National Semiconductor Application Note 2009 Steve Solanyk February 4, 2011 Introduction Key Features This application note describes an evaluation board consisting of the LM3421 controller configured as a SEPIC constant current LED driver. It is capable of converting input voltages from 8V to 18V and illuminating up to six LEDs with approximately 350mA of drive current. Additional features include analog and pulse-width modulated (PWM) dimming, over-voltage protection, under-voltage lockout and cycle-by-cycle current limit. A bill of materials is included that describes the parts used in this evaluation board. A schematic and layout have also been included along with measured performance characteristics. • • • • • • • Designed to CISPR-25, Class 3 limits 0 to 10V analog dimming function PWM dimming function Input under-voltage protection Over-voltage protection Cycle-by-cycle current limit NoPB and RoHS compliant bill of materials Applications • • • Emergency lighting modules LED light-bars, beacons and strobe lights Automotive tail-light modules Performance Specifications Based on an LED Vf = 3.15V Symbol VIN VIN(MAX) Parameter Min Typ Max Operating Input Supply Voltage 8 12 18 Input Supply Voltage Surge Voltage - 50 V - VOUT LED String Voltage - 18.9V (6 LEDs) - ILED LED String Average Current - 345 mA - Efficiency (VIN=12V, ILED=345mA, 6 LEDs) - 85.4% Switching Frequency - 132 kHz LED Current Regulation - < 1% Variation ILIMIT Current Limit - 2.5 A - VUVLO Input Undervoltage Lock-out Threshold (VIN Rising) - 7.2V - Input Undervoltage Lock-out Hysteresis - 1V - VOVP Output Over-Voltage Protection Threshold - 37 V - VOVP(HYS) Output Over-Voltage Protection Hysteresis - 3.5 V - fSW - VUVLO(HYS) - Demo Board AN-2009 30107546 © 2011 National Semiconductor Corporation 301075 LM3421 SEPIC LED Driver Evaluation Board for Automotive Applications LM3421 SEPIC LED Driver Evaluation Board for Automotive Applications www.national.com AN-2009 current which is sufficient to drive a variety of available high brightness (HB) LEDs on the market. In order to comply with EMI requirements for automotive applications, an input filter and snubber components have also been designed into the circuit. This minimizes the time needed to optimize the design for specific EMI qualifications pertaining to individual automobile manufacturers and ensures faster product time to market. The demo board consists of a 1.6” x 2.4” four-layer PCB board. Test terminals in the form of turrets are available to connect the input power supply and an LED string as well as apply an analog or PWM dimming signal. General Information This evaluation board uses the LM3421 controller configured as a SEPIC converter for use in automotive based LED lighting modules. The described circuit can also be used as a general starting point for designs requiring robust performance in EMI sensitive environments.(Note 1) The design is based on the LM3421 controller integrated circuit (IC). Inherent to the LM3421 design is an adjustable highside current sense voltage which allows for tight regulation of the LED current with the highest efficiency possible. Additional features include analog dimming, over-voltage protection, under-voltage lock-out and cycle-by-cycle current limit. The operating input voltage range is from 8V to 18V. The design however is able to withstand input voltages up to 50V to account for power surges and load dump situations. (Note 2) Up to