Electronicdesign 21077 Picocamera

Pico Projectors Liberate Images From Tiny Displays

June 8, 2012
Intersil's Tamaara Schmitz explains the technology behind Pico Projector displays for handheld devices.

Americans are as enthralled by huge video displays as they once were by large cars. On my last trip to Costco, I was greeted by an 80-in. television, a graphics Godzilla. Portable devices need big screens, too, while still fitting in our pocket or briefcase. But the largest practical display for an easily carried notebook is about 17 in. Several companies have broken this limit with tiny pico projectors that can be connected to almost any device, projecting and enlarging anything that appears on their displays.

Pico Projector Parts

A pico projector has four main parts: the battery, electronics, light source, and optics. The battery is usually lithium-ion (Li-ion), like the ones in most portable electronics. The electronics convert battery power into the regulated voltages needed by each projector system; process the video input into signals appropriate for projection; and drive the light source with these signals.

A typical pico projector relies on several components. For example, the ISL9230 supports simultaneous battery charging and projector operation. Next, the ISL9307 is a power-management integrated circuit (PMIC) that converts the charger’s output to the regulated voltages needed by the other components.

A pico projector’s components manage power, acquire and process the video signal, generate light, and focus the image on the screen.

The PMIC comprises four voltage regulators. Two provide linear, low-noise outputs. The other two are highly efficient switching regulators for electronics that don’t need low-noise power. One of those is an ISL9111, which raises VOUT to the voltage needed by the LEDs.

The TW8835 is a video decoder that generates the pixel clock and video signals from a variety of sources, such as HDMI, S-video, or PC VGA. The ISL97901 drives the LEDs. The optional ISL58333 opto-electronic IC (OEIC) sensor (not shown) provides automatic, real-time white balance to correct for variations in source quality and eliminate the need to calibrate each projector’s color temperature.

LEDs are the light source for most current pico projectors. Lasers are an emerging option. There are significant differences between LEDs and lasers. The typical figure of merit for efficiency for LED illumination is 10 lumens/watt, but customers generally want a brighter image than 10 lumens/watt provide.

Unfortunately, LED efficiency declines as output increases. For example, tripling LED output reduces efficiency by two-thirds. (That is, nine times as much power is needed to produce three times as much light.) A laser-based system has similar efficiency (10 lumens/watt), but it stays relatively constant as drive power increases.

Efficiency Counts

Small devices necessarily have small batteries, so the typical pico projector power budget is a Scroogish 2 W. (Current tablet and notebook displays pull 0.25 to 20 W.) Surprisingly, some pico projectors can project an image larger than 17 in. on that 2-W budget—some up to 50 in. In comparison, a 10-in. tablet needs 5 to 6 W. Turning off that display and using a pico projector would significantly extend battery life.

Efficiency is important not just to conserve battery power. A projector necessarily produces more heat than an MP3 player, and we don’t want components to reach temperatures above their Absolute Maximum specs. But in such a small device, there’s no room for a fan, heatsinks, or an open layout that would let the heat passively dissipate. A joule-pinching power budget helps.

Lasers On Stunning

Laser-based projection offers significant advantages that offset their higher cost. Lasers produce high brightness levels using less power than LEDs. Lasers also have virtually infinite depth of focus, so the lens might not need a focusing mechanism.

In addition to higher cost, lasers have other disadvantages. Green lasers emit somewhat less light than red and blue lasers. To compensate, “RGGB” systems include a second green laser. And unlike LEDs, lasers require a minimum drive current before they produce light. This reduces image contrast by making dark areas of the image lighter than they should be.

Yet another problem is speckle. Because laser light is coherent (single-wavelength and phase-coherent), any irregularity in the projection surface (such as the texture of a wall’s finish) creates visible interference patterns of light and dark spots. Speckle-reduction techniques have been developed to reduce this visual annoyance.

Room At The Table

There’s a place for both LED and laser projection. LED systems are preferable where cost is the driving factor, particularly for projectors in mobile phones, toys, or cameras. Laser systems are preferred if brightness is the main concern, as when the image has to be big enough to be viewed by a large number of people. As costs decline, laser projectors will become more popular in all applications.

Regardless of the technology, pico projectors will find their way into more products as their size and cost decline. The “tipping point” of widespread use is estimated to be at a parts cost of $50 (ideally, $25). Pico projectors are likely to become a “must-have” component of all but the smallest portable devices.

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