APPLICATION NOTES: LED Electrical Driving Consideration Introduction: THE NEEDS OF A CONTROLLED CURRENT SOURCE The IV curve for LED is similar to the IV curve for normal diode, except that it has steeper slope of If versus Vf in the high current region. As an example from the figure 1 below, variation of 0.1V supply voltage will cause the forward current differs by approximately 50mA. Since the luminous intensity of LED varies with forward current, this causes the relative intensity to vary by >10%. Therefore, LED circuitry should be designed to drive by controlled current source rather than controlled voltage source. With rapid advancement in semiconductor technology, various IC component suppliers are capable of producing constant current LED driver with output current accuracy +-3.5% between drivers and current matching smaller than 0.5% within driver ( for LED driver with more than 1 output channel drive). Figure 1– SPNovaLED InGaN White 350mA If vs Vf and Iv vs If Characteristic 06/05/10 -1- Ver A LED Electrical Driving Consideration GENERAL LED CIRCUIT DESIGN OVERVIEW Current Limiting Method Resistor ( most common in existing automotive circuit design ) Advantage • • Easy to design. Inexpensive. Disadvantage • • • Linear Current Regulator • • Switching Regulator Control • • • • 06/05/10 Allow dimming through current control. Linear control-loop circuitry accurately controlled the LED current. Efficient operation over wide range of input voltage. Able to regulate LED current precisely. Allow dimming by PWM. Lower power dissipation. -2- • • • • • • It cannot control current accurately. Current varies to some extent with supply voltage. High power dissipation in the resistor especially on high power LED. More expensive than simple resistive current limiter. High power dissipation. Might require heat sink for active pass device. More complex design. More expensive compare to linear current supply. Need to design for electromagnetic compatibility. Ver A LED Electrical Driving Consideration Current Limiting Method : Resistor Series LED Drive Configuration : Pros : • Matching guaranteed with well regulated LED current • Easy implementation with inductive boost • Only 1 ballast element, most power efficient, ballast loss= VBallast x ILED Cons : • High output voltage drive required for long string of series LED. For low input voltage application, boost type regulator might be required • A single defective LED would cause the whole string to fail Parallel LED Drive Configuration : Pros : • Single defective LED will not cause all LED network to fail • Lower output voltage drive required Cons : • Require regulated current source to ensure good matching, 1 for each LED • More power loss compare to series implementation. Total ballast loss = N x VBallast x ILED 06/05/10 -3- Ver A LED Electrical Driving Consideration Parallel Strings of LEDs Configuration : Pros : • Good for driving lots of LEDs • Reduce the output voltage drive requirement compare to the series implementation for the same LED count. • Lower power loss ( ballast loss ) compare to pure parallel implementation for the same LED count. Cons : • Only the 1st string has regulated current. Diode voltages set the current for all secondary strings. • LED voltage mismatches would lead to brightness difference from one string to another. Current Limiting Method : Linear Current Regulator Figure 2, a typical LED circuit design using linear current regulator NUD4001 produce by On Semiconductor. Vin Iout 1 8 Iout 4 Ohm Boost Rext 2 NUD4001 7 Iout 3 6 4 5 GND Iout ILED =175mA 12 V SPNovaLED 0.5W LED1 - LED4 Features : • Up to 30V operating range • Adjustable output current level up to 500mA • Minimum component count • Suitable for automotive application in tail light, CHMSL and internal lighting