six LEDs can be powered with approximately 350mA of www.national.com Note 1: Although this evaluation board can be used as a reference design for automotive applications, it is up to the user to verify and qualify that the final design and BOM meets any AECQ-100 requirements. Note 2: Analog dimming circuit must not be connected when applying surge voltages greater than 21V. 2 AN-2009 30107501 Demo Board Schematic 3 www.national.com AN-2009 Bill of Materials Designator Value Package Description Manufacturer Part Number C4 1.0 µF 1206 Ceramic, C Series, 100V, 20% TDK C3216X7R2A105M C5 - - DNP - - C6 10 µF 2220 CAP, CERM, 50V, +/-10%, X7R TDK C5750X7R1H106K C7 10 µF 2220 CAP, CERM, 50V, +/-10%, X7R TDK C5750X7R1H106K C8 0.10 µF 805 Ceramic, X7R, 100V, 10% TDK C2012X7R2A104K C10 4.7 µF 2220 Ceramic, X7R, 100V, 10% MuRata GRM55ER72A475KA01L C11 0.10 µF 805 Ceramic, X7R, 50V, 10% Yageo America CC0805KRX7R9BB104 C12 0.22 µF 805 Ceramic, X7R, 50V, 10% TDK C2012X7R1H224K C13 1000 pF 805 Ceramic, C0G/NP0, 50V, 1% AVX 08055A102FAT2A C14 2.2 µF 805 Ceramic, X5R, 16V, 10% AVX 0805YD225KAT2A C15 47 pF 805 Ceramic, C0G/NP0, 50V, 5% MuRata GQM2195C1H470JB01D C16 0.1 µF 805 Ceramic, X7R, 25V, 10% MuRata GRM21BR71E104KA01L C20 1.0 µF 805 Ceramic, X7R, 25V, 10% MuRata GRM216R61E105KA12D C21 1.0 uF 805 Ceramic, X5R, 25V, 10% MuRata GRM216R61E105KA12D C22 68 µF Radial Can - SMD CAP ELECT 68UF 63V FK Panasonic EEE-FK1J680UP C23 0.01 µF 805 CAP, CERM, 100V, +/-10%, X7R TDK C2012X7R2A103K C24 4.7 µF 2220 CAP, CERM, 100V, +/-10%, X7R TDK C5750X7R2A475K C25 1000 pF 805 CAP, CERM, 100V, +/-10%, X7R TDK C2012X7R2A102K C26 1.2 nF 1206 CAP, CERM, 100V, +/-20%, X7R AVX 12061A122JAT2A C27 0.10 µF 805 Ceramic, X7R, 25V, 10% TDK C2012X7R1E104K C28 2.7 nF 1206 CAP, CERM, 100V, +/-20%, X7R AVX 12065C272KAT2A D6 - SOD-123 Diode Schottky, 60V, 1A Rohm RB160M-60TR D10 - SOD-123 Vr = 100V, Io = 0.15A, Vf = 1.25V Diodes Inc. 1N4148W-7-F D12 - SOD-123 SMT Zener Diode Diodes Inc. MMSZ5231B-7-F J1 - Through hole Header, 100mil, 1x2, Gold plated, 230 mil above insulator Samtec Inc. TSW-102-07-G-S J2 - Through hole Header, 100mil, 1x2, Gold plated, 230 mil above insulator Samtec Inc. TSW-102-07-G-S J3 - Through hole Header, 100mil, 1x2, Gold plated, 230 mil above insulator Samtec Inc. TSW-102-07-G-S J4 - Through hole Header, 100mil, 1x2, Gold plated, 230 mil above insulator Samtec Inc. TSW-102-07-G-S L1 100 µH SMD Coupled inductor Coilcraft MSD1278-104ML L2 - 1206 6A Ferrite Bead, 160 Ohm @ 100MHz Steward HI1206T161R-10 L3 10 µH SMD Inductor, Shielded Drum Core, Ferrite, 2.1A, 0.038Ω Coilcraft MSS7341-103MLB Q1 - DPAK MOSFET N-CH 100V 6.2A Fairchild Semiconductor FDD3860 Q2 - SOT-23 MOSFET, N-CH, 30V, 4.5A Vishay-Siliconix SI2316BDS-T1-E3 Q3 - SOT-23 MOSFET, N-CH, 60V, 0.24A Vishay-Siliconix 2N7002E-T1-E3 R1 40.2 kΩ 805 1%, 0.125W Vishay-Dale CRCW080540K2FKEA R2 40.2 kΩ 805 1%, 0.125W Vishay-Dale CRCW080540K2FKEA R5 174 kΩ 805 1%, 0.125W Panasonic ERJ-6ENF1743V R6 1.0 kΩ 805 1%, 0.125W Vishay-Dale CRCW08051k00FKEA R7 1.0 kΩ 805 1%, 0.125W Vishay-Dale CRCW08051k00FKEA R8 0.2 Ω 2010 1%, 0.5W Vishay-Dale WSL2010R3000FEA R9 10 Ω 805 1%, 0.125W Yageo America RC0805FR-0710RL www.national.com 4 Value Package Description Manufacturer Part Number R10 21.5 kΩ 805 1%, 0.125W Vishay-Dale CRCW080521K5FKEA R11 100 Ω 805 5%, 0.125W Vishay-Dale CRCW0805100RJNEA R13 174 kΩ 805 1%, 0.125W Panasonic ERJ-6ENF1743V R14 4.32 kΩ 805 1%, 0.125W Vishay-Dale CRCW08054K32FKEA