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Optoelectronics/Displays??

Save power in LCD TVs with LED driving techniques

Posted: 29 Jan 2013 ?? ?Print Version ?Bookmark and Share

Keywords:direct backlit? LED backlighting? HD plasma? OLED?

Both of these solutions offer good efficiency, work with every type of SMPS with voltage feedback, and can be implemented by attaching feedback lines from more than one driver to the same SMPS, as is required in mixed- architecture systems.

However, the second, digital implementation provides some special advantages. As well as not requiring an output capacitor, the digital circuit also gives the designer the freedom to define the feedback system's attack and decay times. By selecting a fast attack time combined with decay latency and relatively slow decay, the display's performance can be improved. This benefit is particularly noticeable in scenes that require brightness to change rapidly. In this case, a fast attack time eliminates perceptible brightness artifacts as the screen changes from dark to full brightness. The analogue solution (figure 8) dims the LEDs' output gradually during a short dark frame, resulting in a visible delay in achieving full brightness for the next bright frame.

This is a noticeable distraction for TV viewers because films and other video content create large dynamics from one frame to another. Such artifacts can be eliminated with digital regulation circuits by inserting latencies of several hundred milliseconds into the decay instruction. Thus, when bright frames are interrupted by a short sequence of dark frames, the second bright frame starts at full brightness because the driver has automatically delayed the voltage ramp-down. Digital feedback algorithms implementing decay latency can be found in products from ams.

Another useful feature integrated in LED driver ICs is a fast Serial Peripheral Interface (SPI). In direct backlit TVs, the LEDs are arranged in a large number of relatively short strings, so that small areas of the panel can be dimmed to save energy. Typically, such arrangements contain 256 channels in a matrix of 16x16 fields, each individually configured through pulse width modulation (PWM). But generating 256 PWM signals with variable PWM width and delay is a hugely intensive processing task, even for the fastest microcontroller.

These backlighting systems therefore use local PWM generators integrated into the LED driver ICs. This enables brightness to be configured with simple SPI data transfers. In an architecture with multiple driver ICs (e.g. 256 channels with 16 channels per IC, and 16 ICs), the LED channels can be configured by daisy-chaining SPI signals and transferring the data that are used in a VSYNC frame to the frame before.

In this arrangement, data transfer over an SPI can reach speeds of 20 Mb/s, or 50 kb/frame at a 400Hz frame rate. This is fast enough to change dimming of each field in sync with the actual frame. So ideal local dimming can be achieved with minimum overhead on the microcontroller.

Smart dimming for edge-lit systems
This local dimming technique is only possible with direct backlit systems. But a certain amount of smart dimming can also be achieved with edge lighting. In particular, PWM dimming can be used to vary brightness without changing the colour temperature of the white LEDs. Instead of having the edge-lit LEDs permanently set to a specific brightness value, the brightness can be dynamically altered via changes to the pulse width.

Another technique for saving power is Dynamic Luminance Scaling (DLS). With this technique, the LCD's white level/ brightness level is increased in certain scenes to allow the backlight LEDs' power output to be reduced.

Yet another method to reduce power consumption is the use of ambient light sensors. If the room where the TV is being watched is fairly dark, the backlight brightness can be reduced (figure 9).

Figure 9: Two Energy saving methodologies using smart LED drivers and smart ALS sensors.

Even more sophisticated methods are being explored by TV manufacturers. For instance, cameras are beginning to be designed into displays to enable consumers to use video-telephone services such as Skype on their TVs. These cameras can also be used to detect if someone is actually watching TV; if the TV is on without anyone being in the room the backlight can be reduced to a minimum brightness level.

Even customised energy consumption patterns can be implemented. While you might prefer watching in the energy friendly eco-mode with reduced backlighting, another member of the household might prefer full brightness.

In sum, considerable power savings can be realised by implementing today's advanced techniques for efficient LED driving. This is important since ever-tougher regulations continue to reduce the maximum power that a new TV can consume.

About the authors
Herbert Truppe is a Product Line Manager at ams.

Peter Rust, Werner Sch?gler and Manfred Pauritsch are all Senior Design Engineers at ams.

To download the PDF version of this article, click here.


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