National Semiconductor Application Note 2056 L.K. Wong September 21, 2010 Introduction DHC also facilitates a single BOM for different number of LED in a string, which is required for backlight panels of different size, thereby reducing overall development time and cost. The LM3492 comes with a versatile COMM pin which serves as a bi-directional I/O pin interfacing with an external MCU for the following functions: power-good, over-temperature, IOUT over- and under-voltage indications, switching frequency tuning, and channel 1 disabling. Other supervisory functions of the LM3492 include precise enable, VCC under-voltage lockout, current regulator Over-Power protection, and thermal shutdown protection. The LM3492 is available in the thermally enhanced eTSSOP-20 package. This application note details the design of a LM3492 evaluation board which drives 2 LED strings, each of which consists of 10 LEDs running at 150 mA and the forward voltage of each LED is typically 3.8V. The input voltage is from 9V to 16V. The evaluation board schematic, PCB layout, Bill of Materials, and circuit design descriptions are shown. Typical performance and operating waveforms are also provided for reference. The LM3492 integrates a boost converter and a two-channel current regulator to implement a high efficient and cost effective LED driver for driving two individually dimmable LED strings with a maximum power of 15W and an output voltage of up to 65V. The boost converter employs a proprietary Projected-On-Time control method to give a fast transient response with no compensation required, and a nearly constant switching frequency programmable from 200 kHz to 1 MHz. The application circuit is stable with ceramic capacitors and produces no audible noise on dimming. The programmable peak current limit and soft-start features reduce current surges at startup, and an integrated 190 mΩ, 3.9A N-Channel MOSFET switch minimizes the solution size. The fast slew rate current regulator allows high frequency and narrow pulse width dimming signals to achieve a very high contrast ratio of 1000:1 at a dimming frequency of more than 3 kHz. The LED current is programmable from 50 mA to 200 mA by a single resistor. To maximize the efficiency, Dynamic Headroom Control (DHC) automatically adjusts the output voltage to a minimum. Evaluation Board Schematic and PCB Layout LM3492 Evaluation Board Reference Design LM3492 Evaluation Board Reference Design 30123301 FIGURE 1. LM3492 Evaluation Board Schematic AN-2056 © 2010 National Semiconductor Corporation 301233 www.national.com AN-2056 30123302 FIGURE 2. LM3492 Evaluation Board Top Overlay 30123303 FIGURE 3. LM3492 Evaluation Board Top View 30123304 FIGURE 4. LM3492 Evaluation Board Bottom View www.national.com 2 AN-2056 Evaluation Board Quick Setup Procedures Step Description Notes 1 Connect a power supply to VIN and PGND terminals VIN range: 9V to 16V 2 Connect 2 LED strings: from VLED1 to IOUT1 terminals, and VLED2 to IOUT2 terminals Each LED string consists of 10 LEDs with a forward voltage of 3.8V per LED at 150 mA 3 The EN terminal should be left open for normal operation. Ground this terminal to shutdown 4 Connect DIM1 and DIM2 terminals to a voltage > 2V, apply VIN = 12V 5 Ground the EN terminal to check the shutdown function Nominal LED current is 150 mA per channel Evaluation Board Performance Characteristic Description Symbol Input Voltage VIN Rail Voltage VOUT LED Current ILED LED Current Regulationt ΔILED Efficiency Condition Min Typ Max Unit 9 12 16 V 39 V 150 ALL VIN conditions -3 mA +3 % VIN = 9V 85.7 % VIN = 12V 88.2 % VIN = 16V 89.1 % L1 = (VIN(MAX) x ton) / 2IIN(MIN) Design Procedure It can be calculated that IIN(MIN), ton, and L1 are 0.363A, 1.17 µs, and 25.8 µH. On the other hand, IIN is maximum when VIN is minimum, which is 9V in this example, and 2 LED strings are turned on. Hence IIN(MAX) is 1.29A. From (3), ton is 1.54 µs when VIN is 9V. Then ILR is The following procedures detail the design of the LM3492 evaluation board driving 2 LED strings consists of 10 LEDs per string. The forward voltage of each LED is 3.8V, and the LED current is 150 mA. The input voltage is ranged from 9V to 16V. The switching frequency fSW is designed to be 500 kHz. Design Parameters: VIN = 9V to 16V, typical 12V ILED = 150 mA Step 1: Calculate the output voltage feedback circuit The nominal voltage of the LED string with 10 LEDs is 38V, and the minimum voltage of the IOUTn pin (n = 1, 2) is 0.75V for an ILED of 150 mA. Hence, VOUT(NOM) is 38.75V. Since the dynamic range of VFB under DHC is from 1.05V to 2V, the nominal voltage on