Technical
Article
How to comply with the Energy Star 6.0 standard for LED
TVs: a demonstration of reduced power consumption
with improved picture quality
Markus Luidolt and David Gamperl
Technical
Article
How to comply with the Energy Star 6.0 standard for LED
TVs: a demonstration of reduced power consumption
with improved picture quality
Markus Luidolt and David Gamperl
A constant tightening of the regulations governing the power consumption of TVs has had its intended effect: modern LED TVs today consume around half as much energy as LCD TVs with a
standard CCFL (cold cathode fluorescent lamp) backlight consumed even as recently as two years
ago.
The squeeze on power usage continues, however, with the next revision of the US Energy Star
regulation, bringing into play the technique of adjustable display brightness optimised for varying
ambient light conditions. In a room with subdued lighting, the TV viewer will be more comfortable
with a relatively dim display output. In a brightly sunlit room, the display screen must be backlit at
high power, in order for the viewer to be able to see the picture in clear detail.
But the ambient light sensors (ALS) generally used in today’s TVs are not able to provide a finely
graduated backlight control, and so fail to take full advantage of the opportunity to reduce power
inputs to LED backlights.
In fact, 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 markedly reducing the power usage of LED TVs. This article describes a design project
by ams to implement both local dimming and ambient light display control in a mainstream LED TV
on the market today. How large an effect on average power consumption is advanced dimming able
to make?
Changes to energy standards
The most important energy standards for TVs are the EU Energy Label, the Chinese energy consumption standard for TVs, and the best known standard, the US’s Energy Star. The current Energy
Star standard for TVs, version 5.3, already sets tight power limits, but version 6.0 planned for 2013
will further cut the maximum allowed power consumption (see Figure 1). Under v6.0, the maximum
allowed power usage for a TV of 46” falls by 20.5% from 93,9W to 74,6W.
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Fig. 1: Energy Star on-power limits for various TV screen sizes
It is already hard enough to reach this standard. But the regulators have also made the standard
tougher in another way. Until now, the TV’s power consumption would be measured under two conditions: in the dark (0 lux) and in a bright room (300 lux).
With version 6.0, the Automatic Brightness Control (ABC) function must be validated as follows, to
define the maximum allowed on power:
Power measurements must be taken at three different luminance levels: 10, 50, 100 lux
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 (See green curve in Figure 1). The ‘ON mode’ power is measured with ABC disabled, and with default home settings.
Without having an ABC, all TVs will not work above the red energy star mode (Figure 1)
Many TVs on the market today fail to comply with this new test (see Figure 2).
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Fig. 2: Energy Star® on-power maximum allowed power vs reported power
This is largely because they were designed to comply with the 5.3 standard, using cheap photodiodes to implement a crude ALS that reduces TV backlight power at 0 lux. Many manufacturers have
used this ABC to reduce backlight brightness to an unrealistic low level, producing poor picture
quality (in order to reduce reported Energy Star power consumption), in the quite justified belief that
viewers rarely watch TV in a completely dark room (0 lux).
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Ideal TV
Green: LED,LCD
Blue: CCFL, LCD
Fig. 3: power consumption of TVs at measured ambient light conditions. As the TVs are similar but not identical in size, the
diagram shows power consumption normalised by measuring Watts per square inch.
As Figure 2 shows, such crude implementations of backlight power control in response to ambient
light will not be sufficient to support compliance with version 6.0.
So how much difference is made when more intelligent, fine-grained dimming is implemented?
Figure 3, a study of TVs on the market today, shows that many TVs fail to take advantage of an
opportunity to save power at low room light conditions of between 10 and 100 lux. This is important
not only for compliance, but also for customers. Three studies carried out independently have come
to the same conclusion: viewers spend more than 80% of their viewing time in ambient light of less
than 85 lux.
Figure 3 also shows that intelligent local edge dimming offers dramatic power savings: of the models studied, only the LG 47W5600 divides the LED edge light into 12 segments, and automatically
dims each of the 12 segments in tune with the picture content. Local dimming means that where the
video content is dark, the backlight output is reduced. This results not only in lower power consump-
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tion but also a better contrast ratio and hence better picture quality. (LG’s precise implementation of
local dimming is achieved using an advanced LED driver from ams, the AS3695C.)
In the same way, only the Vizio TV implements intelligent ABC (using an ams ambient light sensor).
