Driving the Golden Dragon LED - Farnell Uk

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February 2, 2005 page 1 of 14

Driving the Golden Dragon LEDApplication Note

INTRODUCTIONThe Golden Dragon LED is OSRAM OptoSemiconductors high performance LED

requiring special considerations in thermalmanagement and electrical implementation.This application note is intended to help thedesign engineer with the special electricalconsiderations of the Golden Dragon LED.

With a higher current there is higher power,and therefore more heat to dissipate. TheGolden Dragon LED package is optimizedfor removing this heat efficiently. With anintegrated heat slug (also known as a heatspreader) the thermal performance is far

superior to standard LEDs.

Golden Dragon LEDs are delivered on tapeand reel. It has a flat top to allow pick-and-place machinery installation. All contacts(including the heat slug) are soldered inplace using standard infrared reflowprocesses (Surface-mount componentprocessing). Up to now the Golden Dragonis the only high power LED in the market

which is capable to be processed accordingto these cost effective standard assemblytechniques. ESD handling guidelines shouldbe followed when handling the GoldenDragon LED.

Basic structureFigure 1 shows the internal structure of theGolden Dragon LED.

Leads

Die Attach

Bond Wire

Die Molding Compound

Solder Pads

Dielectric Heat Slug

SolderAluminum Plate

Figure 1 Structure of the Golden DragonLEDThere are large leads for a strong mechani-cal attachment to the printed circuit board(PCB), and to assist proper orientation of thepart during reflow soldering.The semiconductor die is directly attached tothe heat slug. This heat slug is cast insidethe molding compound, which forms areflector cup around the die. The heat slug isexposed on the bottom of the part for IR

reflow soldering to the PCB to provide a verylow thermal resistance from the die to thePCB it is mounted to. The die is coveredwith an optically transparent encapsulationmaterial to protect the die from the ambientenvironment.


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DESIGN CONSIDERATIONSThermal designBecause the Golden Dragon LED has a highpower rating, special consideration must bemade to optimize the thermal performanceof the entire system.OSRAM Opto Semiconductors has releasedan application note specifically addressingthermal design for the Golden Dragon LED.(The application note is titled ThermalManagement of the Golden Dragon). Formore details, please consult that applicationnote.

Optical designThe scope of this application note does notinclude details of optical design. However itis an important step in the lighting systemdesign and should not be ignored. Opticaldesign must target the highest efficiency toreduce the LED light output requirementsand therefore the driver and heat-sinkrequirements.

Electrical designSemiconductor technology differencesThere are two technologies used to produceLEDs: InGaN and InGaAlP.Different colors can be achieved with thesetwo technologies. InGaAlP is used toproduce colors from Green (570nm) toSuper Red (632nm). It has a forward voltage

around 1.8V to 2.3V, depending on thecolor. InGaN is used to produce colors fromBlue (460nm) to True Green (528nm) andphosphor based colors like White (typ.3250K or typ. 5600K). It has a higherforward voltage around 3.2V to 3.8V,depending on the color. Be sure to checkthe data sheet for the specific LED you areusing to get the correct information.

High currentThe Golden Dragon LED is a high currentLED capable of operation at current levels inthe hundreds of milliamps. InGaAlP products(Amber-Red and Yellow) can operate from100mA up to 750mA. InGaN products (Blue,Verde Green, True Green, and White) canoperate up to 500mA.This high current develops a great deal ofpower to dissipate in the LED. This powercan be up to two Watts in specific products.OSRAM Opto Semiconductors will continual-ly develop improvements to the GoldenDragon LED. Please check the data sheetsfor the latest performance data.

Steep If vs. Vf slopeThe Forward Current vs. Forward Voltagecurve of the Golden Dragon LED is verysimilar to any other LED. It has a steeperslope of the If vs. Vf curve in the high currentarea. This makes for rapid changes inforward current with small changes inforward voltage. The graph in Figure 2shows this characteristic. Increasing thecurrent in the diode will not increase theforward voltage by a significant amount.

