Ultra-Low Power Displays

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Ultra-low-power displays consume very little energy, and the two primary technologies used for these types of displays are bistable and low refresh rate displays. They are used when there is a need for a battery-powered device, want maximum life between charges, and the content being displayed does not change very frequently.

Table 1.0 is a comparison between bistable and low-refresh rate displays:

     
  Bi-stable Low-Fresh Rate
Steady-state power Advantage  
Dynamic power   Advantage
Speed   Advantage
Temperature range   Advantage
Contrast ratio Advantage  
Sunlight readability Advantage  
Color   Advantage
Drive   Advantage
Cost   Advantage

The common uses for ultra-low-power displays are e-readers and electronic price tags. Some of the other applications we have seen are secondary displays for handheld devices and battery-powered products like locks, remote-mounted homes, and industrial products. 

Probably the most well know bi-stable low power display is e-paper technology historically used on e-readers. This technology is available in both monochrome and color.

In a monochrome e-paper display, millions of tiny liquid-filled capsules contain black and white charged ink particles. These capsules are sandwiched between a grid of electrodes. Applying a charge to the electrodes causes the ink particles to migrate to the top of the capsule, and depending on the polarity of the charge, it changes the color of the surface of the display. See Figure 1.

 

 

E-paper is a reflective technology and, with good ambient light, has an excellent contrast ratio. One of the characteristics of e-paper is that the background is white, whereas many reflective display technologies like LCD have a gray or greenish background. One disadvantage is that E-paper requires front lighting if used in low light conditions.

Displaying static images on an e-paper display uses very little energy (uW). However, it can require more power (mW) to update the screen than other technologies like LCD in the same size and resolution.

Low power LCDs get their power savings in 2 ways; eliminating the backlight and moving to a very low refresh rate.

The backlight uses most of the power in a standard TFT display. For example, on a 7” TFT panel, the backlight uses almost 80% of the energy consumed for an average brightness display. The digital circuitry utilizes the remaining power to sustain the picture.

The first step of building a low-power TFT is to move to a reflective or transflective display and eliminate the power consumption of the backlight when the display can use ambient light. 

Choosing a Transflective display is a good trade-off since it comes with a backlight that, when turned, the display becomes reflective. However, there is some trade-off in that the reflectance of a transflective display is lower than a pure reflective display. We use an advanced LCD driver chip to reduce the power further to drive the display at different refresh rates.

We use an advanced LCD driver chip to reduce the power further that allows the display to be driven at different refresh rates. The drivers have two modes; a standard TFT mode that enables the display to operate like a standard TFT being able to do video rate, 60Hz, updates, and a low-power mode where the display refreshes at a rate of 1Hz. This mode is excellent for holding static images and using very little energy. Figure 2.0 depicts the driving methodology. Using these drivers, you can reduce the power of the digital portion of the display by 60%.  

 

Table 2.0 shows a comparison study that we did for a thermostat application to compare different low-power technologies. In this study, the display is active for 15 minutes, and then it shows static images for the remainder of the day. 

Technology Size Resolution Mode

Power Consumption

(15 min/day)

Estimated Lifetime with 2 AA Batteries
Low Power TFT 2.13" 122x250 Reflective Type 72.6uW 11.97 years
E-Paper 2.13" 250x122 Reflective Type

Standby: 17uW

Active: 26.4mW

1.76 years

Active mode: 15min/day

Conclusion: Depending on your application, either low-power TFT or e-paper may be suitable. If power is critical for your application and requires maintaining an image on display for long periods, consider these great technologies.

US Micro Products has designed displays with both technologies for special low-power applications and can do the same for your product. So let us help you with your display requirements; we have the expertise that spans multiple markets and technologies.

If you have a project that is considering taking advantage of any display technology, US Micro Products can provide a solution designed for your application. Send us an email at sales@usmicroproducts.com.

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