R15 6.04 kΩ 805 1%, 0.125W Panasonic ERJ-6ENF6041V R16 0.10 Ω 2512 1%, 1W Vishay-Dale WSL2512R1000FEA R18 60.4 kΩ 805 1%, 0.125W Vishay-Dale CRCW080560K4FKEA R19 40.2 kΩ 805 1%, 0.125W Vishay-Dale CRCW080540K2FKEA R20 40.2 kΩ 805 1%, 0.125W Vishay-Dale CRCW080540K2FKEA R21 22.1 kΩ 805 1%, 0.125W Vishay-Dale CRCW080522K1FKEA R25 40.2 kΩ 805 1%, 0.125W Vishay-Dale CRCW080540K2FKEA R26 11.8 kΩ 805 1%, 0.125W Vishay-Dale CRCW080511K8FKEA R27 0Ω 1206 1%, 0.25W Yageo America RC1206JR-070RL R28 10.0 Ω 1206 1%, 0.25W Vishay-Dale CRCW120610R0FKEA R29 590 Ω 1210 1%, 0.5W Vishay/Dale CRCW1210590RFEA R30 10 Ω 805 1%, 0.125W Vishay-Dale CRCW080510R0FKEA R31 2.2 Ω 1206 1%, 0.25W Vishay-Dale CRCW12062R20FKEA R32 0Ω 1206 5%, 0.25W Yageo America RC1206JR-070RL R33 4.99 kΩ 805 0.1%, 0.125W Yageo America RT0805BRD074K99L R34 10.0 kΩ 805 1%, 0.125W Vishay-Dale CRCW080510K0FKEA R35 1%, 0.5W Vishay/Dale CRCW1210590RFEA TP1 - TP8 590 Ω - 1210 Through Hole Terminal, Turret, TH, Double Keystone Electronics 1573-2 U1 - National Semiconductor LM3421MH U3 - TSSOP-16 N-Channel Controller for Constant EP Current LED Drivers SC70-6 2.4V R-R Out CMOS Video OpAmp National with Shutdown Semiconductor 5 LMH6601MG www.national.com AN-2009 Designator AN-2009 LM3421 Device Pin-Out Top View 30107502 Pin Description 16-Lead TSSOP EP Pin # Name 1 VIN Description Bypass with 100 nF capacitor to AGND as close to the device as possible in the circuit board layout. 2 EN Connect to AGND for zero current shutdown or apply > 2.4V to enable device. 3 COMP 4 CSH Connect a resistor to AGND to set the signal current. For analog dimming, connect a controlled current source or a potentiometer to AGND as detailed in the Analog Dimming section. 5 RCT External RC network sets the predictive “off-time” and thus the switching frequency. 6 AGND 7 OVP Connect to a resistor divider from VO to program output over-voltage lockout (OVLO). Turn-off threshold is 1.24V and hysteresis for turn-on is provided by 23 µA current source. 8 nDIM Connect a PWM signal for dimming as detailed in the PWM Dimming section and/or a resistor divider from VIN to program input under-voltage lockout (UVLO). Turn-on threshold is 1.24V and hysteresis for turn-off is provided by 23 µA current source. Connect a capacitor to AGND to set the compensation. Connect to PGND through the DAP copper pad to provide ground return for CSH, COMP, RCT, and TIMR. 9 DDRV Connect to the gate of the dimming MosFET. 10 PGND Connect to AGND through the DAP copper pad to provide ground return for GATE and DDRV. 11 GATE Connect to the gate of the main switching MosFET. 12 VCC 13 IS 14 RPD Connect the low side of all external resistor dividers (VIN UVLO, OVP) to implement “zero-current” shutdown. 15 HSP Connect through a series resistor to the positive side of the LED current sense resistor. 16 HSN Connect through a series resistor to the negative side of the LED current sense resistor. EP (17) EP www.national.com Bypass with 2.2 µF–3.3 µF ceramic capacitor to PGND. Connect to the drain of the main N-channel MosFET switch for RDS-ON sensing or to a sense resistor installed in the source of the same device. Star ground connecting AGND and PGND. 6 AN-2009 Evaluation Board Connection Overview 30107519 Wiring and Jumper Connection Diagram Name I/O Description VIN Input Power supply voltage. PGND Input Ground. DIM Input PWM Dimming Input Apply a pulse-width modulated dimming voltage signal with varying duty