LCD Design Basics
Interactive LCD Design Information
Now that you have downloaded 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 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 the Pacific Display Devices. 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, the Pacific Display Devices 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.
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.
A further discussion of display technology can be found in or technical section under
"LCD Operating Modes"
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.
Additional information can be found in or technical section under
"LCD Viewing Modes - Polarizers"
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.
A further discussion of viewing modes can be found in or technical section under
"LCD Viewing Angles"
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.
The drive methods are discussed in the
following applications sections:
"Static
Drive LCD Technology" and "Multiplex
Drive LCD Technology"
Sections.
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 Pacific Display Devices 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.
More detailed information regarding the display temperature
ranges can be found in the
"LCD Temperature Range Selection" Section.
LCD 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).
Additional information is discussed in the "LCD
Interconnection; Heatseals, Elastomers, & Pins" Section.
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).
Additional information is discussed in the
"LCD Backlighting" Section.
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
Pacific Display Devices 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.
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
Completion
You have reached the end of your
Custom LCD Glass Design Worksheet or the
Custom LCD Module Design Worksheet, and
hopefully have filled in most, if not all, of the information you need to specify a
design. Now that you are finished,
you can Submit Request to Pacific Display
Devices, or if necessary, get additional information from our
Technical Information and Application Notes.
