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Comprehensive System Solutions

Mon, 05/04/2009 - 1:56pm
Peker Arkadly and Tom Kapucija, Microsemi Corporation

ec95smharkadiyAlthough the LCD TV display market has not experienced as quick a conversion rate to LED backlighting as the laptop/notebook segment, the TV segment is experiencing similar transitional forces. As the migration to full 1080P (1920x1080/50P or 1920x1080/60P) TV panel resolution continues and the display size increases, the overall quality of the displayed image becomes more and more dependent on the quality of the TV backlight unit. Image quality that is acceptable in a 15.6-inch notebook display may not be so acceptable in a 46- or 55-inch class television.

ec95smhKapucija
The large-screen TV display may have more stringent demands than laptops and notebooks for the following backlight unit (BLU) characteristics:

• Luminance uniformity from edge-to-edge and center-to-corner;
• High dynamic contrast ratio and deep black levels;
• Color gamut at or exceeding NTSC requirements;
• Moving-picture quality as indicated by MPRT (Motion Picture Response Time);
• High luminous efficiency indicative of low power consumption.

ec95sm100fig1The LED transition in notebook applications has yielded the primary benefit of improved efficiency and extended battery operating time, but the primary implementation is with Edge-Lit LED BLUs. The TV or TV-panel manufacturer has a number of different technology options available that are not typically used in the majority of notebook displays. The LCD TV with an LED BLU can also use an Edge-Lit implementation (D0), but will have higher levels of picture quality and increased efficiency with D1 scanning dimming or D2 local or zone dimming. Optimum picture quality would be achieved in an RGB LED BLU implementation of D3 (local or zone dimming of individual R, G, or B LEDs). 

Using LED backlighting in TVs can offer many of the above advantages and improvements in image quality, but there are significant cost and implementation issues, including:

• Selection of the optimum and most cost-effective power system architecture. Edge-Lit and Direct-Lit LED BLUs each have different requirements, which will result in different system level power efficiency as well as cost;
• Selection of the optimum driving methods for the LED strings;
• Lighting control and management of LED current accuracy over a wide range of input voltages and operating temperatures, as well as thermal protection;
• Accurate color management in RGB LED BLU implementations, as well as synchronization of the BLU with the system video processor to implement the BLU addressing schemes (including D0, D1, D2, or D3 dimming approaches).

Many solutions to overcome challenges inherent in multi-array LED BLUs have been proposed. 2, 3, 4, 5.

In general an LED based BLU, contains the following blocks:

• Multiple strings of White or R, G, B LEDs;
• Lighting control and management function/controller, which includes the video processor interface for dimming, as well as an ambient light sensor (for W-LED BLU) or RGB color sensor (RGB-LED BLU) and diagnostics;
• Power section, which could be single or multiple power supply units (PSUs) with thermal management and interface to the BLU lighting control and management block;
• The LED drivers and associated protection schemes.

The main power system requirements are low cost, excellent power efficiency and small size.  Certain trade-offs will be required depending on the type of dimming implementation.  As an example, an Edge-Lit BLU (using D0 dimming) will require multiple PSUs with a very thin form factor, whereas a Direct-Lit BLU (using D2 local or zone dimming) will be able to use a smaller number of larger and thicker PSUs.


ec95sm100fig2aLEDs are current-driven devices with output brightness proportional to the forward current, which is driven by a constant DC current source in order to maintain uniform luminosity.  A constant current source provides independence from the LED forward voltage differences and forward voltage (VF) variations over temperature.  Figure 2a shows a conceptual drawing of circuitry for controlling RGB LED arrays which are powered by a constant voltage power supply (PS) and controlled by individual current sources. This concept also applies to W-LED strings, where each of the colored LEDs would be replaced by White LEDs.

