The Anatomy of a COB Character LCD

Character LCDs maintain their popularity and will continue to be the display of choice for new designs thanks to their simplicity. Their simplicity produces an LCD module that is inexpensive, easy to program and almost always in stock. A COB LCD (chip-on-board liquid crystal display) involves the LCD controller or driver being directly connected to the printed circuit board.

The character display consists of a few basic components that have a very low probability of being discontinued. This is a big plus for OEM’s (original equipment manufacturer) that produce a product for several years.

So, what components are used to build a character LCD?

PCB

PCB aka the Printed Circuit Board, is a FR4 material that contains traces, mounting holes and pads. The pads contain resistors, capacitors, jumpers and other electronic components. But for those who don’t want all this technical jargon, just think of it as the green thing that comes on the back of your LCD.

The mounting holes allow the LCD module to be attached to the customer’s product.

Note: The mounting holes are normally reinforced with copper that is connected to the ground plane. If metal screws are used to connect the PCB/LCD to your product, it may short out. If this happens, use a nylon washer to break the connection.

The PCB’s job is to hold the glass and bezel together to withstand strong vibration.

Focus LCDs works with several PCB suppliers. If one halts production, we have others that can build the exact PCB. This means you will have zero downtime if a PCB is discontinued.

ITO Glass

ITO aka Indium tin oxide glass is a coated glass that is one of several types of transparent conductive oxide coated materials. Don’t try to remember all these details, or even try to pronounce some of the words. Just call it ITO glass.

The glass has a transmittance (which is a measure of how much light passes through) between 85% and 90%.

The ITO glass is used in monochrome displays such as: character, segment and graphic displays. Displays contain two pieces of glass with a nematic liquid between them.

Fluid type

Between the two layers of ITO glass is a nematic fluid that rotates when an electrical field is applied.

Note: Nematic is just a fancy word for liquid crystals.

There are several types of fluid, but the most popular are: TN, STN and FSTN.

Let’s start with the cheapest option.

TN LCD (Twisted Nematic)

Monochrome displays utilizing TN fluid have the lowest contrast, but a faster switching time. This translates to excellent performance in colder temperatures. They behave even better in cold ambient temperatures next to a hot cup of dark roast coffee.

TN fluid displays black letters on a grey background.

STN LCD (Super Twisted Nematic)

Displays containing the STN fluid, rotate over a greater arc, which produces a sharper contrast than TN. Although, not as sharp as FSTN.

The added expense is 5% more than a TN. This may be important in higher volume production runs where fractions of a penny count.

Since STN is the most popular option, most of the character displays that Focus LCDs maintains in sample stock are STN. If you are in a rush and need to save money. Order a STN sample and then once designed in, order production quantities of the TN version.

Basic STN color options are gray, blue and yellow/green (most common). Other colors are available using filters.

FSTN LCD (Film compensated Super Twisted Nematic)

Last, but not least, is FSTN fluid. Used widely in medical, aviation, test & measurement and other industries where a sharp contrast is key.

FSTN LCD monochrome displays contain a retardation film applied to a STN display to produce a black and white appearance. This produces a higher contrast and wider viewing angle then STN or TN.

The added expense is around 8% more than STN.

Basic color options are black/white and blue/white, but with modifications, other color options are available.

LED Backlight

Monochrome displays require a backlight to be readable in the dark, but thanks to their polarizer (see below) they are readable in direct sunlight.

LED backlights are standard features for all character and monochrome graphic displays. Segment displays require a one-time tooling (NRE) to build and attach the LED backlight.

Years ago, EL was another option, but this technology is less common due to its short life-time and the EM noise generated.

LEDs are available in red, yellow, white, blue, yellow/green and many others. The RGB (red, green, blue) option allows engineers to generate a unique color.

The backlight can draw from 15mA up to 90mA+ depending on the number of LEDs and their brightness. The backlight will require ten times (10x) the power required for just the LCD.

The backlight can add from 3mm to 7mm to the overall thickness of the module.

Polarizer

Monochrome displays must have some light to be readable. Unlike TFTs or OLEDs that generate their own light, monochrome displays must have ambient light (sun, indoor lamps or the power button on your coffee maker) to be seen.

The polarizers job is to regulate the light source. There is no price difference between the three types of polarizer.

Reflective:

The reflective polarizer is a mirror that is applied to the bottom layer of glass. It reflects 100% of the ambient light entering through the top layer of glass back toward the user.

This is the best option for use in direct sunlight because the sun light is reflected, making the segments, characters, and icons easier to read.

It is not possible to add a LED backlight because it will sit behind the polarizer, which will block all the light. In this case you will need a transflective polarizer or to convert your LED from a backlight to a side-lit (aka edge-lit) display.

Transflective Polarizer (Most Popular)

Transflective polarizer displays are the most popular and combines the functions of both reflective and transmissive polarizers. In other words, it allows the display to be readable in direct sunlight and allows the light from an LED backlight to pass through.

It produces as sharp of contrast as a reflective polarizer, but the backlight does not appear as bright as the transmissive option. Still, for most applications, it is the best fit.

If you need the brightest backlight possible and the display doesn’t need to be sunlight readable, then choose Transmissive.

Transmissive Polarizer:

The transmissive polarizers provide the brightest backlight possible but are difficult to read when the backlight is off. These displays can be seen in credit card readers, scales and other products that must stand out to catch the user’s attention.

This is a poor option for battery powered products because the backlight must be on to read the display, and backlights use a lot of power.

Transmissive is a poor choice for direct sun light since the backlight is not bright enough to overpower the sun.

Controller/driver

The controller/driver, aka C/D is the brains of the LCD. It converts the customer’s code to letters, numbers and icons. There are several manufacturer’s and, for the most part, the C/Ds are interchangeable in case one gets discontinued.

In a small percentage of cases, one manufacture’s C/D is not interchangeable with another C/D. When this happens, the customer may need to modify the firmware code.

Staring a New Design?

Not sure which LCD display polarizer to select? Need help with your new LCD design?

Feel free to contact our US-based technical support staff by calling 480-503-4295 or contact us by using the Contact Form.