National Semiconductor Application Note 2076 SH Wong August 10, 2010 Introduction Standard Settings of the LM3414HV Evaluation Board The LM3414HV is a 65V floating buck LED driver that designed to drive up to 18 pieces of serial High Brightness LEDs (HBLEDs) with up to 1000mA LED forward current. With the incorporation of the proprietary Pulse-Level-Modulation (PLM) technology, the LM3414HV requires no external current sensing resistor to facilitate LED current regulation. The LM3414HV features a dimming control input (DIM pin) that allows PWM dimming control. The LM3414HV is available in LLP-8 (3mm x 3mm outline) and ePSOP8 to fulfil the requirements of small solution size and high thermal performance respectively. In order to demonstrate the performance of the LM3414 family, the LM3414HV is selected for the evaluation boards because of the wide input voltage range (4.5V to 65V) providing the best flexibility to fit the requirements of different applications. Two versions of evaluation board with identical schematic are available with the LM3414HV in either LLP-8 or PSOP-8 package. The board with LLP-8 package demonstrates the high power density of the device. The board with PSOP-8 package demonstrates the functionality of the LM3414HV with enhanced thermal performance. The schematic, bill of materials and PCB layout for the evaluation boards are provided in this document. The evaluation boards can be adapted to different application requirements by changing the values of a few components only. This evaluation board is also suitable for the LM3414 with maximum acceptable input voltage reduced to 42VDC. Vin range: 4.5V to 65V No. of LEDs: 1 - 18 LED current: 1A Switching frequency: 500 kHz LM3414HV 1A 65V LED Driver Evaluation Board LM3414HV 1A 65V LED Driver Evaluation Board 30128701 AN-2076 FIGURE 1. Standard Schematic for the LM3414HV Evaluation Board © 2010 National Semiconductor Corporation 301287 www.national.com AN-2076 Board Connectors and Test Pins 30128702 FIGURE 2. Connection Diagram www.national.com Terminal Designation Description VIN Power supply positive (+ve) connection GND Power supply negative (-ve) connection LED+ Connect to cathode of the serial LED string LED- Connect to anode of the serial LED string DIM PWM dimming signal input (TTL signal compatible) THM+ Connect to PTC thermal sensor for thermal foldback control THM- Connect to PTC thermal sensor for thermal foldback control 2 Design Example The LM3414HV evaluation board can be powered by a DC voltage source in the range of 4.5V to 65V through the banana-plug type connectors (VIN and GND) on the board as shown in figure 2. This evaluation board is designed to provide 1A (ILED)output current to a LED string containing up to 18 pieces of serial HBLEDs. The anode and cathode of the LED string should connect to the LED+ and LED- bananaplug type connectors on the board respectively. By default, the LM3414HV on the evaluation board is enabled. The LEDs will light up as long as appropriate input voltage is applied to the evaluation board. Assuming a LED string containing six serial HBLEDs is being driven by the board with 700mA (ILED). The forward voltages of one HBLED with 700mA driving current under different operation temperatures are: Vf(60C) @700mA = 3.0V Vf(25C) @700mA = 3.2V Vf(-10C) @700mA = 3.5V Step 1. Defining input voltage range Because the LM3414HV is a floating buck LED driver, the input voltage to the LED driver must be higher than the total forward voltage of the LEDs under all conditions. As the forward voltage of a common HBLED could increase as the driving current increases or the operation temperature decreases, it is essential to ensure the minimum supply voltage is at least 10% higher than the possible highest forward voltage of the LED string. For example, assuming the forward voltage of a HBLED is 3.2V at TA = 25°C and 3.5V at TA = -10°C at 700mA driving current. When 6 pieces of LED are connected in series, the total forward voltage of the LED string at 25°C and -10°C are 19.2V and 21V respectively. In order to secure current regulation under -10°C, the input voltage should not be lower than 23.1V. In this example, a standard 24V DC power supply with no more than +/– 3% output voltage variation can be used. Adjusting the Output Current The resistor RIADJ defines the output current of the LM3414HV evaluation board. The default value of RIADJ is 3.09kΩ, which sets the LED driving current to 1A. The LED current can be changed by adjusting the value of RIADJ with equation (1): (1) Table 1 shows the suggested value of RIADJ for common output current settings: ILED (mA) RIADJ (kΩ) 350 8.93 400 7.81 500 6.25 600 5.21 700 4.46 