Four Criteria When Choosing a TFT for a Mobile Device
TFTs are still the most popular multi-color LCD technology because of the following features:
- Able to generate up to 64M unique colors
- Display video (depending on the chosen interface)
- Viewability from all angles
- Longer half-life than alternatives such as AMOLED
- High resolution
- Smartphone-like user interface
When considering a TFT for a mobile product such as a GPS or diagnostic device, there are a few key criteria to pay attention to.
1. POWER CONSUMPTION
Most times, mobile devices run on battery power. A display that draws a lot of power could quickly drain the battery of your product, resulting in many battery changes or a lot of time waiting for a recharge.
Note: Smaller screens typically require less power.
As with other LCD technologies, LED backlights are the leading factor in power consumption for TFT’s. If low power is the most important design consideration, one option to choose from is Transflective TFT’s. Transflective TFT’s offer sunlight readability even with the backlight powered off. Transflective TFT’s, however, also have a higher price tag and the display color tends to wash out when used in high brightness environments. For more information on this, see the following application note about transflective TFT’s.
Most TFTs are transmissive. These types of displays are less expensive and are only readable while the backlight is powered on. When using this style of display, it may be important to implement sleep and dimming functions into the software to save power. Dimming the display can greatly reduce power drain.
2. SUNLIGHT READABILITY
If dimming the display will reduce power consumption, it may seem obvious that increasing a displays brightness will also increase power consumption. Unfortunately, if you are using a transmissive TFT in a high brightness application, the only way to improve the readability is to make the display brighter.
To compete with the sun, TFT’s require a brightness of 700 Nits or higher. Some displays are rated as high as 2,000 Nits.
While increasing the displays brightness or using a transflective TFT will always be the first steps in making a display readable in high ambient light conditions, another useful step is to incorporate a coating to deflect some of the sun’s rays and reduce reflections on the display. Anti-glare and anti-reflective coatings can help to make the display more readable in high brightness situations since they can drown out some of the reflections that make the display unreadable.
TFTs have several different interface options available.
HDMI is the easiest interface for integrating an LCD to your mobile product. The cost is higher but may be worth it for small production runs or quick development.
MIPI-DSI is becoming an increasingly popular interface due to it’s low pin count and high speed.
SPI also uses few pins similar to MIPI, however, at a slower data rate which will not work for displaying video or driving high resolution displays.
MCU and RGB (Red/Green/Blue) have a fast data exchange capable of video, but more time consuming with more I/Os (in/Outs) to program.
One way for a user to interface with your mobile device is through the use of Touch Panels (TP) , which are standard equipment on most TFTs. The two most common are Resistive and Capacitive. Additionally, the user can interface the device though components like tactile switches.
RTP (Resistive Touch Panel)
Resistive touch panels detect touches by sensing pressure on the screen making them usable with gloves, or any other materials. These touch panels often lack multitouch and gesture capabilities.
CTP (Capacitive Touch Panel)
Capacitive touch panels are the type of touch screens used on most smart phones and feature multitouch and gesture capabilities. They sense inductive materials such as a finger or stylus, which allow them to work without needing to add force.
4. PROTECTIVE COVER
The top layer of TFTs is made of glass and is prone to scratching and cracking. Many LCD suppliers can add a protective cover to protect the glass and, in some cases, the entire LCD module.
Common thickness of the protective cover ranges from .4mm to 6mm.
Silk screening added to the cover lens can display a company name or model number. Multi-color silk screening is growing in popularity.
If a protective cover is not an option, there are a variety of coatings that the LCD supplier can add to the TFT module.
AF treatment repels skin oils on LCD glass. AF finishes do not prevent fingerprints from sticking to the glass, instead they bead or collect the skin’s oils so they can be easily wiped away.
Scratch Resistance (SR)
SR surfaces reduce scratches caused by car keys, tools, and any other sharp object. SR does not mean scratch proof; only scratch resistant.
Ultraviolet light causes damage to displays over extended periods of exposure, as can be seen in faded displays, color shifts, cracked glass and even burn-in where the segments are difficult to read.
AG treatment or AG coating or AG glass reduces the glare from high ambient light sources such as the sun and indoor lighting.
AR coatings on LCDs are manufactured to a set thickness producing a wave that is out of phase and cancels that wavelength.