Forwardcurrent(Amps)

Forward Voltage (Volts)

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2

0.0

0.1

0.2

0.3

0.4

0.5

Figure 2: Graph of Forward Voltage versusForward Current for a typical yellow GoldenDragon LED.


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Note that a 0.1 Volt change in forwardvoltage is marked, and the indicated forwardcurrent changes approximately 100mA. Thisis approximately a 40% change in currentwith a 5% change in forward voltage.

The intensity of the Golden Dragon LED isclosely linked to the forward current. With a40% change in current, the intensity willchange approximately 40%. To properlycontrol the LED intensity, current control orcurrent limiting is mandatory.

Temperature coefficient of forwardvoltageAll LEDs exhibit a change in forward voltage

as the junction temperature changes. Thistemperature coefficient of forward voltage ispublished in each data sheet of individualLEDs. InGaAlP LEDs (Yellow and AmberRed) have a coefficient of between -3.0mV/Kto -5.2mV/K, and the InGaN LEDs (Blue,Verde Green, and White) have a coefficientof between 3.6mV/K and -5.2mV/K. Checkthe data sheet for the specific part you areusing to find this coefficient for your designs.

Intensity changes over temperaturevariationsAll LEDs also exhibit a change in intensity asthe junction temperature changes. This is aresult of changing efficiencies in thesemiconductor, and not a result of thechange in the forward voltage over tempera-ture changes. This temperature change isnon-linear, but is represented in graph formin all data sheets. Check the data sheet forthe particular LED you are using for thisgraph.

Example of critical data sheetinformationThe published Forward Voltage of the thinfilm amber-red Golden Dragon LED (LAW5SF) is provided as a minimum (2.05V), atypical (2.4V) and a maximum (2.65V). Thisis the range that the LEDs can be delivered

from production. This voltage is tested at aspecific current. It is best to use any LED asclose to the test current as possible. It isimportant to verify operation over thisvoltage range to be sure operation is in thesafe range.The published thermal coefficient of forwardvoltage for the thin film amber-red GoldenDragon LED is -5.2mV/K. The publishedmaximum junction temperature of the Thinfilm amber-red Golden Dragon LED is125C. It is important to verify operation overthe specified operating temperature range toassure that the maximum junction tempera-ture is not exceeded.The published maximum current of the thinfilm amber-red Golden Dragon is 750mA.

All conditions (input supply variations,temperature variations, and productionvariations) must be evaluated to assure themaximum current is not exceeded.

HOW TO DRIVE A HIGH CURRENT LEDLIKE THE GOLDEN DRAGON LEDLED circuit arrangementsDue to the high slope of the Forward Current

vs. Forward Voltage graph (Figure 2) it isstrongly recommended to only connect theGolden Dragon LED in a series arrangementwith some current control for each seriesstring in the system. As described in theapplication note titled Comparison of LEDCircuits, a matrix circuit has uncertainties inthe LED current that result from a mismatchof the LED forward voltages. The GoldenDragon LED will have this behavior but moreso.

Series resistor current limitingStandard LEDs, like the Power TOPLED,typically employ a series resistor to limit theforward current. With 350mA through aseries resistor, and a 12V supply, theresistor power dissipation can easily exceed3 Watts when used with a single GoldenDragon LED.


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Putting more LEDs in the string, and thusreducing the resistor value, will reduce thepower dissipation in the series resistor. Withthe small resistances resulting, the supplyvoltage variations will cause larger currentvariations in the LEDs. Figure 3 shows thedifferent effect on the current with supplyvoltage variations. (A typical automotivelighting application will see a variation from9V to 16V)

400

350

300

250

200

150

100

509 10 11 12 13 14 15 16

Supply voltage (V)

LEDcurrent(mA)

1 Dragon

3 Dragons

4 Dragons

(37.5 )

(18.0 )

(27.0 )

Figure 3: Comparison of LED currentvariations with supply voltage variationsThe smaller resistor creates a larger currentvariation in the LEDs from the minimum tothe maximum supply voltage. This variationin current will create a variation in lightoutput of the LED. There is a possibility thatthe maximum forward current (as publishedin the data sheet) will be exceeded when thesupply is at its maximum. To minimize thisvariation, maximize the resistance byreducing the number of LEDs in each string.