800 3.91 900 3.47 1000 3.13 Step 2. Defining switching frequency fSW When the maximum LED forward voltage and minimum input voltage are identified, the switching frequency of the LM3414HV can be defined. The switching frequency of the LM3414HV must be set in the range of 250kHz to 1MHz. Because the LM3414HV is designed to operate in continuous conduction mode (CCM) with 400ns minimum switch ON time limit, the maximum allowable switching frequency is restricted by the minimum input voltage, VIN(MIN) and maximum LED forward voltage, Vf(MAX) according to equation (3): TABLE 1. Examples for RIADJ Setting Adjusting the Switching Frequency The resistor RFS defines the switching frequency of the LM3414HV evaluation board. The default value of the RIADJ is 40kΩ that sets the switching frequency to 500kHz. The LED current is adjustable by altering the resistance of RFS according to the equation (2): (3) In this example, because a 24V DC power supply with +/- 3% output voltage variation is used, VIN(MAX) is 24.72V. The minimum forward voltage of the LED string Vf(MIN) is 18V because the forward voltage of the LED string will be at the lowest level when the operation temperature rises to 60°C. According to equation (3), with VIN(MAX)=24.72V and Vf(MIN)=18V, the switching frequency, fSW should not set higher than 1.82MHz. However, because the switching frequency of the LM3414HV must set in the range of 250kHz to 1MHz, 1MHz switching frequency is selected. (2) Table 2 shows the suggested value of RFS for different switching frequencies: fSW (kHz) RFS (kΩ) 250 8.93 500 7.81 1000 6.25 Step 3. Inductor Selection The inductance of the inductor, L1 can be decided according to the switching frequency and output current settings determined in step 1 and step 2. The inductance must be adequate to maintain the LM3414HV to operate in CCM. The minimum inductance can be calculated by following equation (4): TABLE 2. Examples for RFS Setting When setting the switching frequency, it is necessary to ensure the on time of the internal switch is no shorter than 3 www.national.com AN-2076 400ns; otherwise the driving current to the LEDs will increase and may eventually damage the LEDs. Connecting to LEDs and Power Supply AN-2076 PWM Dimming Control The average LED current can be controlled by applying PWM dimming signal across the DIM and GND terminals of the LM3414HV evaluation board. The board accepts standard TTL level dimming signal. The output of the board is enabled when the DIM terminal is pulled high. The average LED current is adjustable according to the ON duty ratio of the PWM dimming signal by equation (6): (4) In equation (4), ILED is the average output current of the LM3414HV circuit to drive the LED string. IRIP(P-P) is the peakto-peak value of the inductor current ripple. Assuming that the required LED current is 700mA, 50% inductor current ripple and 1MHz switching frequency, the inductance should be no less than 14uH. Because common power inductor carries +/20% inductance tolerance, a standard 18uH inductor with +/-20% tolerance can be used. Other than deciding a suitable inductance value, it is essential to ensure the peak inductor current is not exceeding the rated saturation current of the inductor. The peak inductor current is governed by the following equation: (6) In equation (6), ILED(AVG) is the average current flows through the LED string and DDIM is the ON duty ratio of the PWM dimming signal being applied to the DIM pin of the LM3414HV. Analog Dimming Control As the output current of the LM3414HV is defined by the current being drawn to GND through RIADJ proportionally, analog dimming control (true output current control) can be accommodated by applying external current to RIADJ of the LM3414HV evaluation board. Figure 3 shows an example circuit for analog dimming control. With analog dimming control. Injecting additional current through the RIADJ to GND can effectively reduce the LED current (ILED). The relationship of ILED and IEXT is governed by equation (7). (5) In equation (5), IL(PEAK) is the peak inductor current. As a 18uH with +/- 20% variation is used, the minimum inductance L (MIN) is 14.4uH. With 700mA LED current, the peak inductor current is 836mA, thus a standard 18uH power inductor with 1A saturation current (ISAT) can be used. 30128720 FIGURE 3. Reducing LED current with external current to the IADJ pin on end application, the components required in this thermal foldback control circuitry are not included in the LM3414HV evaluation board. Physical pads and connections for R1, R2 and Q1 have been reserved on the board for component mounting. In order to detect the temperature of the LED string, a