While announcing the Apple Watch Series 5, Apple did a very Apple thing: they touted a new feature, hinted at some of the technology behind it, and left some of us with a lot of questions. The biggest one for us under-the-hood types: what is an “LTPO” display, and how does it allow for an always-on face?
We won’t know everything about how Apple’s newest Watch works until we tear it down soon after its Sept. 20 release. In the meantime though, let’s dig into baking crystals, making backplanes, and tweaking refresh rates.
The LTPO Display
The display industry says that LTPO stands for Low-Temperature Polycrystalline Oxide, but Apple says its watch is powered by a “low-temperature polysilicone and oxide” display (the extra “e” on silicon is in Apple’s original press release). It might just seem like a typo, but Apple is actually using its own blend of materials here. Their display tech is based on three patents it received between 2015 and 2018, as reported by the watchful blog Patently Apple.
Here is a medium-distance overhead view of what Apple has. The display on an Apple Watch, and many newer phone screens, uses AMOLED technology. An AMOLED display is made up of a whole bunch of layers. The most crucial layers are the organic light-emitting diodes (OLED)—a layer of organic material that emits light, through a pixel etched into the glass above it, when stimulated with electrical current—and a dense array of thin film transistors (TFTs) below them, which are arranged into a marvelously complex array of circuits called backplanes. Put more simply: there’s a layer of pixels suspended above an extremely dense set of switches, and those switches control whether each pixel is on or off, how bright they are, and what shade of color comes through.
Apple’s latest advancements are in the materials and control of some of the TFT circuits. They’re using somewhat standard low-temperature polycrystalline silicon (LTPS) materials for the “switching” circuits that, essentially, turn pixels on and off for individual frames every fraction of a second. But Apple is implementing newer Indium Gallium Zinc Oxide (IGZO) for the “driving” circuits that keep the pixels powered with a certain voltage during that frame, determining how bright each pixel should be, and what combination of red, green, and blue to display.
The analysts at IHT Markit estimated that this dual TFT makeup could save 5 to 15 percent in power draw for a screen. There are challenges to manufacturing and implementing IGZO transistor films, including the need for larger transistors that would imply a lower display density, but Apple seems to be comfortable with their implementation, at least comfortable enough to use it in two different Apple Watch generations.
The Other Parts of the Apple Watch Display
Two different Apple Watch generations, we say? Indeed, Apple implemented LTPO tech into the display of the Series 4, but the always-on display is an exclusive feature to the Series 5.
During the Apple Watch portion of its event, Apple implied that their LTPO tech is the reason the watch can drop its refresh rate and save battery life when it’s not actively being used. But it’s the other components briefly mentioned during the segment, and listed on the news release, that likely make the real difference: “an ultra-low-power display driver, efficient power management integrated circuit, and a new ambient light sensor.” The Series 4 had an ambient light sensor, and notably had a smaller battery than the Series 3, as seen in our teardown—maybe the LTPO savings alone helped Apple get that slimmer profile.
Before we can see them up-close, we’ll have to assume the new power management IC and display driver handle the dropping of refresh rates when the watch is inactive. Which is a really important job.
Refresh Rates Are a Real Power Savings
Higher refresh rates are something gamers and high-definition fiends want on their TVs and monitors, but not something you need on the tiny little computer that you’re only actively looking at during select moments of your day.
The refresh rate, stated in Hertz (Hz), or cycles per second, is how often the display—be it on a computer display, television, or tiny watch face—checks with its input to see if there’s something new it should be showing, and then subsequently… refreshes, to show that thing. This is not the same thing as the “frame rate,” or “frames per second” (fps), which is how many frames per second the source of the display can serve up new images. With TVs and computer monitors, it’s not that big a deal if a display is refreshing faster than it needs to, and it’s usually better for the display to always be at the ready. But when you’re not looking at an Apple Watch, there’s no need for the display to be buffering for smooth 60Hz motion, just to show a second hand that ticks once per second.
There’s no need, and it’s a big energy drain. It’s far from a perfect example, but PC Perspective showed how one gaming monitor, with a 165Hz refresh rate, could more than double the power draw from a gaming computer. Apple says it can lower the Apple Watch refresh rate from 60Hz down to 1Hz when the watch is inactive. It’s not the first time Apple has introduced variable refresh rates to save on battery life, either. The iPad Pro’s ProMotion display can vary its rate from 24 up to 120Hz, accommodating slower standard video or fast-reaction drawing with the Apple Pencil, optimizing battery life where possible.
Add this scaling refresh rate to the overhead savings of its LTPO transistors, along with the Apple Watch’s black-framed watchfaces that are geared toward OLED power-saving, and that might just explain how Apple found the room for an always-on display hooked to a tiny battery.