LED_Display_Backlighting_monitor_appl

8/7/2019 LED_Display_Backlighting_monitor_appl

1/7

May 19, 2005 page 1 of 7

LED Display Backlighting Monitor Applications using 6-lead MULTILEDApplication Note

AbstractThis application note describes tworeference designs for LCD backlightingusing the 6-lead MULTILED LRTB G6SGThe feasibility of the designs is shown ontwo LCD sizes of 19 and 24 with a typicalbrightness of 250cd/m required for officeapplications.

The uniform illumination of very largescreens used in LCD televisions whichrequire a typical brightness of 350-500cd/mis described in a separate application note"LED Display Backlighting Large Screenand TV Application".

This document builds upon the fundamentalconcepts of LED backlighting described inthe application note "LEDs, New LightSources for Display Backlighting.

IntroductionThe continual increase in LED brightness inrecent years has opened up new marketsegments and application areas. RGB SMD-LEDs have become particularly important forthe LCD display market and offer severaladvantages over existing CCFL (coldcathode fluorescent lamp) technology foruse in backlight units (BLUs). The increased

color gamut (beyond the NTSC standardand 50% larger than CCFL, as shown inFigure 1) and the compensation of blurringeffects by means of pulsed LEDs provideexceptional image quality.

Figure 1: Color gamut of LED and CCFLcompared to NTSC standard (NTSC:National Television System Committee)

The color point can be set to any locationwithin the color triangle, and can be colorstabilized and dimmed with the appropriateLED driver circuitry.In addition, LED technology provides a highlevel of reliability with a lifetime of over50,000 hours and the components are freeof hazardous materials such as mercury or

lead.If very fast switching LCDs become availablein the future, the color filter in LCD displayscan be entirely eliminated, and the colordisplay can be achieved directly by means of"sequential color".


2/7

May 19, 2005 page 2 of 7

6-lead MULTILED LRTB G6SG

Figure 2: 6-lead MULTILEDThe 6-lead MULTILED LRTB G6SG (seeFigure 2) is used for both designs. Thisdevice contains three chips for the colors red

(R), true-green (G) and blue (B) using ourhighly efficient Thinfilm technology.This LED is distinguished by the high opticalefficiency of the chips used and theincreased lifetime (> 50,000h) due to thesilicon packaging, allowing the LED to beused in demanding backlightingapplications.When all three chips of this LED are drivensimultaneously with the appropriate RGBintensities, a white light is produced.Depending on requirements, the resulting

color coordinates can be positionedanywhere within the color triangle.

Details concerning the optical characteristicsand the current performance of the LED canbe found in the corresponding data sheet.

DESIGN 1: Example of indirect LEDbacklighting for a 19-inch LCDMonitor

Figure 3: 19-inch TFT MonitorA commercially available 19-inch TFTmonitor (see Figure 3) was obtained andmodified with LED backlighting. Only thelight source was modified; the rest of thedesign (housing, light guide, optical filmsetc.) remained unchanged.The two cold cathode fluorescent lamps

(CCFL) located on the long sides of the lightguide were removed and were eachreplaced with a strip of IMS-PCB (insulatedmetal substrate) containing 77 LEDs. For theentire backlight, 154 LEDs were used, with apitch of 5mm (see Figures 4 & 5).

Figure 4: Section of the LED strip whichreplaces the CCFL tubes


3/7

May 19, 2005 page 3 of 7

Figure 5: Upper left corner of the LEDbacklight in operationIn future a less number of LEDs will beneeded due to the continuous increasingbrightness of the semiconductor chip.As a consequence of the light guideapproach, the backlight can be made verythin (approx. 10mm).

The LED primary colors vary in intensitydepending on the desired color coordinatesfor the RGB mixture. The ratio of red, greenand blue must be specifically adjustedaccording to the type of LEDs used and theirbrightness and wavelength groups.

In this specific example the chip brightnesswas within the level of 400-502mcd for red,900-1120mcd for green and 140-180mcd forblue, measured at 20mA according to thefloating bins of OSRAM OS.For white (x=0.33 & y=0.33), the intensityratios of R30%, G 60% and B 10% shouldbe employed. The original backlight was setto color coordinates of x=0.30 & y=0.34which resulted in ratios of R 25%, G 67%and B 8%.

For the LEDs, this results in red with460mcd (at 20mA, 2V), green with 1230mcd(at 30mA, 3V) and blue with 150mcd (at20mA, 3V).The power consumption per LED is around190mW (30 W for all LEDs) and provides aluminous flux of about 5.5lm for thebacklight, leading to 250nits at the LCD.This value was verified by measurements onthe modified monitor (see Figure 6).

Figure 6: Display with conventionalbacklighting (left) and with LED backlighting(right) the strengths lie in the red andgreen areasFor this modification, the PCB strips weremounted in the existing housing. In order toguarantee stable continuous operation of theLEDs, thermal dissipation must also betaken into account.As seen in our thermal simulation (Figure 7)of the LED setup and verified by themodified backlight the heating is below theadmissible max. junction temperature of125C.In this case, passive cooling by means of

ventilation slots in the housing and themounting of the IMS-PCBs on thin heatsinks is sufficient.

Figure 7: Thermal simulation map of 19LED setup


4/7

May 19, 2005 page 4 of 7

For each PCB strip, a group of 11 red, 7green and 7 blue LEDs were driven inseries, so that a supply voltage of 24V couldbe used. Appropriate series resistors wereused to limit the maximum current to theLEDs; the fine adjustment of the RGB ratioswas accomplished by means of PWM (pulse

width modulation).