With high power LEDs, the resistor is kept ata minimum to minimize power dissipation.These are mutually exclusive requirements;therefore a balance must be achieved with acompromise. High power resistors can beexpensive, and assembly of a high powerresistor can add significant cost. (i.e. handsoldering, selective soldering, clinching, anti-vibration mounting.) These factors must also

be considered when determining thebalance.

Example series resistor calculationsThere are many factors that affect current inthe LED during operation:

Supply variationFirst, the supply voltage has some variation.(Typically 5% to 10%, automotive experi-ences a variation from 9V to 16V withnominal being in the 12.5V to 13.5V range.)As we discussed previously, the supply

variation can add a significant currentvariation in the LEDs.

For example, lets start with a low cost 5%regulator supplying 12V (Vsupply). This wouldreduce the large voltage swings typical in anautomotive lighting application. If we putthree LEDs in a string, each with 2.4V (V ftypical for a thin film amber-red goldendragon LED), the series resistor will have a4.8V (Vresistor) drop at 0.350A (Idiode). Thisresults in a 13.7Ohm resistor dissipating

1.68W (Presistor).

WattsAVP

IVP

A

VR

I

VR

VVVV

VnVV

diode

resistor

68.1350.0*8.4

*

7.13350.0

8.4

8.44.2*312

*

resistor

dioderesistorresistor

resistor

diodesupplyresistor

==

=

==

=

==

=

At the limits of the regulator tolerance, thesupply voltage increases only 0.6V (Vsupply =12.6V maximum). The voltage droppedacross the resistor increases to 5.4V, andthe current increases by 0.044A. The LEDnow passes 394mA.


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AVI

VI

VVVV

VnVV

394.07.13/4.5

Resistance/

4.54.2*36.12

*

diode

resistordiode

resistor

diodesupplyresistor

==

=

==

=

Temperature VariationThe second factor affecting LED current isthe temperature coefficient of the forwardvoltage of the LED. The data sheet for everyLED gives a coefficient for the forwardvoltage with respect to the junctiontemperature. At higher temperatures, theforward voltage of the LED will decrease.

For the InGaAlP thin film amber-red LED,the coefficient is 5.2mV/K.

VLEDsV

VKKV

KT

9.033.0

3.060/0052.0

60

=

=

=

With a temperature rise of 60K above roomtemperature, the forward voltage of eachLED drops 0.3V. With three LEDs in a string,(in an attempt to reduce power dissipation in

the series resistor) the forward voltageacross the string will drop 0.9V as a result ofthe temperature change.The effects of supply variation and tempera-ture variation add. with a 5% tolerance on a12V supply, and a 60K temperatureincrease, there is a possible total variation of1.5V across the series resistor. Thisincreases the current in the LEDs by a totalof 0.11A. The LED now is passing 0.46A.

Production variationThe third factor affecting LED current isproduction variation of its forward voltage.The data sheet of the Thin film amber-redGolden Dragon LED gives a roomtemperature forward voltage variation of0.6V. With a design targeting the nominalvalue, this can be seen as a 0.3V

tolerance. This voltage change adds with thefirst two effects creating a possible totalvariation of 1.84V across the series resistorin this application.

0.3V----Production0.9V----Temperature

0.6V----Supply

0.3 + 0.9V + 0.6V = 1.8V

So, the voltage across the resistor canincrease by 1.8V. The current in the LED isnow 0.48A. This is not yet at the maximumcurrent published for the thin film amber-redGolden Dragon LED, but heat dissipation atthis current level may cause the maximumjunction temperature to be exceeded. This isstill significantly above the design intent ofthe LED. The design must account for thismuch variation to prevent LED damage. Thepower dissipated in the LED and resistor willincrease substantially, and must be takeninto consideration. The current could alsodecrease when these tolerances move in theopposite direction. If all the tolerances werein the opposite direction, the LED currentwould drop to 0.2A. This could create

problems from intensity variation and thespecification may not be satisfied.