application Figure 2 – On Semiconductor NUD4001 linear current regulator 06/05/10 -4- Ver A LED Electrical Driving Consideration Current Limiting Method: Switching Regulator Control A. SWITCHING POWER SUPPLY CURRENT CONTROL DRIVER With ever demanding for higher brightness LED and higher power efficiency especially for portable power application, there is trend for major IC component manufacturer to introduce more advance switching LED current control driver with various functionality and better current matching/regulation. When choosing the switching power supply LED current control driver, consideration should be taken on the supply input voltage level versus the output LED load voltage level. Basically the are 3 type of topologies for switching power supply : 1. Buck Regulator It can function if the input voltage always exceeds the sum of the maximum forward voltages of every LED in a string. It can reduce the output voltage to a lower level to minimize power loss. 2. Boost Regulator When the sum of all the forward voltages drop in a string will always exceed the maximum input voltage, a boost regulator is needed to amplify the output voltage to the required range in order to drive the LED network. 3. Buck/Boost Regulator When the input voltage might swing above or below the sum of all the forward voltage drop in a string, then a buck/boost regulator should be use to drive the LED network. 06/05/10 -5- Ver A LED Electrical Driving Consideration Various LED driving circuitry is depicted as below, please refer to respective IC manufacturer for detail design consideration when using their LED driver. Switching Buck Regulator Figure 3, a high power LED driving circuit design using National Semiconductor LM3402 0.5A constant current buck regulator. Vin range from 6V to 42V ( LM3402 ). Figure 3 – LM3402, an example of buck regulator Figure 4, another LED circuit design using Texas Instrument bq24105. The bqSWITHER series can achieve 10% current regulation accuracy for 100mV to 200mV sensing resistor voltage. Figure 4 – bq24105, an example of buck regulator 06/05/10 -6- Ver A LED Electrical Driving Consideration A list of market commonly available buck regulator is as tabulated below : Type Buck 06/05/10 Supplier Part Number Vin (DC) Exar SP6652 2.7 to 5.5 5 1000 Exar SP6655 2.7 to 5.5 5 400 STMicroelectronics Melexis National Semiconductor National Semiconductor National Semiconductor National Semiconductor National Semiconductor National Semiconductor National Semiconductor National Semiconductor National Semiconductor National Semiconductor Sipex STMicroelectronics Zetex Zetex Zetex Zetex Zetex Zetex Zetex Zetex National Semiconductor Sipex Catalyst L6926 2 to 5.5 5 800 MLX10803 6 to 32 32 External LM3401 4.5 to 35 35 External LM3407 4.5 to 30 27 350 LM3402 6 to 42 41 500 LM3402HV 6 to 75 74 500 LM3404 6 to 42 41 1200 LM3406 6 to 42 37 1500 LM3406HV 6 to 75 67 1500 LM3405A 3 to 22 21 1000 LM3404HV 6 to 75 74 1500 LM3489 4.5 to 35 Adjustable External SP6137 3 to 20 3 to 15 External L6902 8 to 36 34 1000 ZXLD1350E5 9 to 30 30 350 ZXLD1350 7 to 30 30 350 ZXLD1360 7 to 30 30 1000 ZXLD1362 6 to 60 60 1000 ZXLD1320 4 to 18 18 1500 ZXLD1352 7 to 30 30 350 ZXLD1356 6 to 60 60 550 ZXLD1366 6 to 60 60 1000 LM2734 3 to 20 18 1000 SP7601 4.5 to 29 29 External CAT4201 6.5 to 20 32 350 -7- Vout(max, DC) Iout (max, mA) Ver A LED Electrical Driving Consideration Switching Boost Regulator Figure 5 below shows a typical switching boost regulator circuit design using Linear Technology LT3466-1, white LED driver and boost converter. It can drive up to 10 white LEDs from a 3.6V supply. It allows +-4% current programming accuracy. Figure 5—LT3466-1, an example of boost regulator Figure 6, another switching boost regulator design using On Semiconductor NCP5010, 500mW boost converter for white LED. It featuring 2.7V to 5.5V input voltage range. 