend designing a part with pins, even if we have to put them on three sides. So, unless your design begs for a heat seal or an elastomer (commonly called a "Zebra-Strip"), the only good reason not to use pins is if we cannot fit them all onto the part, i.e., there are more segments to drive than there are pins on the part. 
Elastomer Strip:  The Elastomer Strip (commonly called a "Zebra-Strip") are small rubber strips that alternate conductive and insulating layers that allows the conductive pads on the surface of the glass (Contact Ledge) to mate with similar pads on the surface of the PCB.  They require a bezel frame or other form of glass restraint that will compress Elastomer Strip between the glass and the PCB. Please see the Links section of the website for information concerning vendors of these strips.
Heat Seal:   The heat seal flex cable is a flexible cable with conductive traces that is bonded to the LCD glass that can either be bonded to the PCB or plugged into a specialized connector on the PCB.  The advantage of a heat seal cable is the high density of conductors that can be used on this cable and it has reach.  However, the setup charges are much higher even though the per piece cost is reasonable (compared to pins and Elastomer Strip along with the interconnection length)

 

 

Cost & Materials
can be added to an LCD in a number of different ways. Examples of colored displays will be found on our pretty pictures page. I will make a few comments about the cost and appearance of each method:

a) Color Filters - We can add a color decal to the back of the display. This decal can have different colors in different areas so as to give different annunciators different colored backgrounds. This method has the lowest cost, but also gives the dimmest colors.

b) - Similar to the color filters, we can Screen Print different colors onto either the front or rear of the display. Each color can be applied in an intricate pattern, thus giving the same flexibility as the decals but with much brighter colors. This method usually costs more than a color decal, but is the preferred method for most applications.

c) Color Polarizers - We can add a colored polarizer to the front of the display. This method will cause the individual segments to be colored when they are activated, i.e. all segments will be red or green or whatever, on a silver background. This method will add only slightly to the cost of the display. I might add that these colored polarizers are not as efficient as normal polarizers, and the resulting segments do not have the same contrast as a normal black on silver display.

It is possible to put anti-reflective or anti-glare filters over the front of a display to improve viewability in harsh lighting conditions. These filters are usually bonded directly to the front polarizer of the display.

An anti-glare filter is one that has its front surface either physically or chemically roughened. This surface scatters specular reflections over a wide area. It produces diffuse, bright reflections reducing the overall contrast, but can improve the display readability.

A better, but more expensive solution, is to use a high efficiency anti-reflective material. These materials do not scatter the light like the anti-glare materials, but rather re-direct the light waves so that they continue traveling forward instead of reflecting back toward the observer. New anti-reflective materials can reduce the front surface reflections to less than 0.3% or less.


LCD Module Backlighting
When developing an LCD Module, a backlight can be added to light the LCD and there are several options available to backlight a LCD module.  The considerations in backlighting a display are the lighting intensity, life of the backlight, and the power it consumes.  He is a quick comparison of the backlights available.
LED:  Offered in both edge and array lit, this technology is preferred due to its variety of colors, intensity, long life (>100K Hrs), wide temperature range, and low voltage requirements.  The downsides are the power consumption of some configurations (large sizes), and the uniformity of the lighting for those same configurations.
EL Panel:  This backlight is very low power, but it requires a high voltage (120VAC @ 400Hz Typically) and it has a relatively short life (half intensity life <4K Hrs), and medium temperature range.
Cold Cathode Florescent Lamp:  This backlight has a lot of intensity for the power consumed, and has its applications for the larger LCD displays.  The downsides are a short life span (<20K Hrs), vibration will reduce the lifespan of the tube, limited temperature range, and the high voltage it needs to operate (>300VAC @ 30-80KHz).

 

 

LCD Glare Filter
It is possible to put an anti-reflective filter over the front of a display to improve viewability in harsh lighting conditions. This filter is bonded directly to the front polarizer of the display and its front surface either physically or chemically roughened. This surface re-direct the light waves so that they continue traveling forward instead of reflecting back toward the observer. New anti-reflective materials can reduce the front surface reflections to less than 0.3% or less.


Physical Size
In general, the larger the display, the higher the price.  The most expensive part of the Glass or Module LCD in most cases is the glass. At Blaze Display we usually use a master laminate (Sheet of glass) which is 14" x 16". We can produce a single display that size, or we can partition the array into hundreds of smaller displays . Our strategy is to maximize the number of individual displays which we can get onto this laminate.  We therefore recommend display sizes that give our customers maximum glass utilization. For example, display widths of 6.75": 4.50", 3.375", 2.70", 2.25" etc., utilize 100% of the 14" dimension.

 

In general, if you specify a display size which we feel can be optimized, we will offer you the optimized size, along with the size you have requested.


LCD Module Considerations:
Chip on Board (COB)
 and Surface Mount Technology (SMT) uses a PCB is mounted behind the LCD Glass.  The support PCB for the LCD Module may be larger than the LCD display and supports the driver ICs and backlight.  It can also be specified to contain any amount of customer circuitry in addition to the LCD Module support circuitry. 
Chip On Glass (COG) uses the actual LCD glass acts as the circuit board and the controller ICs can be mounted off to any side of the display as required by the mechanical packaging.  The COG LCD then is electrically connected to the user circuitry through Edge Mount Solder Pins, Heat Seal Cable, or Elastomer Strip.

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