Positive Temperature Coefficient (PTC) thermistor, RPTC should be connected across the THM+ and THM- terminals of the LM3414HV evaluation board. In figure 4, the bipolar transistor, Q1 is biased by a potential divider composes of R1 and RPTC. When the temperature of the LEDs rises, the volt- (7) In equation (7), IEXT is the external current being injected into RIADJ. As IEXT increases, ILED decreases. Figure 4 shows a practical thermal foldback control circuit which reduces the LED current when the temperature of the LED sting is exceeding certain preset threshold. Because the temperature threshold for thermal foldback control depends www.national.com 4 mal foldback control is activated and the LED current reduces according to IEXT. 30128721 FIGURE 4. Thermal Foldback Control with PTC thermistor RPTC(25C) = 330Ω RPTC(80C) = 1.2kΩ RPTC(100C) = 10kΩ Design Example The LM3414HV evaluation board is used to drive a LED string at 700mA and thermal foldback control is needed to take place when the temperature of the LED strings exceeds 80° C as presented in Figure 5. In Figure 5, the LED current with the LED temperature below 80°C (ILED(normal)) is 700mA. As the temperature of the LED goes up to 80°C, thermal foldback begins and reduces the LED driving current with respect to the increase of resistance of RPTC. As the temperature of the LEDs reaches 100°C, the LED current reduces to zero. Provided that the resistance of the thermistor RPTC under 80C and 100°C are 1.2kΩ and 10kΩ respectively, the values of R1 and R2 can be calculated following the steps listed below. At 80°C: 30128722 FIGURE 5. Reduction of LED current with thermal foldback control (8) Assume the resistance of the PTC thermistor under 25°C, 80° C and 100°C are: 5 www.national.com AN-2076 age drop across RPTC increases as the resistance of RPTC increases. As the emitter voltage of Q1 reaches 1.255V, ther- AN-2076 At 100°C: Tiny Board Outline The tiny packages of the LM3414 family are exceptionally suitable for the applications that require high output power in limited space. In order to demonstrate the high power density of the LM3414HV, the core circuitry of this evaluation boards are completed in compact form factors: 22mm x 19mm for LLP-8 package, 26mm x 19mm for PSOP-8 package. The schematic of the core circuitry is as shown in Figure 6. The core circuitry can be extracted by cutting out from the PCB frame of the board as shown in Figure 7. (9) 30128725 FIGURE 6. Core Circuitry of the LM3414HV Evaluation Boards 30128726 FIGURE 7. Extracting the core circuitry from the LM3414HV evaluation boards www.national.com 6 AN-2076 30128727 FIGURE 8. Connecting to the core circuitry The board of the core circuitry features four connection pads for connections to DC power supply and LED string, as shown in Figure 8. To ensure thermal performance of the board, a heatsink attaches to the bottom layer of the board may be required depending on actual operation environment. Bill of Materials Designation Description Package Manufacturer Part # U1 LED Driver IC, LM3414HV LLP8 / PSOP8 LM3414MH Vendor NSC D1 Schottky Diode 100V 2A SS2PH10-M3/84A Vishay L1 Power Inductor 47 µH MMD-08EZ-470M-S1 MAG.Layers CIN Cap MLCC 100V 2.2 µF X7R 1210 1210 GRM32ER72A225KA35L Murata CVCC Cap MLCC 10V 1 µF X5R 0603 603 GRM185R61A105KE36D Murata RIADJ Chip Resistor 3.09 kΩ 1% 0603 603 CRCW06033K09FKEA Vishay RFS Chip Resistor 40.2 kΩ 1% 0603 603 CRCW060340K2FKEA Vishay VIN, GND, LED+, LED- Banana Jack 5.3(mm) Dia 5.3 (mm) Dia. 575-8 KEYSTONE VIN, GND, LED+,LED-, THM+, THM-, DIM, Turret 2.35(mm) Dia 2.35 (mm) Dia. 1502-2 KEYSTONE PCB LM3414EVAL PCB 85 X 54 (mm) 85 X 54 (mm) Q1 NPN Bipolar Transistor SOT23 R1,R2,RFS_1, RFS_2,RIADJ_1 NA 603 JP1,JP2,JP3 NA 603 7 NSC www.national.com AN-2076 Typical Performance Characteristics All curves taken at VIN = 48V with configuration in typical application for driving twelve power LEDs with four output channels active and output current per channel = 350 mA. TA = 25°C, unless otherwise specified. Output Current (A) Efficiency (%) 30128730 30128731 ILED(A) vs RIADJ(kΩ) fSW(kHz) vs RFS(kΩ) 30128732 30128733 ILED with VDIM rising ILED with VDIM falling 30128708 www.national.com 30128709 8 AN-2076 Evaluation Board Layout (LLP-8 Package) 30128712 Top Layer and Top Overlay 30128713 Bottom Layer and Bottom Overlay 9 www.national.com AN-2076 Evaluation Board Layout (PSOP-8 Package) 30128714 Top Layer and Top Overlay 30128715 Bottom Layer and Bottom Overlay www.national.com 10 AN-2076 Notes 11 www.national.com LM3414HV 1A 65V LED Driver Evaluation Board Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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