84% Efficiency for 5 LEDs at 30mA and Vin =4.2V Figure 6—NCP5010, an example of boost regulator 06/05/10 -8- Ver A LED Electrical Driving Consideration A list of market commonly available boost regulator is as tabulated below : Type Boost Supplier ON Semiconductor ON Semiconductor Exar Exar Exar Zetex Zetex Zetex National Semiconductor National Semiconductor National Semiconductor National Semiconductor STMicroelectronics Part Number Vin (DC) Vout(max, DC) Iout (max, mA) NCP1422 1.0 to 5.0 5 800 NCP1450A 0.8 to 6.0 6 1000 SP6641B 1.0 to 5.0 6 500 SP6648 0.7 to 4.5 4 400 SP7648 2.7 to 4.5 2.7 to 5.5 800 ZXSC310E5 0.8 to 8.0 8 1100 ZXSC400E6 8 1100 ZXLD1321 1.8 to 8.0 1.2 to 12.0 18 1000 LM2623A 0.8 to 14 14 1000 LM2700 2.2 to 12 17.5 1000 LM3551 2.7 to 5.5 11 1000 LM3553 2.7 to 5.5 19 1200 L6920 0.6 to 5.5 8 500 Switching Buck/Boost Regulator Figure 7 below shows a typical switching buck/boost regulator circuit design using Linear Technology LTC3453, Synchronous Buck Boost High Power White LED Driver. It is optimized to drive up to 4 white LEDs at a combined current of 500mA from a single Li-Ion battery input. The regulator operates in either buck, boost or buck-boost mode, depending on the input voltage and LED maximum forward voltage. Figure 7-- LTC3453, an example of buck/boost topology regulator 06/05/10 -9- Ver A LED Electrical Driving Consideration Figure 8, another switching buck-boost regulator circuit design using MAXIM MAX1759, Buck-Boost Charge Pump Regulator. In the design below, the regulator biases white LEDs with 15mA current from a wide 1.6V to 5.5V voltage range. Figure 8—MAX1759, an example of buck/boost topology regulator A list of market commonly available buck/boost regulator is as tabulated below: Type Buck/Boost 06/05/10 Supplier Part Number Vin (DC) Vout(max, DC) Iout (max, mA) STMicroelectronics STMicroelectronics STCF02 2.7 to 5.5 2.5 to 5.3 600 STCF03 2.5 to 5.3 800 Zetex ON Semiconductor ON Semiconductor ON Semiconductor National Semiconductor National Semiconductor Zetex ZXLD1322 2.7 to 5.5 2.5 to 15.0 15 700 NCP5030 2.7 to 5.5 5.5 1200 NCP3163 2.5 to 40 40 3000 NCP3065 3.0 to 40 40 1500 LM3423 4.5 to 75 75 External LM3421 4.5 to 75 75 External ZXLD1322 2.5 to 15 15 700 - 10 - Ver A LED Electrical Driving Consideration B. ADVANCE LIGHT MANAGEMENT UNIT (ALMU) With the increasing popularity of LED application in various fields, especially in mobile handheld application and display backlighting, several major integrated circuit supplier have already starts to produce the advance light management IC. This ALMU not only regulate the LED current, but also provide various functionality to manage the brightness, sequence, and to synchronize the LEDs with audio input. The ALMU usually will come with an I2C or SPI interface. This ALMU will further reduce the space required for LED control circuit and will simplify the circuit design effort. Several examples are depicted below. Please consult respective IC manufacturer’s website for detail information. National Semiconductor, LP3954, Advance Light Management Unit Features : • • • • • • Advance Light Management Unit for handheld product Audio synchronization for color/RGB LEDs High current driver for flash LED with built-in timing Six constant current white LED driver with 8-bit programmable adjustment SPI/I2C compatible interface Possibility for external PWM dimming control Figure 9—LP3954 Advance Light Management Unit 06/05/10 - 11 - Ver A LED Electrical Driving Consideration MAXIM MAX1707, Light Management Unit Figure 10 – MAX1707 Light Management Unit Features : • • • • • 06/05/10 The MAX1707 provides complete light management for main display backlight, sub-display backlight and white LED camera flash with regulated constant current up to total 610mA. An I2C serial port is used for on/off control and setting the LED currents in 32 linear steps. 