Fine-grained response to ambient light conditions again offers both power savings and an improved
viewing experience – the display brightness is automatically reduced 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’ shown in Figure 3 – combining both forms of intelligent backlight dimming –
provide a means to comply with Energy Star v6.0?
Combining smart local dimming and ambient light sensing
As a demonstration, ams modified a mainstream 42” LG TV (model number LW5500), implementing
intelligent ambient light sensing alongside the TV’s local edgelight dimming (see Figure 4). The
modified TV was then submitted to an independent Energy Star-certified laboratory for Energy Star
tests, using the standard 10-minute video clip in the three room light conditions laid down by the
v6.0 standard.
Fig. 4: power consumption of the LG LW5500 modified by ams
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Local dimming is not implemented in most TVs on the market today. When turned off in the
LW5500, the TV consumes an average of 93W in the standard ‘TV HOME’ setting of the Energy
Star 6.0 test.
By enabling 12-segment local dimming, the TV benefits from a power reduction of 16%. Picture
quality and contrast are also enhanced.
For result 3 in Figure 4, 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 made 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 slightly reduces the 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 energy star 6.0.
The tests showed that, with only a combination of local edge light dimming, precise ambient light
sensing and a mild power-saving mode, an existing mainstream TV can be modified to comply with
the stringent requirements of the Energy Star standard v6.0, while improving the viewing experience. What is more, this has been achieved without any modification to the TV’s architecture or
power circuit.
Design engineering considerations
This dramatic improvement in power consumption makes particularly demanding calls on the performance of the ALS. The dimming of the display panel’s brightness must match very closely the
response of the human eye to changes in ambient light levels.
The ALS used in the modified LW5500 demonstration is superior in two main respects to the crude
ALS devices typically found in TVs today: first, it maintains high accuracy and precision across its
measurement range, even at low light levels. Second, it features filters which reject the IR portion of
the light spectrum incident on the sensor. IR is invisible to the human eye, but distorts the measurement results of ambient light sensors unless filtered out.
The LW5500 demonstration design uses a CT820 ALS from ams, which also offers high sensitivity.
This means it can operate from a concealed location behind the TV’s bezel, which makes drills
through the TV bezels obsolete.
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The choice of ALS device, then, is extremely important in the implementation of this energy-saving
design. Another key element of the design is local dimming. This requires the LED driver to be synchronised with the TV picture, using VSYNC and HSYNC input signals. In order to make the LED
light output match precisely the video signals, the demonstration design uses an ams LED driver,
the [AS3695C / AS3820] with completely 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 High time (that is, the duty
cycle) and delay. This gives a great deal of flexibility to the system designer, and supports all LED
backlight architectures (3D, edgelight, direct backlighting).
With this scheme, interaction between the main video processor (TCON) and the LED driver chipset
can be minimised, allowing the use of a relatively slow SPI interface operating at well below 1MHz.
TCON
local dimming information
LED Backlight
SPI
TCON
local dimming information
SPI
LED Backlight Driver
ambient light information
analog dimming
MCU
Algorithm
ALS
12C
Fig. 5: ambient light control bypassing the video processor
I2C
ALS
Fig. 6: ambient light control via the video processor
In the LW5500 demonstration design, the ALS is connected to a separate MCU, which reads out
the ambient light information, processes this information via a proprietary algorithm and converts it
this into an analog signal (see Figure 5). This analog signal then directly dims the LED backlight.
The advantage of this topology is that dimming control can be implemented completely independently of the main video processor’s software, and the ABC function is autonomous of the local
dimming protocol.
Figure 6 shows a possible different topology, in which the ambient light signal is routed via the main
video processor.
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Conclusion
The LW5500 demonstration design from ams shows that relatively simple methods of intelligent
backlight dimming can produce power savings of ~30% in an LED TV, which already featured an
advanced power system design. Reducing a TVs’ energy consumption by 30% would eliminate
around 8 megatons of CO2 emissions per year worldwide. What is more, this energy saving can be
combined with better picture quality and an improved viewing experience.
ams is the leading provider of driver and lighting sensor solutions for LED TVs and has a complete
portfolio of LED driver and lighting sensor products supporting edge lighting, direct lighting, ambilight and 3D TVs. The aim of ams is to provide its customers with outstanding performance in
power-efficient systems with outstanding LED TV picture quality.
For more information on power saving in LED TVs, including a video demonstrating the effect of the
design described in this article, http://www.ams.com/eng/Lighting/60WattTV
For further information
ams AG
Tel: +43 (0) 3136 500
info@ams.com
www.ams.com
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