cycle. Maximum dimming voltage level is 20V. Maximum dimming frequency is 1kHz. DIM_GND Input PWM dimming ground. ADIM Input 0 - 10V Dimming Input Apply a 0 - 10V analog dimming voltage signal. See "Theory of Operation" section for more details. ADIM_GND Input Analog dimming ground. LED+ Output LED Constant Current Supply Supplies voltage and constant-current to anode of LED array. LED- Output LED Return Connection (not GND) Connects to cathode of LED array. Do NOT connect to GND. Evaluation Board Modes of Operation Overview The available modes of operation for this evaluation board are enabled utilizing the jumper configurations described in the following table. J1 J2 J3 J4 - OPEN - - OPEN CLOSED CLOSED OPEN Mode of Operation LM3421 is disabled and placed into low-power shutdown. LM3421 is enabled and powered on. The evaluation board will now run under standard operation. CLOSED CLOSED CLOSED CLOSED LM3421 is enabled and powered on. The analog dimming function is now enabled. OPEN CLOSED OPEN OPEN LM3421 is enabled and powered on. The PWM dimming function is now enabled. 7 www.national.com TA = 25°C and LED Vf = 3.15V unless otherwise specified. Efficiency vs. Input Voltage fSW = 132kHz, ILED = 345mA Efficiency vs. Switching Frequency VIN = 12V, ILED = 345mA 100 100 95 95 EFFICIENCY (%) EFFICIENCY (%) 6 LEDs 90 85 80 75 2 LEDs 4 LEDs 70 65 6 LEDs 90 85 80 75 4 LEDs 2 LEDs 70 65 60 60 6 8 10 12 14 VIN (V) 16 18 20 50 100 150 200 250 SWITCHING FREQUENCY (kHz) 30107516 LED Current vs. Input Voltage fSW = 132kHz, 6 LEDs, VOUT = 18.8V 100 400 95 350 ILED=345mA 90 300 ILED (mA) 85 80 75 ILED=207mA 70 RSNS=0.3Ω 250 200 150 RSNS=0.5Ω 100 ILED=104mA 65 50 60 RSNS=1.0Ω 0 6 8 10 12 14 VIN (V) 16 18 20 6 30107517 www.national.com 300 30107515 Efficiency vs. Input Voltage fSW = 132kHz, 6 LEDs, VOUT = 18.8V EFFICIENCY (%) AN-2009 Typical Performance Characteristics 8 10 12 14 VIN (V) 16 18 20 30107518 8 350 300 300 250 250 ILED (mA) ILED (mA) PWM Dimming VIN = 12V, fSW = 132kHz, 6 LEDs, VOUT = 20.4V 350 200 150 150 100 50 50 0 1 2 3 4 5 6 7 8 0 9 10 ADIM VOLTAGE (V) fDIM = 100 Hz 200 100 0 AN-2009 Analog Dimming VIN = 12V, fSW = 132kHz, 6 LEDs, VOUT = 20.4V fDIM = 1 kHz 0 20 40 60 80 100 DUTY CYCLE (%) 30107513 Steady-state Waveforms Top Plot: VSW, Bottom Plot: ILED (VIN =12V, ILED = 342mA, 6 LEDs, VOUT = 20.4V) 30107514 Start-up Waveforms Top Plot: VSW, Bottom Plot: ILED (VIN =12V, ILED = 342mA, 6 LEDs, VOUT = 20.4V) 30107520 30107521 Shutdown Waveforms Top Plot: VSW, Bottom Plot: ILED (VIN =12V, ILED = 342mA, 6 LEDs, VOUT = 20.4V) Over-voltage Protection Response Top Plot: VSW, Middle Plot: VOUT, Bottom Plot: ILED (VIN =12V, ILED = 342mA, 6 LEDs, VOUT = 20.4V) 30107523 30107522 9 www.national.com AN-2009 100Hz, 50% Duty Cycle PWM Dimming Top Plot: VSW, Middle Plot: VDIM, Bottom Plot: ILED VIN = 12V, fSW = 132kHz, 6 LEDs, VOUT = 20.4V 100Hz, 50% Duty Cycle PWM Dimming (rising edge) Top Plot: VSW, Middle Plot: VDIM, Bottom Plot: ILED VIN = 12V, fSW = 132kHz, 6 LEDs, VOUT = 20.4V 30107525 30107524 1 kHz, 50% Duty Cycle PWM Dimming Top Plot: VSW, Middle Plot: VDIM, Bottom Plot: ILED VIN = 12V, fSW = 132kHz, 6 LEDs, VOUT = 20.4V 1 kHz, 50% Duty Cycle PWM Dimming (rising edge) Top Plot: VSW, Middle Plot: VDIM, Bottom Plot: ILED VIN = 12V, fSW = 132kHz, 6 LEDs, VOUT = 20.4V 30107526 www.national.com 30107527 10 AN-2009 PCB Layout Top Layer 30107528 Mid-Layer 1 30107529 Mid-Layer 2 30107530 Bottom Layer 30107531 11 www.national.com