0.3% LED current accuracy and matching. The I2C port provides 32k colors and programmable ramp up/down rates. The camera flash may be turned on/off by the I2C port - 12 - Ver A LED Electrical Driving Consideration LED CIRCUIT PRACTICAL DESIGN GUIDE For automotive/general lighting application, the most common input supply voltage would be 12V & 24V DC. The most common LED drive current level would be 120mA -150mA for SPNovaLED 0.5Watt and 320mA - 350mA for SPNovaLED 1Watt. Two examples of reference design to drive a series of LEDs at Vin =12V, using On Semiconductor NUD4001, High Current LED Driver is depicted below. Please refer to NUD4001 datasheet for further technical details. Reference Design 1 : Driving 4 Series Connected SPNovaLED 0.5Watt At Forward Current of 150mA , Supply Voltage, Vin = 12V Vin Iout 1 8 2 7 Iout 4 Ohm Boost 4.7 ohm Rext NUD4001 Iout 3 6 4 5 GND Iout ILED =175mA ILED =150mA 12 V SPNovaLED 0.5W LED1 - LED4 Bill Of Materials: Part Manufacturer Part Number Rext Vishay CRA06E0803472JRT1 4.7 Ohm resistor NUD4001 On Semiconductor NUD4001DR2G High Current LED Driver LED1-LED4 Dominant Semiconductor NPR-SSS-XY2-1 SPNovaLED 0.5W Red 06/05/10 - 13 - Attribute Ver A LED Electrical Driving Consideration Several key parameters for NUD4001 are as follow : 1. Vsense = 0.7 V at Tj= 25 ºC 2. P D_control, Power dissipation for internal circuitry of NUD4001 is 0.055W at 12V input, 0.25W at 24V input. 3. Maximum power dissipation of NUD4001 is 1.13W. 4. It supplies constant LED current for varying input voltage up to 30V max 5. Output current level is defined by the resistor, Rext Electrical Design Guide: 1. LED current required, ILED is 150mA 2. Resistor value for Rext = Vsense / ILED = 0.7V/0.150 Ohm = 4.7 Ohm Thermal Design Guide : Base on Dominant Semiconductor datasheet for SPNovaLED, for example NPY-SSSXY2, the minimum forward Voltage Vfmin =1.9V, typical forward voltage Vftyp= 2.2V. The power dissipation in the current regulator IC should be calculated for Vfmin, where the voltage drop across the current driver would be maximum. This will prevent the NUD4001 from being killed due to overheat. Vfmin = 1.9V Vftyp = 2.2V Total voltage across LED, VLED = 1.9V x 4 = 7.6V VLED = 2.2V x 4 = 8.8V Voltage drop across NUD4001, Vdrop = Vin – Vsense- VLED = 12V – 0.7V -7.6V Vdrop = 12V -0.7V – 8.8V = 3.7V = 2.5V Power dissipation across NUD4001, PD_driver = Vdrop x Iout = 3.7V x 0.175A PD_driver = 2.5V x 0.175A = 0.4375W = 0.6475W Total power dissipation, PD_total = PD_driver + PD_control = 0.6475W + 0.055W PD_total = 0.4375W + 0.055W = 0.7025W = 0.4925W Base on both case of Vf above, the total power dissipation is < 1.13W, which means the circuit above can be operated without overheating the NUD4001. 06/05/10 - 14 - Ver A LED Electrical Driving Consideration Reference Design 2 : Driving 3 Series Connected SPNovaLED 1 Watt At Forward Current of 350mA , Supply Voltage, Vin = 12V Vin Iout 1 8 2 7 Iout 2 Ohm Boost Rext NUD4001 Iout 3 6 4 5 GND Iout ILED =350mA 12 V SPNovaLED 1W LED1 - LED3 Bill Of Materials: Part Manufacturer Part Number Attribute Rext Vishay CRCW12062R00FRT1E3 2 Ohm resistor NUD4001 On Semiconductor NUD4001DR2G High Current LED Driver LED1-LED3 Dominant Semiconductor NPW-USD-AD-1 SPNovaLED 1W White Several key parameters for NUD4001 are as follow : 1. Vsense = 0.7 V at Tj= 25 Deg C 2. P D_control, Power dissipation for internal circuitry of NUD4001 is 0.055W at 12V input, 0.25W at 24V input. 3. Maximum power dissipation of NUD4001 is 1.13W 4. It supplies constant LED current for varying input voltage up to 30V max 5. Output current level is defined by the resistor, Rext Electrical Design Guide: 1. LED current required, ILED is 350mA 2. Resistor value for Rext = Vsense / ILED = 0.7V/0. 