Understanding the LCD Development process
When discussing LCD development it’s important to understand there are several types of LCD and display technologies available; each one offering several options for modifications and customizations to meet the design requirements of the customer.
Current LCD display technology allows you to design your LCD to produce a prototype display able to meet engineering requirements. When developing your LCD display, the first question is what job will your LCD need to perform?
Will you need:
- Color or Monochrome?
- Graphics, Characters or Segments?
- A display to operate in extreme environments?
- Low Power?
Let’s begin the display design process by introducing the six types of LCD and display technologies.
Six major LCD Display technologies
To simplify, all displays can be broken down into two categories: Monochrome (Single Color) or Multi-Color.
We will start with the lowest cost and most basic type: monochrome segment displays.
Segment LCD displays, also called static displays or glass only displays are constructed of two pieces of ITO glass with a twisted nematic fluid sandwiched in between. There are three types of Nematic fluid:
TN (twisted nematic)
STN (Super twisted nematic)
FSTN (Film Super twisted nematic)
Power consumption of a segment display
Segment displays have the lowest power consumption of any of the six technologies, which makes them a good option for battery or solar powered products. Most static displays can be driven at 3.3V or 5V since microprocessors can operate at both voltages.
3.3V is popular since two double ‘AA’ batteries can produce between 3.0V and 3.3V.
They require the lowest power to drive, an estimated 2uA per Centimeter squared; in fact, glass only displays require an estimated 10% of the power that is required for a LED backlight. In other words, a segment display without a backlight will draw around 1mA; the same display with a LED backlight will demand 10mA up to 25mA.
Segment LCDs offer lower cost
Lowest tooling cost
The tooling cost for a custom static display is the lowest of all the technologies and allows the customer to receive a LCD that is manufactured to the exact dimensions requested including custom icons and segments.
Lowest unit cost
Segment displays have the lowest unit cost of the six LCD technologies; and cost is influenced based on the size and quantity ordered.
The MOQ (Minimum Order Quantity) for a segment display is 1K units.
Interface options for a segment LCD
The most popular interface options for a segment display is: 4:1, then direct drive, then SPI (Serial Peripheral Interface), and then I2C (I-two-C).
Segment display with 4:1
A segmented display with a 4:1 multiplex means that four segments are attached to each lead or pin. In other words, a segment display with 100 segments would only require 29 pins to drive.
The number of pins is calculated by taking the total number of segments (100) and dividing by four (100/4) to arrive at 25. Then you add on four more pins for each of the four commons or backplanes for a total of 29 pins.
If the number arrived at is an odd number, round up to an even number with the extra pin being a NC (no connect). The pins are spaced evenly on both sides of the glass, but can also be placed alone just one side.
A segment display driven as a 4:1 multiplex means that each segment is on 25% of the time, this may seem like a small amount of ‘on’ time or low duty cycle, but the human eye cannot see the flicker or refresh rate.
Segment display with direct drive
A segment display with a direct drive (also called a static display) contains one pin for every segment.
This is a good option if the LCD contains less than 25 segments. Once you exceed 25 segments, than the number of pins required can make the display difficult and expensive to integrate in final assembly.
Direct drive means that the segment is on 100% of the time or off 100% of the time. The segments will be a little darker than a 4:1 multiplex and the display will respond quicker in colder temperatures, but the difference in cost between a 4:1 and direct drive is very minor.
There is no difference in tooling cost between a 4:1 and a direct drive display.
Segment displays with a SPI interface
This is becoming more and more popular for segment displays. SPI allows the display to be easily integrated into a microprocessor and reduces the number of connections necessary.
The tooling cost is higher than a 4:1 or direct drive since a controller/driver chip needs to be added to the glass.
It does require a FPC/FFC cable which is custom built to the length required by the customer.
Segment displays with an I2C interface
This is a rare occurrence for segment displays; the majority of design engineers we talk too prefer SPI over I2C.
Like a SPI, the I2C interface will require a higher NRE or tooling cost to add the required controller.
Segment displays with a backlight
It is poplar to add a LED backlight to a segment display if the display needs to be readable in dark environments. Since the power drain of the LED backlight is much greater than the segment display alone, it is suggested that the backlight is turned on only for short periods of time.
There is a one-time NRE cost to design, build and attach the backlight to the segment display. There are two common methods for attaching the backlight to the LCD glass: Adhesive and frame.
Using an adhesive to attach the LED backlight to the segment display is the lowest cost and does not require a tooling fee. The downside to the adhesive option is that it can slowly deteriorate over time and under high humidity conditions.
If the product is disposable, than adhesive is a good option.
If you require a more stable method to hold the backlight to the LCD, than we recommend a frame or PCB. The frame is custom built to match the size of the LCD glass and the backlight and requires a one-time NRE.… please check back!