Special consideration must be given to thesefactors to be sure the LEDs maximumcurrent rating and the maximum junctiontemperature are not exceeded at any time inthe application when using a series resistor.This means the LED must be used at anominal level far below its capacity. Usingthe Golden Dragon LED at a reducedcapacity with a series resistor will require

more LEDs. This can significantly increasesystem costs. In most applications, the costsaved by using only the needed GoldenDragon LEDs and eliminating the specialassembly costs of a high power resistor, willeasily cover the cost of a current controlsupply.


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CURRENT CONTROLThe current control supply can often beassembled on the same board as the LEDsto save on assembly costs. When doing this,

ensure the current control supply does notexcessively heat the LEDs.

In this example, the voltage regulator usedcould have been configured as a currentsource with no added cost to the system. Ifthe regulator had been eliminated from theexample, then the wide variations typical foran automotive application (9V to 16V) wouldhave overstressed the LED more than whatwe saw in the example.Designing in a current control supply has

several benefits: Reduced LED count and thus lower

costs

The current control supply can be allSMT components, and a high powerresistor can be eliminated. Thissignificantly reduces assemblycomplexity. Again this will give a costsavings.

Reduced power dissipation at the

higher temperature, higher supplyvoltage and lower LED forwardvoltage conditions (as compared to aresistor drive).

Constant intensity output will give abetter, more uniform appearance,and can better satisfy a tighttolerance specification.

Current control methodsThere are two technologies to considerwhen designing a current control driver forthe Golden Dragon LED. The first is a linearcurrent control driver, and the second is aswitch mode current control driver. Thisapplication note will consider the basics ofusing both, but will not detail designs.Please seek information from the individualIC manufacturers for designing with theirparts.

Linear current supplyVery similar to a voltage regulator, the linearcurrent supply uses a linear pass elementwith a feedback mechanism that regulatesthe current in a path rather than the voltageat a node (see Figure 4).

Bias Control

Feedback

Vsupply

Figure 4: Principle of a linear current controlMany adjustable voltage regulators can beconfigured to operate as a current regulator.(The IC manufacturer will have suggestionson maintaining regulator stability in theseconfigurations) There are specialty partsdesigned as a current source or sink forother applications that can be enlisted todrive an LED. (battery chargers, solenoiddrivers, etc.)

The linear current supply can only be usedwhen the input voltage is always higher than

the output voltage. Headroom must also beadded to account for drop voltages in thedriver circuits. Otherwise there is not enoughvoltage available to operate the LEDs.

The advantages of a linear current supplyare its simplicity of design, and lowcomponent cost. A linear current supply hasa disadvantage in that in regulating current,a large amount of power must be dissipatedby the supply. This can occur when there isa large voltage drop across the supply. (Thisdissipation will always be lower than aresistor drive where the current will increaseas the voltage increases.) This oftenrequires a heat sink. Since the GoldenDragon LED requires a heat sink in mostapplications, the two can be combined into asingle heat sink.


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Linear current control supply designexampleLets consider an application examplerequiring three Golden Dragon LEDs in a

string. (Three LEDs are needed based onminimum light requirements) We will use aninput supply voltage of 15V (VSupply).With 3 LEDs (n) each having a 2.2V forwardvoltage (Vf), the current control supply (ordriver) is left to drop the remaining voltage(Vdriver).