350 Ohm = 2 Ohm 06/05/10 - 15 - Ver A LED Electrical Driving Consideration Thermal Design Guide : Base on Dominant Semiconductor datasheet for SPNovaLED, NPW-USD-AD-1, the minimum forward Voltage Vfmin =3.0V, typical forward voltage Vftyp = 3.6V. Vfmin = 3.0V Vftyp = 3.6V Total voltage across LED, VLED = 3.0 V x 3 = 9.0V VLED = 3.6V x 3 = 10.8V Voltage drop across NUD4001, Vdrop = Vin – Vsense- VLED = 12V – 0.7V -9.0V Vdrop = 12V –0 .7V – 10.8V = 2.3V = 0.5V Power dissipation across NUD4001, PD_driver = Vdrop x Iout = 2.3V x 0.350A PD_driver = 0.5V x 0.350A = 0.175W = 0.805W Total power dissipation , PD_total = PD_driver + PD_control = 0.805W + 0.25W PD_total = 0.175W + 0.25W = 1.055W = 0.425W Base on both case of Vf above, the total power dissipation is < 1.13W, which means the circuit can be operated without overheating the NUD4001. Please note that for the both reference design above, care should be taken to ensure minimum input voltage, Vinmin > Vsense + VLED(Max) , in order to supply enough forward voltage drive for the LEDs. 06/05/10 - 16 - Ver A LED Electrical Driving Consideration Reference Design 3: Using National Semiconductor LM3402 ( 0.5A Constant Current Buck Regulator ). Driving 6 Series Connected SPNovaLED 1 Watt at Forward Current of 330mA, Supply Voltage, Vin = 24V ( Please refer to National Semiconductor LED Reference Design Library : http://www.national.com/webench/ledrefdesigns.do for further details and circuit simulations. ) ILED =330mA Bill Of Materials: Part Cb Cf Cin Co D1 L1 Ron Rsns U1 LED1-LED6 06/05/10 Manufacturer Vishay Vishay TDK TDK Central Semiconductor TDK Vishay Panasonic National Semiconductor Dominant Semiconductor Part # VJ0805Y103KXXAT VJ0805Y104KXXAT C3225X7R1H225M C3225X7R1E475M CMSH1-40M SLF7045T-470MR75 CRCW08054873F ERJ6BQFR56V LM3402 NPW-TSD - 17 - Value 10nF 100nF 2.2uF 4.7uF 0.5V 47uH, 0.15 Ohm 487k Ohm 0.56 Ohm SuperNovaLED 1 W Ver A LED Electrical Driving Consideration LED DRIVER The following is a list of integrated circuit suppliers that produce LED driver/LED Management device or regulator that can be configured to drive LED circuits. Please refer to their website for detail design consideration and specification. ( Note : The name of the IC manufacturers below is directly hyperlink to their respective website ) IC Supplier Analog Devices Fairchild Semiconductor Holtek Semiconductor Inc. Infineon Intersil Corporation Linear Technology Maxim Integrated Products Micrel Semiconductor MICRO ANALOG SYSTEMS MicroChip Microsemi Corp. Mitsubishi Semiconductor National Semiconductor SIPEX SUPERTEX STMicroelectronics Texas Instruments ZETEX With the rapid growth of LED industry, several premier analog IC suppliers have compiled a comprehensive LED driver guide for their products that would provide the designer with a vast choice of LED drivers based on their application needs. Four of the LED driver guide selections can be downloaded from the link below: National Semiconductor : http://www.national.com/appinfo/power/files/lighting_solutions.pdf Linear Technology : http://www.linear.com/ then select Power Management For LEDs Arrow Electronics : http://www.arrownac.com/industry_solutions/lighting/selector_guide.html Supertex : http://www.supertex.com/products/selector_guides SUMMARY : To ensure good long-term reliability and brightness uniformity, the LED shall be driven by constant current source. Since there are a wide range of LED drivers available in the market from different electronic IC manufacturers, the implementation of constant current drive becomes very simple without the need of extensive circuit design while minimizing the total component count. 06/05/10 - 18 - Ver A