Boosting LCD brightness for mobile video without increasing power

The latest application challenges to design engineers are the new generations of phones and portable devices capable of playing videos and TV programs. These applications create a conflicting challenge because they not only use substantially more power, but also need considerably more luminance to perform optimally, a function that in turn requires even more power. This design requirement further encumbers engineers to effectively balance performance and power requirements while still monitoring overall device costs.

Mobile video display demands
The display industry has long recognized that current display technology could not meet the power requirements of next-generation devices without hindering new technology adoption, and that there should be greater focus on finding ways to optimize performance and manage excess power consumption through display modifications. Often it is achieved by simply lowering the backlight power, sacrificing brightness but still performing well enough for most current applications. With the advent of mobile TV, however, the emerging need for significantly greater luminance adds another element to the power/performance challenge.

Video-enabled devices will not satisfy the expectations of the consumer to clearly see all the fine details and images on-screen unless the luminance level is sufficiently high. Many small displays designed into cell phone and portable devices currently on the market exhibit an average of 200 nits of luminance; however, to see the fine details of mobile television content and view video satisfactorily, it is critical that brightness levels increase by two- to three-fold to 400 nits or higher -- closer to the luminance of a TV set in a home.

Figure 1: 250 nit display


Figure 2: 500 nit display

While the role of luminance has long been understood, it is not commonly applied to handheld products largely due to insufficient battery life to support such high-brightness displays.

Consider, for example, the kayaker video being shown on a typical 250 nit display (Figure 1) as compared with a 500 or 600 nit display (Figure 2) appropriate for viewing mobile TV. Video applications require luminance to be about 400 nits or higher, causing power requirements for the display backlight to increase by at least 2x.

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