OLED Displays

Start with how often the screen changes.

ePaper holds an image with no power at all and only draws current when it updates. If the display shows a reading that changes every few seconds or less often, ePaper beats everything else on battery life by a wide margin, and it is the most readable of the three in sunlight. It cannot do video, updates take a noticeable fraction of a second, and color options are limited and slower still.

OLED / PMOLED is self-emissive, so its power depends on how many pixels are lit and how brightly. For sparse content on a black background it is very efficient. For a mostly-white screen it is not. It gives excellent contrast and a wide viewing angle and needs no backlight; its weaknesses are limited lifetime at high brightness and image retention with static content.

TFT LCD is the answer when you need full color, a real interface or moving content. Power is dominated by the backlight, which is on whenever the display is on regardless of what is shown. A transflective TFT recovers much of the outdoor readability and some of the power.

A useful test: if the device spends most of its time showing the same thing, use ePaper. If it shows a little bright information on a dark screen, use OLED. If it shows a colour interface, use a TFT.

Passive matrix (PMOLED) drives one row at a time, with the drive current shared across the row. That keeps the panel simple and inexpensive and limits practical size, resolution and brightness. It suits small displays with modest information content, which is the bulk of embedded use.

Active matrix (AMOLED) gives every pixel its own thin-film transistor and storage capacitor, so each pixel holds its state continuously. That enables large, high-resolution, brighter, faster panels at higher cost and complexity, and it is what phones use.

For an embedded product with a small display, PMOLED is usually the right technology and AMOLED is usually over-specified. Crystalfontz supplies passive-matrix OLED, AMOLED, character and graphic, and transparent OLED. Standard operating range is 0 °C to +70 °C, with some parts rated to -40 °C.