The following sections provide basic
information on LCD glass and LCD modules, which is useful when you start
designing your own LCD or submit requests to LCD manufacturers.
Types of LCD Products - Glass or Modules
The most fundamental decision to make is whether 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 the manufacturer. 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.
A glass only design puts the design burden on the end user. You will need to
learn a great deal about LCD's in order to complete your design. Luckily,
our website provides Application Information and Web
Links to assist you in the search for the information you may need.
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.
Types of
LCD Images
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.
Types of Liquid and
Technology - TN - STN - FSTN - DSTN - CSTN
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
Multiplex 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.
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 and Polarizers
The viewing mode is controlled by the
rear polarizer, and how much it does or does not reflect light. We offer
three modes.
- 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 Direction
The viewing direction of a LCD part is defined as the angles above, below,
left, and right of the point-of-view that is perpendicular to the center of
the display.
- 6:00 viewing direction has optimum contrast from below the perpendicular viewing
plane (most popular).
- 12:00 viewing direction has optimum contrast from above the perpendicular
viewing plane.
- 9:00 viewing direction has optimum contrast to the left
side of the perpendicular viewing plane (uncommon).
- 3:00 viewing direction has optimum contrast to right
side of the perpendicular viewing plane (uncommon).
Please keep in mind that viewing direction is less important for a direct drive
display. As a general rule, the higher the multiplex rate, the more
important the viewing direction 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 direction 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.
- 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.
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 manufacturer 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: Standard
Temp Range: -10°C to +60°C, Wide Temp Range: -40°C to +80°C, Specialized
Temp range: down to -55°C, or up to +110°C
- LCD TN Modules / Multiplexed:
Standard 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:
Standard Temp Range: 0°C to +50°C, Wide Temp Range: -20°C to +70°C, Specialized
Temp range: -40°C to +80°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.
Connection Method for
LCD Glasses
There are three ways to bring the conductive traces on the surface of the
glass to your control circuitry.
- 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 that 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.
- 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).
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. Here 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. Manufacturers 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.
LCD Module Considerations
Chip on Board (COB) and Surface Mount Technology (SMT) uses a PCB that 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
that acts as the circuit board and
the controller ICs can be mounted on 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
|
