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# Driving LEDs - Resistors and Linear Drivers: LED Fundamentals

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In 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.

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### Driving LEDs - Resistors and Linear Drivers: LED Fundamentals

1. 1. LED FundamentalsDriving LEDs –Resistors and LinearDrivers08-19-2011
2. 2. Introduction Normalized Spectrum Behavior at 24, 35, 50, 70 Degree Celsius 1,00 TY INTENSIT Proper driving of LEDs is required to 0,80 address some of the fundamental 0,60 variations that all LEDs may have due Spectrum Behavior at 24, 35, 50, 70 Degree Celsius 6,00E-05 INTENSITY 0,40 to manufacturing tolerances. 0,20 4,00E-05 5,00E-05 WAVELENGTH [nm] 0,00 580 590 600 610 620 630 640 650 660 670 680 3,00E-05 There are different methods used to 2,00E-05 drive LEDs. These methods can be 1,00E-05 WAVELENGTH [nm] 0,00E+00 , very simple or complicated, depending 380 430 480 530 580 630 680 730 780 on the application. S Some of the key parameters needed f th k t d d to choose proper driving method include expected Tj (Junction Temperature), expected Vf mismatch between LEDs, color accuracy b t LED l required at the system level, and if dimming of the LEDs is required for the application.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 2
3. 3. Need for Current Regulation in LED Systems The I-V characteristics of an LED plays a key role in deciding what type of regulation is best suited for driving LEDs. Due to the fact that a small increase in voltage, once th threshold is reached, will lt the th h ld i h d ill significantly increase current through an LED, regulating current is more appropriate for driving LEDs. Also, current regulation is required in LED systems 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. 4. Driving Methods Resistor driveSwitching regulatordrive is not includedin this presentation pLED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 4
5. 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 the If resistor: If (12V – 3.2V) / .350A = 25.1 Ohms The power dissipation of the resistor would be Ohms 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 and Vf is LED’s forward voltage Vf of the LED, the less power wasted on the and Vbatt is the supply voltage resistor. LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 5
6. 6. Resistor Drive - Change in Vf of LEDs Vf difference can come from two sources: » Vf difference comes from process variation during 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 a For th i it the i lid ith supply voltage of 5V, a change in Vf of 250mV will result 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. 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 g p (13.2V – 3.2V) / 25.1 Ohms = 398.4mA » Color shift due to LED current » Efficacy of the LED/ circuit/ system With 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. 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 is assumed to be 100lm: » Flux at 302mA is ~90lm » Flux at 398mA is ~115lm 115lm If resistor based regulation is used in a system where the source voltage has a tolerance of +/- 10%, the light output can vary.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 8
9. 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 b t 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. 10. A Slightly Different Approach to Resistor Drive Fixed Voltage Source How 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 its ti i t LED equal to the positive input. This means that Vref = VR (voltage across resistor “R”). Iled = Vref / R OP-AMPAVref 5 4 2 NPN 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 same y , current that is developed through R must also flow through the LED. GNDLED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 10
11. 11. Advantage and Disadvantages of Resistor DriveThe advantages of resistor based regulation includes: • Cost effective solution • Very simple and ideal for space constrained applications •P Possible solution f applications th t have a l t of variables and i ibl l ti for li ti that h lot f i bl d issues such as fl h flux and Vf variations that don’t 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 •S Supply voltage variation i not addressed l lt 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. 12. Linear Regulators Linear regulators can either be fixed voltage or constant current Use of a resistor to regulate current through the LED is required in fixed voltage regulators If is LED current (V out  V f ) I f  R  Fixed voltage regulator compensates for source voltage variations, but does not compensate 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. 13. Linear Regulator Based Driving Constant current linear regulators are very simple to design and are widely used in LED systems. National Semi, Infineon Technologies, Zetex, and TI 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 in Maintains t t t d b i ht i LEDs. minimal passives, a capacitor • Compensates for changes in LED forward and a resistor in most cases voltage.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 13
14. 14. Linear Regulator Based Driving optionalTLE 4309G Adjustable Output Current up to 500 mA Features Low dropout voltage PWM Input (dimming) I t (di i )  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-263-7 PG TO 263 7LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 14
15. 15. Linear Regulator Based Driving BCR40X Total forward voltage must be lower g than the supply voltage (more like a • Output current adjustable by buck type) external resistor from 10mA to Efficiency is not as good as a switching 65mA regulator. • Suitable for PWM for dimming Some of the features of BCR40X are listed on the right • Negative temperature coefficient Linear regulators are ideal for low serves as protection for LED’s LED s current LED applications • Pretested output current If higher current is needed, then • Low voltage drop external circuitry i required t l i it is i dLED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 15
16. 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. 17. How Stable is Linear Regulator Based Driving? Linear regulators are very stable over a wide range of temperatures. The temperature characteristics of the BCR4X family from Infineon Technologies is shown on the right. The reference voltage, which is directly proportional to LED current at two different temperatures (20°C and 100°C), 100°C) was examined. i d The delta between 20°C and 100°C is only about 1mV, hi h 1 V which means th t th LED current will b very that the t ill be stable over a wide range of temperature. LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 17
18. 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. 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 compared to resistor drive • Can only be used when total Vf of LEDs less than the supply voltage • If it is a fixed voltage type linear regulator, use of a resistor is still required g yp g , qLED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 19
20. 20. Summary In summary, the table below compares resistor drive and linear drive in LED systems/ circuits. Resistor Drive Linear Regulator Vf mismatch between LEDs addressed NO YES Vf change due to temperature addressed NO YES Source voltage variation addressed NO YES Tight current regulation NO YES Simple solution YES YES Costly solution NO YES (compared to resistor driver) Efficient solution NO NO Stable over wide-range of temperature NO YESLED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 20
21. 21. Disclaimer All information contained in this document has been checked with the greatest care. OSRAM Opto Semiconductors GmbH can however, not be made liable for any damage that occurs in connection with the use of these contents. OSRAM Opto Semiconductor GmbH makes no representations and warranties as to a possible interference with third parties intellectual property rights in view of products originating f i i ti from one of OSRAM Opto Semiconductor GmbHs partners, or in view of f O t S i d t G bH t i i f products being a combination of an OSRAM Opto Semiconductor GmbHs product and a product of one of OSRAM Opto Semiconductor GmbHs partners. Furthermore, OSRAM Opto Semiconductors GmbH cannot be made liable for any damage that occurs in p y g connection with the use of a product of one of OSRAM Opto Semiconductor GmbHs partners, or with the use of a combination of an OSRAM Opto Semiconductor GmbHs product and a product of one of OSRAM Opto Semiconductor GmbHs partners.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 21
22. 22. Thank you for your attention.LED Fundamentals | Driving LEDs - Resistors and Linear Drivers| Page 22