driving leds - resistors and linear drivers: led fundamentals
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DESCRIPTIONIn this presentation, on Driving LEDs – Resistors and Linear Drivers, we will look at simple resistor based current regulation for LED systems and the use of linear drivers to regulate current in an LED system.
- 1. LED FundamentalsDriving LEDs Resistors and LinearDrivers08-19-2011
2. IntroductionNormalized Spectrum Behavior at 24, 35, 50, 70 Degree Celsius 1,00 TYINTENSIT Proper driving of LEDs is required to0,80address some of the fundamental0,60variations that all LEDs may have due Spectrum Behavior at 24, 35, 50, 70 Degree Celsius6,00E-05INTENSITY 0,40to manufacturing tolerances. 0,204,00E-055,00E-05 WAVELENGTH [nm] 0,00580 590 600610 620630640 650660670 6803,00E-05 There are different methods used to2,00E-05drive LEDs. These methods can be1,00E-05 WAVELENGTH [nm]0,00E+00 ,very simple or complicated, depending 380 430 480530580630680730780on the application. SSome of the key parameters neededf th k t d dto choose proper driving methodinclude expected Tj (JunctionTemperature), expected Vf mismatchbetween LEDs, color accuracyb t LEDlrequired at the system level, and ifdimming of the LEDs is required forthe application.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 2 3. Need for Current Regulation in LED Systems The I-V characteristics of an LED plays akey role in deciding what type of regulationis best suited for driving LEDs. Due to the fact that a small increase involtage, once th threshold is reached, will lt the th h ld i h d illsignificantly increase current through anLED, regulating current is moreappropriate for driving LEDs. Also, current regulation is required in LEDsystems to control and maintain: C l shift vs LED current Color hift t Flux or light output vs LED currentLED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 3 4. Driving MethodsResistor driveSwitching regulatordrive is not includedin this presentationpLED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 4 5. Resistor Drive Very Inefficient If LED current of 350mA is required: Using the Vf VS If Graph, the Vf of the LED at 350mA would be 3.2V Using Ohms Law we calculate the value of theIf resistor:If(12V 3.2V) / .350A = 25.1 Ohms The power dissipation of the resistor would beOhms Law (.350A*.350A)*25.1 Ohms = 3.08 WattsVery simple (Vbatt V f )solution If Very inefficient R This is just an example. Power wasted on Where If is LED current,the resistor is a function of source voltage. The smaller the difference between voltage source andVf is LEDs forward voltage Vf of the LED, the less power wasted on theand Vbatt is the supply voltage resistor.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 5 6. Resistor Drive - Change in Vf of LEDs Vf difference can come from two sources: Vf difference comes from process variationduring manufacturing.(typically in the range of ~250mV)250mV) Change in Vf due to junction temperature (Tj)of the LED. Graph on the right shows change in Vf due to Tj. F the same circuit on th previous slide, with aFor thi it the i lid ithsupply voltage of 5V, a change in Vf of 250mV willresult in the following LED current:Resistor = (5V 3 2V)/350mA = 5 2 ohms (5V-3.2V)/350mA 5.2(5-3.45)/5.2 = 304mA OR(5-2.95)/5.2 = 394.2mALED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 6 7. Resistor Drive - Source Voltage Tolerances The tolerances on a voltage source will impact LED current in resistor based current regulation. Assuming a +/-10% tolerance, which is very reasonable, reasonable on the voltage source for the same circuit, the LED current can vary between 302.8mA and 398.4mA. If the voltage source had a tolerance of +/- 10%:+/ The change in LED current will impact the 10% of 12V is 1.2V following: With tolerance of +10%, LED current would be Flux or light output gp (13.2V 3.2V) / 25.1 Ohms = 398.4mA Color shift due to LED current Efficacy of the LED/ circuit/ systemWith tolerance of -10%, LED current would be ( (10.8V 3.2V) / 25.1 Ohms = 302.8mA)LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 7 8. Resistor Drive - Flux Varies with LED Current Flux at 302mA is ~90% of flux at 350mA Flux at 398mA is ~115% of flux at 350mA For calculation purposes, if flux at 350mA isassumed to be 100lm: Flux at 302mA is ~90lm Flux at 398mA is ~115lm115lm If resistor based regulation is used in a systemwhere the source voltage has a tolerance of +/-10%, the light output can vary.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 8 9. Resistor Drive Drive-Color Point Can Vary with LED Current Consider the same circuit analysis on the previous slide, in which the LED current can be either 302mA or 398mA. Based on the graph on the right, the Cx, Cy shift at an LED current of 302mA can be ~0.00125 and ~0.00250, respectively. At an LED current of 398mA, the Cx, Cy shift can bt f 398 A th C C hift be -0.00125 and -0.00250, respectively. The color shift due to varying LED current cannot be addressed in a resistor drive.