DESIGN 2: Example of direct LEDbacklighting for a 24-inch LCDmonitorA commercially available 24-inch monitorwas modified with a direct LED backlight,

because for diagonals of 24 inches andmore, direct CCFL backlighting ispredominantly used as well. A direct setuphas the advantage that the backlight can berelatively easily scaled up or down,corresponding to the LCD size and thebrightness requirements, due to theunnecessary light guide.The schematic construction is shown inFigure 8. The LEDs are evenly distributedwithin a reflector box (backlight unit = BLU)which is coated with a white reflective

material. The required optical films are alsoadded to the BLU, followed by the LCDitself.Specifically, the backlight consisted of 468LEDs in a 26 x 18 matrix within an area of530mm x 350mm. The LEDs were arrangedon IMS-PCBs in 26 vertical strips, with 18LEDs per strip. The LEDs for each strip wereconnected in series and the maximumcurrent for each color was limited byappropriate series resistors. The strips wereconnected in parallel this has theadvantage that in spite of the high LEDcount, the required supply voltage can bekept low. The LEDs were positioned with aspacing of 20mm. To further homogenizethe color uniformity, especially at the bordersof the backlight, it is possible to rotate e.g.every other LED by 180.

Figure 8: Schematic construction of direct backlighting with LEDs


5/7

May 19, 2005 page 5 of 7

As already mentioned, an advantage is thescalability of this arrangement as long asthe number of LEDs for a given arearemains constant, this approach can also beused for smaller or larger displays (seeTable 1).

BLU size Number of 6-leadMultiTOPLEDs20 325

21 358

24 468

26 550

27 593

Table 1: Estimation of LED count for variousdisplay sizes (250 nits on LCD)Increased brightness (TV LCDs typicallypossess a brightness of 350 500nits) canbe achieved either by a higher LED count orincreased LED performance or acombination of both measures.There is actually no limitation in size andbrightness, as long as one is able to drivethe necessary amount of LEDs.

An additional advantage is the equaltemperature distribution of the backlight(Figure 9) as a result of the equal spacing ofthe LEDs.

Figure 9: LED temperature distributionAs seen in the simulation and by the mockupas well, passive cooling by means ofventilation slots in the housing and the

mounting on the IMS-PCBs which is used asheat sink is sufficient to keep the temper-ature below the admissible junctiontemperature.

In order to assure homogeneous illuminationof the display, the standard optical films inthe monitor were not suitable. In this case,the LEDs would create noticeable spots oflight on the backlight.Therefore, a special diffractive diffuser wasused which creates a satisfactoryhomogenization of the light at a distance of20mm from the LEDs. This optical film hasan additional advantage: it does not onlyreplace the diffuser, but also offersincreased brightness by reducing the

emission angle of the backlight.Thus, the diffractive diffuser effectivelyreduces the number of boundary layersrequired and further increases the efficiency.The excellent transmission of around 90%also contributes to the light emittingefficiency (see Figure 10).

Figure 10: Transmission of various diffuserfilms (Source: POC)The diffractive diffuser can be obtainedunder the name LSD (Light ShapingDiffuser) from Physical Optics Corporation(www.poc.com) in various formats(dimensions, thickness, and transmissioncharacteristics).


6/7

May 19, 2005 page 6 of 7

For the 24 inch backlight in our exampleapplication, an LSD 80 and a DBEF-D wereused.With the appropriate LED supply current, thecolor mixing of the primary colors from theLEDs was adjusted to white so that acontribution of about 4.5lm at 60mW perLED was assumed (the total LED poweramounts to about 75W), resulting in 250nitson the LCD. Details of the mechanical setupare shown in the following images (Figures10a, b & c).

Figure 10a: Photo of the backlight (withoutoptical films)

Figure 10b: Close-up of the LED strips

Figure 10c: BLU-Overlay of the reflectormask

SummaryThe 6-lead MULTILED is ideally suited forbacklighting in LCD monitors.Beside the ecological and economicaladvantages the LED backlight provides anexceptional image quality on LCDs with ahigher color gamut and compensatedblurring effects.Either indirect or direct backlighting can beused. Direct backlighting can be scaled forvarious display sizes and brightnesses; the

light guide solution permits a very thindesign.Driving and switching of LEDs in an LEDBLU is also very flexible, allowing numerousapproaches such as pulse operation,unlimited dimming by PWM, brightness andcolor control as well as sequential coloroperation to eliminate color filters in the LCD

The 6-lead MULTILED is available in thelatest generation of Thinfilm technologywhich typically delivers around 6lm at

200mW (30lm/W) at the white point (x=0.33;y=0.33); this value will significantly increasein the near future (38lm/W by the end of2005).


7/7

May 19, 2005 page 7 of 7

Links to manufacturers:http://www.poc.com/(light shaping diffuser from POC)http://www.kimoto.ch/(diffuser and reflective film from Kimoto)http://cms.3m.com/cms/US/en/2-136/izezrFG/view.jhtml (optical films from 3M)

Author: Josef Httner

About Osram Opto SemiconductorsOsram Opto Semiconductors GmbH, Regensburg, is a wholly owned subsidiary of Osram GmbH, one ofthe worlds three largest lamp manufacturers, and offers its customers a range of solutions based onsemiconductor technology for lighting, sensor and visualisation applications. The company operatesfacilities in Regensburg (Germany), San Jos (USA) and Penang (Malaysia). Further information isavailable at www.osram-os.com.All information contained in this document has been checked with the greatest care. OSRAM OptoSemiconductors GmbH can however, not be made liable for any damage that occurs in connection withthe use of these contents.

Documents

LED_Display_Backlighting_monitor_appl