For an RGB-LED BLU implementation (as shown in Figure 2a), the PSU must support the LED array (string) with the maximum total VF, which would result in excessive power dissipation due to the VF difference of colored LEDs and the VF difference of same-color LEDs, both of which result from variations in LED manufacturing.

ec95sm100fig2b
For example, a Lumiled Red Luxeon Emitter has typical VF of 2.95V, and the Green LED has a typical VF drop of 3.99 V.  Assuming there are 10 LEDs in an array and the nominal current is 0.35 A, the red array voltage drop is 29.5V and the green array voltage drop is 39.9 V. Therefore, the PSU maximum voltage should be at least 39.9 V, which would result in excessive power dissipation for the red array.  The calculation is as follows:  Pd= (39.9 V-29.5 V)*0.35 A=3.64 W.

A more power-efficient — but also more expensive — solution is to use an individual DC/DC converter on each R, G, and B array/string, and transform the convertor to a constant current source by regulating the voltage across an LED string current sense resistor as illustrated in Figure 2b. This implementation is also suitable for W-LED implementations where each of the colored LEDs is replaced by a white LED.

Another key requirement in an LED BLU is to manage the positive thermal co-efficient of the LEDs, which results in decreased luminance flux with increasing temperature.  For example, in an RGB BLU, if the PWM duty cycle increases faster than the decrease in LED VF drop, then the LED backlight can go into thermal runaway. It is the responsibility of the lighting control and management block(s) to measure the LED temperature(s).  If the temperature increases above a predetermined value, the lighting control and management system must start to decrease the duty cycle of all RGB PWM signals in order to keep the LEDs below thermal runaway conditions.  This approach will protect LED backlights and the LCD display from failing in case of uncontrollable ambient conditions.

In an RGB-LED BLU system, it is also important to provide closed color-loop management of the preset or user-selected white point, while also maintaining the overall brightness of the display.  In a W-LED implementation, only a light sensor will be required, but the RGB-LED system will also require a more complicated RGB sensor plus a closed color-management loop.
To take full advantage of the image quality improvements possible with LED based BLUs, it is necessary to implement advanced features such as D1 scanning dimming, D2 local or zone dimming, or D3 RGB-based zone or local dimming (See Figure 3).ec95sm100fig3

Accurate frame-by-frame synchronization of the LED BLU with a video processor is a very critical requirement. Each light zone of the LED BLU needs to be synchronously updated with the data addressing of the panel.  Failure to precisely synchronize the zones and panel data will significantly impact image quality. Communication with video will require a high-speed serial bus such as an SPI bus, and special communication protocols may be required with the following information communicated on the SPI bus:

• Dimming data for every zone for D2 local- or zone-dimming operation, or dimming data for every RGB string for D3 RGB-based local or zone dimming;
• The ability to enable/disable information in order to control every zone in D1 scanning dimming mode;
• Clock signal;
• Communication VSYNC for synchronization with panel data.
In summary, to capitalize on the image quality improvements possible with LED BLU implementations, careful attention must be paid to implementing an optimum lighting control and management scheme that optimizes power efficiency as well as image quality.

References
1 Munisamy Anandan-ADEAC’06 Tutorial Notes on “LED Backlight”, October 2006.
2 Sang-Yun Lee, Chang-Hoon Baek, Do-Hun Kim, Hyung-Suk Kim, Myoung-Soo Choi, Bang-Won Oh “Integrated Power-LED Module Driving System for LED Backlights,” SID 07 Digest, pp.1802-1805 (2007).
3 Sang-Yun Lee, Jae-Wook Kwon, Choul-Ho Lee, Myoung-Soo Choi, Kyung-Soo Byun “A Novel Application of High Efficiency RGB-LED Driving and Control System for LED Backlight in LCD,” ADEC 06 Digest, pp. 179-182 (2006).
4 Ahmed Masood “Driving Schemes for LED Backlighting Large–area LCD Displays,” SID 07 Digest, pp. 308-311.
5 A.A.S Sluyterman “System aspects of LED backlighting,” ICDL2007, pp.15-18.
6 Marc Dyble, Nadarajan Narendan, Andrew Bierman,Terence Klein “Impact of Dimming White LEDs: Chromaticity Shifs Due to Different Dimming Methods,” Fifth International Conference on Solid State Lighting, Proceeding of SPIE5941, pp. 291-299.



 

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