WattsAVP

IVP

VVVV

VnVV

94.2350.0*4.8

*

4.82.2*315

*

driver

diodedriverdriver

driver

diodesupplydriver

==

=

==

=

The driver needs to drop 8.4V, so the powerdissipated is 2.94W, which can be verydifficult to dissipate depending on themaximum ambient temperature.To reduce this dissipation, add an LED tothe string so the power is used by theadditional LED to generate light, not beingwasted in the supply. The power will stillneed to be dissipated by the additional LED,

but more light is generated for use.This additional light reduces the requiredcurrent. This means the Dragons only needto run at about 280mA, and there is still asurplus of light. By adding one LED to thestring, the supply now has to drop 6.2V. Inaddition, with the LEDs operating at 280mA,the power to dissipate in the current controlsupply is 1.74W. This is manageable withonly PCB copper area around the driver IC,or a small heat sink depending on themaximum ambient temperature of the

system.From this example we can see that adding asingle LED to the string can make the wholesystem more thermally manageable. Thisimproves the overall system thermal man-agement by reducing power loss in thedriver and increasing margin of the design.This will improve the reliability of the entirelighting system. In every case, the designer

must evaluate all three parts of the design:thermal system performance, electricalsystem performance, and system assemblyefficiency, to properly optimize the system.

Examples of Linear current controlsupply circuits:

Figure 5: TLE4242G, an example of a linearcurrent controlFigure 5 is an example of a linear currentcontrol, the TLE 4242G from InfineonTechnologies. This circuit offers simplicity,and is suitable for use in automotiveapplications. There is a PWM input to controlLED brightness, or with a constant low, thepart will shut down and consume less than1A. There is also a status feedback fordiagnostics. The part includes over-temperature and short circuit protection.

Figure 6: LM2941, an example of a linearcurrent control


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Figure 6 is an example of a linear currentcontrol, the LM2941 from NationalSemiconductor. It offers simplicity in designand application.

National LM20VinGND

VoutNC

10 F 100 F

9V to 16 V

On/Off

05

+-

National L 2941

Au

OO

G IN O

LF442

LF442

+-

1K 9K

6.98K

499K69K

2.87K 10K

1K

Figure 7 LM2941, an example of a linear current controlFigure 7 is another example of a linearcurrent supply using the LM2941 fromNational Semiconductor. It offers atemperature compensation of the currentlevel. The current is reduced at highertemperatures to allow use of a smaller heatsink while not violating the maximum

junction temperature of the LED at higheroperating temperatures. Both circuits aresuitable for automotive applications and offerover-temperature protection.

Switching current supplyThe switching power supply is well known inthe mobile appliance marketplace. Typicallyused to conserve battery power, the

switching power supply is desired for itsefficiency, which is its primary advantage.This efficiency allows a switching powersupply to control the voltage for a largecurrent with little power dissipation. This alsoapplies to controlling the current in a loadwith very little power dissipation. A switchingcurrent control supply has a secondaryadvantage that it often does not need a heatsink to keep cool. This becomes valuable

when large numbers of LEDs are used in asingle application.

A switching current control supply is afrequency-based device. This adds to thecomplexity of the design. It must be carefullydesigned to be frequency stable, and not

radiate electromagnetic noise in undesirablespectrums. The latter is known as designingfor electromagnetic compatibility (EMC).Individual manufacturers of switching powersupply integrated circuits can provideassistance in both of these requirements.

Compared to a linear current control supply,the switching current control supply canhave a higher cost and more components.This cost disadvantage can often be offset

by not needing a heat sink and redundantlinear current control drivers, but this mustalways be evaluated for each application.


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Different topologies of switching powersupply current controlThere are three primary topologies ofswitching power supplies:

The buck regulatorThe boost regulatorThe buck-boost regulator

Determining which topology to use in thedesign is based on the input and outputvoltage levels:The buck regulator can only function whenthe supply voltage is always higher than theload supply plus some voltage drop from thebuck circuitry.

The boost regulator can only function whenthe supply voltage is always lower than theoutput voltage minus some voltage dropsfrom the boost circuitry.The buck-boost regulator can function whenthe supply voltage can vary above or belowthe load voltage, or when the load voltagecan vary above and below the supplyvoltage. The former can occur when multiplesources are used, or a source varies widely.The later can occur with a current control

supply.