AN-2009 Theory of Operation INPUT EMI LINE FILTER 30107509 FIGURE 1. Input filter circuit A low-pass input filter (highlighted in Figure 1) has been added to the front-end of the circuit. Its primary purpose is to minimize EMI conducted from the LM3421 circuit to prevent it from interfering with the electrical network supplying power to the LED driver. Frequencies in and around the LED driver switching frequency (i.e. fSW = 132 kHz) are primarily addressed with this filter. The ferrite bead, L2, has been chosen to help attenuate EMI frequencies above 10MHz in conjunction with snubber circuitry that has been designed into the driver circuitry which will be discussed in the next section. This low pass filter has a cut-off frequency that is determined by the inductor and capacitor resonance of L3 and C22 as described in the following equation, The input filter needs to attenuate the fundamental frequency and associated harmonics of the demo board’s switching frequency which is designed to be 132kHz. Plugging the chosen values of L3 and C22 as 10µH and 68uF respectively gives a roll-off frequency of 6.1kHz. The ferrite bead chosen has a nominal impedance of 160 Ohm at 100Mhz for 1A of current and will help attenuate higher frequency noise. Conducted EMI scans of an earlier prototype evaluation board with and without an input filter are shown in Figure 2 and Figure 3. (NOTE: These scans were originally done per CISPR-22, however for the purpose of evaluating filter performance this EMI data is acceptable. The actual EMI performance for this evaluation board will be discussed later in this document.). Frequencies from 300kHz to 10 MHz show noticeable attenuation of peak frequencies with the input filter in place. Harmonics of the driver switching frequency are reduced up to 22dBµV/m. 30107541 FIGURE 2. Conducted EMI scan (peak) WITHOUT input filter and with snubber circuitry www.national.com 12 AN-2009 30107540 FIGURE 3. Conducted EMI scan (peak) WITH input filter and with snubber circuitry SNUBBER CIRCUITRY 30107510 FIGURE 4. Snubber circuitry Snubber circuitry (highlighted in Figure 4 has been added around the switching elements of Q1 and D6 in the form of series resistor-capacitor (RC) pairs. The purpose of these snubbers is to reduce the rising/falling edge rate of the switching voltage waveform when Q1 and D6 transition from an “on” to “off” state and vice versa. This helps reduce both conducted and radiated EMI in the higher test frequency ranges. For lower EMI frequencies particularly during conducted EMI testing, the input filter is utilized as the primary EMI attenuator as previously discussed. Conducted EMI scans of an earlier prototype evaluation board with and without snubber circuitry are shown in Figure 5 and Figure 6. (NOTE: These scans were originally done per CISPR-22, however for the purpose of evaluating filter performance this EMI data is acceptable. The actual EMI performance for this evaluation board will be discussed later in this document.). From 10 MHz to 30 MHz, the snubbers reduce peak power for all frequencies with noticeable attenuation of peak power between 20 MHz and 30 MHz. 13 www.national.com AN-2009 30107543 FIGURE 5. Conducted EMI scan (peak) with input filter and WITHOUT snubber circuitry 30107540 FIGURE 6. Conducted EMI scan (peak) with input filter and WITH snubber circuitry Radiated EMI scans of the demo board with and without the snubber circuitry are shown in