Transflective Graphic LCDs
Transflective displays can be used with or without a backlight by combining transmissive and reflective modes to illuminate the display. Transflective Graphic LCDs are a great solution for conserving power by removing the demand of the backlight. This application note will review the different aspects of using Focus LCDs transflective graphic LCDs in different environments and at different operating conditions.
1. Introduction
Transflective displays can be used with or without a backlight by combining transmissive and reflective modes to illuminate the display. Transflective Graphic LCDs are a great solution for conserving power by removing the demand of the backlight. These transflective LCDs maintain great pixel contrast and display a clear image with and without the backlight. Monochrome graphic LCDs are especially favorable for transflective use because of the pronounced contrast of the black pixels from the background color.
This application note will review the different aspects of using Focus LCDs transflective graphic LCDs in different environments and at different operating conditions. The display used in this application is a 128×64 dot graphic LCD G126FLGFGSW64T33XAR with a transflective polarizer. Below is a summary of the display and its properties.
- 128×64 dots
- 80.00×57.0mm
- Transflective Polarizer
- ST7565P Controller IC
- White LED backlight
- FTSN positive
- Serial interface
- 6 o’clock viewing
Another bonus of transflective graphic LCDs is the use of internal voltage regulators and boosters. Graphic LCDs, transflective or otherwise, typically have internal driver ICs that can adjust voltage and currents provided to display the pixels. This is beneficial because it allows you to increase or decrease the contrast of the pixels to the background without having to add external components. This feature can be used to adjust the contrast depending on environment and which transflective property is being utilized. The voltage provided to the pixels can be increased using the voltage boosting circuit or decreased using the voltage divider circuit.
2. Pin Assignments
Pin definitions and connection points are described in the table below. We will use the 4-wire serial interface for this example to save data pins on the microcontroller. There are established 4-wire SPI pins labelled on the microcontroller. Any alternative pins used must be declared in code when programming the device. A more in-depth description of each of the pins can be found on the datasheet. All unused pins are connected to ground.
The backlight LED’s are connected to an external battery and are separate from the 3.3V logic voltage. The voltage that is applied to the backlight LEDs can be adjusted through variable resistors, such as a potentiometer or through programming if connected to an additional PWM pin on the microcontroller.
Below is a diagram representing the pin connections to the microcontroller in accordance with the table above. The pins labeled ‘D’ represent the digital pin terminals on the microcontroller. All unused pins are connected to ground. The diagram represents the 4-wire serial communication interface between the microcontroller and the displays internal IC controller. The backlight LED circuit is connected to an external battery and not powered by the microcontroller. An additional variable resistance can be added between the backlight terminals to adjust the backlight current to dim or brighten the LEDs.
3. Example Lighting Environments
The forward voltage of the backlight LEDs has a typical value of 3.3V and the current can be adjusted to increase or decrease the backlight of the display.
This example will demonstrate the power consumption of the 6 backlight LEDs at different lighting levels. In an outdoor application the backlight could be turned off and in indoor applications the backlight can be turned on. Below are various lighting conditions and their corresponding backlight power.
| No. | Ambient Condition | Backlight Power | Transmissive or Reflective | Example |
|---|---|---|---|---|
| 1 | Bright Indoor Lighting | 100% | Transmissive |  |
| 2 | 0% | Reflective |  | |
| 3 | Dark Environment | 100% | Transmissive |  |
| 4 | Full Sunlight | 100% | Transmissive & Reflective |  |
| 5 | 0% | Reflective |  |
4. Energy Efficiency
Backlights are often the biggest power drain for a display. By turning off the backlight in outdoor environments energy can be conserved. This example is using 6 backlight LEDs which require 3.3V and 50mA of current to illuminate the display in the transmissive mode. The power cost from the backlight becomes substantial in larger displays that have brighter LEDs to compensate for bright environments.
Below is a table comparing the power consumption of different sizes of transflective graphic LCDs. For battery powered devices the battery is typically measured in milli-Watt hours (mWh) and milli-Amp hours (mAh). For reference, one AA battery at 1.5V provides 200-400 milli-Watt hours.
| Display | Size (dots) | # of LEDs | Voltage (V) | Current (mAh) | Watts (mWh) | Power (kJ) |
|---|---|---|---|---|---|---|
| Transflective TFT’s | ||||||
| G126FLGFGSW64T33XAR | 128×64 | 6 | 3.3 | 50 | 165 | 0.594 |
| G126GLGFYSY6WT | 128×64 | 8 | 4.7 | 80 | 376 | 1.35 |
| G160BLGFGSW6WTC3XAM | 160×100 | 3 | 3.3 | 60 | 198 | 0.712 |
5. Summary
In summary transflective graphic displays are a great option for battery powered devices. Transflective graphic displays can display high contrast images with or without the backlight. In bright environments the display will use the ambient lighting to create this contrast. This gives the option to conserve backlight power when the bright ambient lighting is available. In dark environments the backlight can be turned on to display the image. Graphic displays have the additional benefit of having built in DC-DC voltage circuits to control the contrast of the display. This gives the option of adjusting the backlight and pixel contrast through the program without having to add external voltage adjusting circuits. This makes transflective graphic LCDs a great solution for a variety of lighting environments and gives extended control over power consumption of the display.
DISCLAIMER
Buyers and others who are developing systems that incorporate FocusLCDs products (collectively, “Designers”) understand and agree that Designers remain responsible for using their independent analysis, evaluation and judgment in designing their applications and that Designers have full and exclusive responsibility to assure the safety of Designers’ applications and compliance of their applications (and of all FocusLCDs products used in or for Designers’ applications) with all applicable regulations, laws and other applicable requirements.
Designer represents that, with respect to their applications, Designer has all the necessary expertise to create and implement safeguards that:
(1) anticipate dangerous consequences of failures
(2) monitor failures and their consequences, and
(3) lessen the likelihood of failures that might cause harm and take appropriate actions.
Designer agrees that prior to using or distributing any applications that include FocusLCDs products, Designer will thoroughly test such applications and the functionality of such FocusLCDs products as used in such applications.