Optoelectronic components help manufacturers comply with new standard for led tvs

4 mins read

Modern led tvs consume around half as much energy as did an lcd tv with a standard cold cathode fluorescent lamp backlight two years ago. Yet the squeeze on power consumption continues, with the next revision of the US Energy Star regulations requiring adjustable display brightness optimised for varying ambient light conditions.

In a room with subdued lighting, the viewer will be more comfortable with a relatively dim display. In a sunlit room, the screen must be backlit at high power so the viewer can see the picture clearly. The ambient light sensors (ALS) generally used in today's tvs cannot provide a finely graduated backlight control, so fail to take full advantage of the opportunity to reduce power inputs to led backlights. Intelligent backlight dimming – local backlight dimming synchronised with picture content, as well as finely graduated dimming in response to ambient light conditions – looks to be a promising method for reducing led tv power consumption. Changes to energy standards The current Energy Star standard for tvs, version 5.3, sets tight power limits, but version 6.0 – planned for 2013 – will further cut the maximum allowable power consumption. Under v6.0, the maximum power consumption for a 46in tv will fall from 93.9W to 74.6W (see fig 1). The regulators have also made the standard tougher in another way. Until now, power consumption is measured in the dark (0lux) and in a bright room (300lux). With v6.0, the Automatic Brightness Control (ABC) function must be validated as follows, to define the maximum allowed ON power: • Power measurements must be taken at luminance levels of 10, 50, 100lux • If the average power measured at 10lux is 5% lower than at 50lux, and the average power at 50lux is 5% lower than at 100lux, a 10% 'On mode' power allowance is granted. The 'ON mode' power is measured with ABC disabled and with default home settings. Without ABC, many tvs on the market today would fail to comply with this new test. This is largely because, to comply with v5.3, they used cheap photodiodes to implement a crude system to reduce tv backlight power at 0lux. Many manufacturers have used this approach to reduce backlight brightness to an unrealistically low level, producing poor picture quality in the belief that viewers rarely watch tv in a completely dark room. Such crude implementations of backlight power control will not be sufficient to support compliance with v6.0. A study shows that many current tvs fail to take advantage of an opportunity to save power at light conditions of between 10 and 100lux. This is important not only for compliance, but also for customers. Three independent studies reached the same conclusion: consumers spend more than 80% of their viewing time in ambient light of less than 85lux. Intelligent local edge dimming offers dramatic power savings: for example, the LG 47W5600 divides the led edge light into 12 segments, and dims each segment automatically in tune with picture content. Local dimming means that where the video content is dark, the backlight output is reduced. This not only results in lower power consumption, but also better contrast and better picture quality. In the same way, only the Vizio TV implements intelligent ABC (using an ambient light sensor from ams). Fine grained response to ambient light conditions offers power savings and an improved viewing experience –display brightness is reduced automatically to comfortable levels when viewing in a relatively dark room. And while a black picture can often appear grey in today's lcd tvs, a dimmed backlight permits the production of a deep black and true colour reproduction. So can the 'ideal TV' – combining both forms of intelligent backlight dimming – provide a means to comply with v6.0? As a demonstration, ams modified a 42in LG LW5500 tv, implementing intelligent ambient light sensing alongside the tv's edgelight dimming. The modified tv was submitted to an independent laboratory for Energy Star tests, using the standard 10min video clip in the three light conditions laid down by v6.0 (see fig 2). Local dimming is not implemented in most current tvs. When turned off, the LW5500 consumes an average of 93W in v6.0's standard 'TV HOME' setting. By enabling 12 segment local dimming, power consumption is reduced by 16%, with enhanced picture quality and contrast. For result 3 in Fig 2, crude ambient light sensing was also turned on, however due to bad implementation, no ALS allowance is granted according to the v6.0 tests. Result 4 shows the improvement when precise ambient light sensing is implemented, graduating the backlight output in response to changes in room lighting. The tv's least aggressive internal power saving mode, which reduces backlight output in certain video picture conditions with a negligible effect on picture quality, was enabled during this measurement. Through implementing ALS in accordance with the Energy Star specification, a 10% maximum on power allowance is granted, which is mandatory to achieve v6.0 certification. The tests showed that, with a combination of local edge light dimming, precise ambient light sensing and a mild power saving mode, a mainstream tv can be modified to comply with the requirements of v6.0, while improving the viewing experience. This can be achieved without any modification to the tv's architecture or power circuit. Design engineering considerations This improvement in power consumption makes demanding calls on ALS performance. Dimming the display's brightness must match closely the response of the human eye to changes in ambient light levels. The ALS used in the modified LW5500 is superior in two respects to those typically found in tvs today. Firstly, it maintains high accuracy and precision across its measurement range, even at low light levels. Secondly, it features filters which reject the IR portion of the light spectrum incident on the sensor. Although invisible to the eye, IR distorts the measurement results of ambient light sensors. The LW5500 demonstration uses a CT820 ALS from ams. Its high sensitivity means it can operate from a concealed location behind the tv's bezel. Another key element is local dimming. This requires the led driver to be synchronised with the tv picture, using VSYNC and HSYNC input signals. In order to match led light output with the video signals, the demonstration design uses an ams led driver (AS3695C/AS3820) with independent PWM generators for each led channel; the PWM generators are synchronised with VSYNC and HSYNC signals. Each PWM generator can be programmed independently in respect of duty cycle and delay. This gives a flexibility to the system designer and supports all led backlight architectures. With this scheme, interaction between the main video processor and the led driver chipset can be minimised, allowing the use of an spi interface operating at less than 1MHz. In the LW5500 demonstration, the ALS is connected to a separate mcu, which reads the ambient light information, processes this via a proprietary algorithm, then generates an analogue signal which dims the led backlight directly (see fig 3). This means dimming control can be implemented independently of the main video processor's software and the ABC function is autonomous of the local dimming protocol. Markus Luidolt, Product Marketing Manager, and David Gamperl, Staff Applications Engineer, are with ams.