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 9 10. A Slightly Different Approach to Resistor DriveFixed Voltage SourceHow does the circuit work?1. The op-amp automatically adjusts its output (NPNs base d i ) t b i it negative input (NPN b drive) to bring itsti itLEDequal to the positive input. This means that Vref = VR (voltage across resistor R).Iled = Vref / R OP-AMPAVref542NPN 2. A simple application of Ohms law would make current through R and the LED = Vref / R.R 3. NPN delivers the current, and because the NPNs currents are related by Ic Ie, the samey , current that is developed through R must also flow through the LED. GNDLED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 10 11. Advantage and Disadvantages of Resistor DriveThe advantages of resistor based regulation includes: Cost effective solution Very simple and ideal for space constrained applications PPossible solution f applications th t have a l t of variables and i ibl l ti for li tithat h lot f i bl d issues such as flh flux and Vf variations that dont need to be addressed, eg. flashlights/ torch lightsHowever,However there may be issues with this type of driving method: Very inefficient Vf mismatch or variation due to Tj cannot be addressed with this method SSupply voltage variation i not addressedllt i ti is t dd d Flux variation cannot be addressedThe OP-AMP + Transistor method: Source/ supply voltage variation issue addressed Vf mismatch or variation due to Tj will be addressed More stable LED current due to corrections related to the previous two issuesLED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 11 12. Linear Regulators Linear regulators can either be fixed voltage or constant currentUse of a resistor to regulatecurrent through the LED isrequired in fixed voltageregulators If is LED current(V out V f ) I f R Fixed voltage regulator compensates for source voltage variations, but does notcompensate for variations in the LED forward voltage. If being used by other circuits in an LED system, can also be used for LED drive.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 12 13. Linear Regulator Based Driving Constant current linear regulators are verysimple to design and are widely used in LEDsystems. National Semi, Infineon Technologies, Zetex, andTI are some of the suppliers to name a few. f th li t f Linear regulators: Eliminates changes in source voltages issue. M i t i constant current and brightness inMaintainst t t d b i ht iLEDs. minimal passives, a capacitor Compensates for changes in LED forwardand a resistor in most casesvoltage.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 13 14. Linear Regulator Based Driving optionalTLE 4309G Adjustable OutputCurrent up to 500 mA Features Low dropout voltage PWM Input (dimming)I t (dii ) Reverse Polarity Protected Over-Temperature Protection Max. Operating Voltage: 24 V Short Circuit Protection to GND and Vsupply Max. Input Voltage: 45 V Package PG-TO-263-7PG TO 263 7LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 14 15. Linear Regulator Based DrivingBCR40X Total forward voltage must be lower gthan the supply voltage (more like a Output current adjustable bybuck type)external resistor from 10mA to Efficiency is not as good as a switching65mAregulator. Suitable for PWM for dimming Some of the features of BCR40X arelisted on the right Negative temperature coefficient Linear regulators are ideal for low serves as protection for LEDs LED scurrent LED applications Pretested output current If higher current is needed, then Low voltage dropexternal circuitry i requiredtl iit isi dLED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 15 16. Linear Regulator Based Driving With an external circuit, the LED current can be made as high as 500mA.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 16 17. How Stable is Linear Regulator Based Driving? Linear regulators are very stable over a wide range oftemperatures. The temperature characteristics of the BCR4X familyfrom Infineon Technologies is shown on the right. The reference voltage, which is directly proportional toLED current at two different temperatures (20C and100C),100C) was examined.i d The delta between 20C and 100C is only about1mV, hi h1 V which means th t th LED current will b verythat the t ill bestable over a wide range of temperature. LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 17 18. How Much Power Wasted in Linear Regulators?Power on TLE4309 ~ (Vin Vout) * Current (LED current = 350mA)~ (12 6.4) * 0.35 (Vf of two LEDs = 6.4V)~ 2WLED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 18 19. Advantages and Disadvantages of Linear Drive Advantages of linear current regulators: Better/more stable regulation compared to resistor regulation Vf mismatch between LEDs addressed Vf change due to temperature is addressed Stable current regulation over a wide temperature range Disadvantages of linear current regulators: Not very efficient y Costly comp