Figure 7 and Figure 8. From 30 MHz to near 200 MHz, the snubbers reduce peak power with attenuation values ranging from 5 to 10dBµV/m. 30107545 FIGURE 7. Radiated EMI scan (peak) with input filter and WITHOUT snubber circuitry www.national.com 14 AN-2009 30107544 FIGURE 8. Radiated EMI scan (peak) with input filter and WITH snubber circuitry Although the snubber circuits help reduce the EMI signature of the evaluation board, they do so at the cost of lowering the maximum achievable driver efficiency. Since each board design and application is unique, it is recommended that the user investigate different snubber configurations and values to provide the optimal balance of EMI performance and system efficiency. When no analog dimming is being applied, the ICSH current is described by the following equation, The value of RCSH is 11.8kΩ and this gives ICSH as 105µA. The method used to adjust ICSH for analog dimming is with an external variable current source consisting of an on-board opamp circuit. When a 0 to 10V voltage signal is applied to the ADIM test point, the op-amp will adjust its output current accordingly. This output current is sourced into the node consisting of the CSH pin and resistors R21 and R26 which adjusts the ICSH current from the original 105µA based on the 0 to 10V analog dimming signal. A low analog dimming voltage will source more current into the CSH pin effectively dimming the LEDs while a high analog dim voltage will source less current resulting in less dimming. ADIM should be a precise external voltage reference. ANALOG DIMMING The analog dimming circuitry is highlighted in Figure 9. Closing jumpers J1 and J4 connects the analog dimming circuitry to the LED driver and thus enables this feature. Analog dimming of the LED current is performed by adjusting the CSH pin current (ICSH) from the LM3421. The relationship between ICSH and the average LED current is described in the following equation, For the demo board RHSP is 1kΩ and RSNS is 0.3Ω and so the equation becomes, 30107511 FIGURE 9. Analog dimming circuit 15 www.national.com AN-2009 PWM DIMMING 30107512 FIGURE 10. PWM dimming circuit The circuitry associated with pulse-width modulation (PWM) dimming is highlighted in Figure 10 and closing jumper J3 enables this function. A logic-level PWM signal can be applied to the DIM pin which in turn drives the nDIM pin thought the MosFET Q3. A pull down resistor (R34) has also been added to properly turn off Q3 if no signal is present. The nDIM pin www.national.com controls the dimming NFET (Q2) which is in series with the LED stack. The brightness of the LEDs can be varied by modulating the duty cycle of the PWM signal. LED brightness is approximately proportional to the PWM signal duty cycle, so for example, 30% duty cycle equals approximately 30% LED brightness. 16 Several automobile manufacturers base their conducted EMI limit requirements on the CISPR-25, Class 3 standard. However each manufacturer in the end specifies their own individual method for EMI qualification, and so there is not at this time a universally adopted set of EMI limits and performance requirements. This makes it challenging to design a single LED driver circuit to comprehensively meet the EMI requirements for each and every auto manufacturer. Therefore the Class 3 limits described by CISPR-25 were used as a reference point for the EMI performance of the LM3421 SEPIC design. From this data, specific auto