The buck topology is used most often whenthe power dissipated in a linear currentcontrol supply would be excessive.The boost topology should be used whenthere are too many LEDs to drive using alinear current control supply. As the numberof LEDs increases, and the cost (and heat)of the linear current control supplies alsoincreases, a cross-over point is quicklyfound where the switching current controlsupply is significantly cheaper than the linear

current control supplies. The LEDs arearranged all in one string, which makes theoutput voltage higher than the input supplyvoltage. Putting more LEDs in a stringreduces the number of strings in the system,and therefore reduces the total number ofcurrent control drivers needed.

The buck-boost topology is often used inapplications where the application ispowered from the AC mains and the rectifiedvoltage varies from 0V to the peak voltage.This topology is also used when theapplication must operate from many varyingsupplies (i.e. 120VAC and 240VAC systems,or 12V, 24V and 48V DC systems).

Most switching power supply integratedcircuits can be used in multiple topologies.The manufacturer can provide optionalconfiguration information of their parts.Again, in every case, the designer mustevaluate all three portions of the design:thermal system performance, electricalsystem performance, and system assemblyefficiency to properly optimize the system.

Examples of a buck topology switchingcurrent control supply:Figure 8 shows an example of a bucktopology switcher, the MLX10801 fromMelexis. This circuit features a digitalcalibration interface for tuning the currentlevel at production end-of-line, and a remotetemperature sense diode input to shut downthe driver when temperatures reach aspecified maximum. This part is suitable forautomotive applications.


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Figure 8: MLX10801, an example of a buck topology switcherExamples of a boost topology switchingpower supply:Figure 9 is an example of a boost topologyswitching power supply, the EL7512 fromIntersil.This circuit features an output over-voltage

protection for applications where the loadmay be removed from the supply, and anextended supply operating range up to 16 V.The Rset (36K) resistor is used to set the

LED current level, which can have a maxrange of 200mA to 500mA depending on theinput supply voltage.

Figure 10 is another example of a boosttopology switcher, the LM2733 from NationalSemiconductor. This circuit features

simplicity, and a 1.6MHz operatingfrequency for small component size.

Figure 9: EL7512, an example of a boost topology switching power supply

10

1K

Figure 10: LM2733, an example of a boost topology switcher


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02.02.05 page 11 of14

Figure 11 is another example of a boosttopology switching power supply, theZXSC400 from Zetex. This circuit featuresdriving a white Golden Dragon LED from 2NiMH or NiCd cells, and an external powerswitch to allow a wide range of transistors tobe selected based on the current andvoltage needs of the load.

1m

Figure 11: ZXSC400, an example of a boost topology switching power supply

Example of a buck-boost topologyswitching power supply:Figure 12 is a concept example of a buck-boost topology switching power supply, theLM2673-ADJ from National Semiconductor.This circuit can take a varying supplyvoltage, and drive a string of LEDs operatingat a voltage in the middle of the range of thesupply voltage.

1m

Figure 12: LM2673-ADJ, an example of a buck-boost topology switching power supply


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Figure 13 is an example of a buck-boosttopology switching power supply, theHV9906 from Supertex. This circuit isactually a buck-boost-buck, but it demon-strates the function of a buck-boost. Thiscircuit offers operation directly from the AC

power mains with power factor correction.

The buck-boost nature allows this circuit todo the power factor correction. There are noelectrolytic capacitors in the circuit, whichimproves long-term reliability. There is a 7Vversion of this part allowing it to function inautomotive applications as well.

100 nF

6o2V

56 H

8M VinVonVddAGND

GateNSPS

PGND70 W

1K

1N4007MURS160 15 H

MU

.4 uF

IR

A

1F

MURS160

10 nF10 nF

Figure 13: HV9906, an example of a buck boost topology switching power supply


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DIMMING THE GOLDEN DRAGON LEDOSRAM Opto Semiconductors haspublished an application note on dimmingInGaN LEDs titled Dimming InGaN LEDs.