manufacturer EMI limits and requirements can be applied to the data to determine if 30107552 FIGURE 11. Conducted "Peak" scan per CISPR-25 with Class 3 limits 17 www.national.com AN-2009 additional optimization of the reference design is required for compliance. Conducted EMI tests were performed with a six LED load running 345mA of LED current with an input power supply voltage of 12V. In the following EMI scan of Figure 11, the CISPR-25 Class 3 "peak" limits are designated as blue and the "average" limits are designated in green. No enclosure was used around the board. Due to limitations in the data gathering equipment only the peak EMI data from 100kHz to 30MHz could be acquired, and so the conducted EMI performance of the evaluation board at other frequencies and versus quasi-peak and average CISPR25 limits can only be roughly interpreted. Conducted EMI Analysis AN-2009 Radiated EMI tests were performed with a six LED load running 345mA of LED current with an input power supply voltage of 12V. No enclosure was used around the board. In the EMI scan of Figure 12, the CISPR-25 Class 3 "peak" limits are shown in blue. For the EMI scan of Figure 13, the CISPR-25 Class 3 "average" limits are shown in green. Some frequency bands have multiple limits associated with them. In these instances, the frequency bands have multiple RF spectrum allocations (e.g. FM, CB, VHF, etc...), and so all applicable limits are being shown even if they overlap. Due to limitations in the data gathering equipment only the peak EMI data from 10Mhz to 1GHz could be acquired, and so the radiated EMI performance of the evaluation board at other frequencies and versus quasi-peak and average CISPR25 limits can only be roughly interpreted. Radiated EMI Analysis Similar to the conducted EMI testing described previously, several automobile manufacturers base their radiated EMI limit requirements on the CISPR-25, Class 3 standard. However each manufacturer in the end specifies their own individual method for EMI qualification, and so there is not at this time a universally adopted set of EMI limits and performance requirements. This makes it challenging to design a single LED driver circuit to comprehensively meet the EMI requirements for each and every auto manufacturer. Therefore the Class 3 limits described by CISPR-25 were used as a reference point for the EMI performance of the LM3421 SEPIC design. From this data, specific auto manufacturer EMI limits and requirements can be applied to the data to determine if additional optimization of the reference design is required for compliance. 30107551 FIGURE 12. Radiated “Peak” scan data per CISPR-25 with Class 3 "Peak" limits 30107553 FIGURE 13. Radiated “Peak” scan data per CISPR-25 with Class 3 "Average" limits www.national.com 18 AN-2009 Thermal Analysis Thermal scans were taken of the stand-alone LED demo board at room temperature with no airflow. Primary hot spots on the top and bottom layers are associated with the snubber resistors R27 and R31. Test Conditions: VIN = 12.1V, IIN=651mA, VOUT = 20.4V (6 LEDs), ILED = 336mA, PIN = 7.88W, POUT = 6.85W, Efficiency = 86.9%, Time = 75 minutes, Ta = Room temp, No airflow, No enclosure Thermal Scan, Top Layer 30107549 Thermal Scan, Bottom Layer 30107550 19 www.national.com LM3421 SEPIC LED Driver Evaluation Board for Automotive Applications 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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