Since the white Golden Dragon LED isInGaN based, this application note shouldbe consulted if the application will bedimming the LED.

Dimming with a switching power supply canbe difficult. Since the supply is frequencybalanced, rapidly switching the supply onand off can create instabilities in the system.Some higher frequency switching powersupply integrated circuits have a dimminginput that allows a PWM input. Others have

an analog input pin that allows dimming bychanging the current level of the LEDs.(This, as mentioned in the dimmingapplication note, can change the hue of thewhite Golden Dragon LEDs emitted light)Care must be taken when PWM dimming aswitching power supply, as some partsaccept a PWM input, but convert this input toa change in the applied forward current ofthe LED rather than pulsing the LED andchanging the average forward current.Consult with the individual integrated circuit

manufacturers on how their parts utilize aPWM input.

SUMMARY To minimize lighting system costs,

and maximize performance, all areasmust be evaluated and optimized:thermal design, electrical design,optical design and system assemblyefficiency.

The Golden Dragon LED can bedriven with a series resistor, but at agreatly reduced capacity. It needs acurrent control supply to maintainreliable and consistent performancein most applications.

The lighting system thermalperformance is key: keep the thermal

resistance of the system as low aseconomically possible for bestthermal efficiency to keep the LEDand current control parts count low.Good thermal management will alsoimprove the reliability of the system.

Design the system to keep the LEDsjunction temperature as low aspossible, because a cool LEDgenerates more light than a hot LED.In all cases design the system toprevent the LEDs junction tempera-ture from exceeding the ratedmaximums given in the data sheet.

A linear current control supply is bestsuited when the load voltage will be

just below the input supply voltage(to minimize power dissipation in thecurrent control supply) and whenthere are only a few strings.

A buck topology current controlsupply is used in applications wherethe input supply voltage will alwaysbe above the load voltage, and thepower dissipated is more than whatcan be reasonably handled with alinear current control supply.

A boost topology current controlsupply is used when the input supplyvoltage will always be below the loadvoltage, or when there are manyLEDs to drive. In the later case, withmany strings, several supplies wouldbe needed, or a current divider wouldbe needed. Putting the LEDs intoone longer string reduces the totalsupply need, but the voltageincreases above the input supply.This then requires a boost topology

current control supply.

A buck-boost topology current controlsupply is used when the supply andload voltages will not consistently behigher or lower than one another. Insome cases, adding LEDs to thestrings and using a boost topologycurrent control supply can offer abetter solution.


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APPENDIXThe following is a list of integrated circuit manufacturers and module manufacturers who have aproduct that is designed for driving LEDs or can be configured to drive LEDs. This is onlyprovided as a reference, and is not exhaustive.

Advanced Analogic Technologies Inc. www.analogictech.com

Advanced Transformer Company www.advancedtransformer.com

Allegro Microsystems Inc. www.allegromicro.com

Intersil Corporation www.intersil.com

Fairchild Semiconductor Corporation www.fairchildsemi.com

Infineon Technologies AG www.infineon.com

IXYS Corporation www.ixys.com

LEDdynamics www.leddynamics.com

Linear Technology Corporation www.linear.com

Lumidrives Ltd. www.lumidrives.com

Maxim Integrated Products www.maxim-ic.com

Melexis Microelectronic Systems www.melexis.com

Microsemi Corporation www.microsemi.com

National Semiconductor Corporation www.national.com

On Semiconductor www.onsemi.com

Power Integrations Incorporated www.powerint.com

ST Microelectronics www.st.com

Supertex Incorporated www.supertex.com

Sipex Corporation www.sipex.com

Texas Instruments Incorporated www.ti.com

Toko Incorporated www.toko.com

Zetex Semiconductors www.zetex.com

Author: Timothy Dunn

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.

Driving the Golden Dragon LED - Farnell Uk

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