the FB pin VFB(NOM) is designed to be around 1.5V. Hence, VOUT(MAX) is designed to be 65V. Since VOUT(MAX) = 2.5V (1 + RFB1/ RFB2) ILR = (VIN x ton) / L1 IL1(PEAK) = IL1 + ILR / 2 ton = (1 – VIN/VOUT) / fSW (3) (6) As a result, IL1(PEAK) is 1.56A. A standard value of 27 µH is selected for L1, and the saturation current of L1 should be larger than 1.56A. Step 3: Determine the diode The selection of the boost diode D1 depends on two factors. The first factor is the reverse voltage, which equals to VOUT in a boost converter. The second factor is the peak diode current at the steady state, which equals to the peak inductor current as shown in (6). In this example, a 100V 3A schottky diode is selected. Step 4: Determine the value of other components CIN and COUT: The function of the input capacitor CIN and the output capacitor COUT is to reduce the input and output voltage ripples. Experimentation is usually necessary to determine their value. The rated DC voltage of capacitors used should be higher than the maximum DC voltage applied. Owing to the concern of product lifetime, ceramic capacitors are recommended. But ceramic capacitors with high rated DC voltage and high capacitance are rare in general. Multiple capacitors connecting in parallel can be used for CIN and COUT. In this example, two 10 µF 25V ceramic capacitor are used for CIN, and two 2.2 µF 100V ceramic capacitor are used for COUT. CVCC: The capacitor on the VCC pin provides noise filtering and stabilizes the LDO regulator. It also prevents false trig- (1) (2) (5) From (5), ILR is 0.53A. The steady state peak inductor current IL1(PEAK) is By designing RFB2 to be 16.2 kΩ, RFB1 is calculated to be 405 kΩ, and a standard resistor value of 402 kΩ is selected. CFB1 is selected to be 10 pF as recommended. Step 2: Determine the inductance The main parameter affected by the inductor is the peak to peak inductor current ripple (ILR). To maintain a continuous conduction mode (CCM) operation, the average inductor current IL1 should be larger than half of ILR. For a boost converter, IL1 equals to the input current IIN. The minimum IIN occurs when VIN is maximum, which is 16V in this example, and only 1 LED string is turned on (the 2 LED strings are individually dimmable). Hence, IIN(MIN) = (VOUT(NOM) x ILED) / VIN(MAX) (4) Also To ensure a CCM operation, 3 www.national.com AN-2056 external components should be placed as close to the LM3492 and each other as possible in order to make copper traces short and direct. In particular, components of the boost converter CIN, L1, D1, COUT, and the LM3492 should be closed. Also, the output feedback capacitor CFB1 should be closed to the output capacitor COUT. The ground plane connecting the GND, PGND, and LGND pins and the exposed pad of the LM3492 and the ground connection of the CIN and COUT should be placed on the same copper layer. Good heat dissipation helps optimize the performance of the LM3492. The ground plane should be used to connect the exposed pad of the LM3492, which is internally connected to the LM3492 die substrate. The area of the ground plane should be extended as much as possible on the same copper layer around the LM3492. Using numerous vias beneath the exposed pad to dissipate heat of the LM3492 to another copper layer is also a good practice. gering of the VCC UVLO. CVCC is recommended to be a 1 µF good quality and low ESR ceramic capacitor. CCDHC: The capacitor at the CDHC pin mainly determines the soft-start time tSS, i.e. the time for the output voltage to reach its maximum. tSS is determined from the following equation: (7) In this example, CCDHC is recommended to be a 0.47 µF good quality and low ESR ceramic capacitor. RRT and RIREF: The resistors RRT and RIREF set the switching frequency fSW of the boost converter and the LED current ILED respectively. From the LM3492 datasheet, RRT is selected to be 274 kΩ if fSW is 500 kHz (Figure 1 of the datasheet), and RIREF is selected to be 8.25 kΩ if ILED is 150 mA (Figure 4 of the datasheet). RCOMM: Since the COMM pin is open drain, a resistor RCOMM of 52.3 kΩ is used to connect the VCC and COMM pins to implement a pull-up function. PC Board Layout The layout of the printed circuit board is critical to optimize the performance of the LM3492 application circuit. In general, www.national.com 4 Item Ref Designator(s) Size 1 Part Number GRM31CR61E106KA12L Mfg name muRata Part Description Cap 10 µF 25V X5R Qty 2 CIN1, CIN2 1206 2 GRM188R71C474KA88D muRata 0603/X7R/0.47 µF/16V 1 CCDHC 0603 3 GRM1885C2A100RA01D muRata 0603/COG/10 pF/100V 1 CFB1 0603 4 GRM188R71C105KA12D muRata 0603/X7R/1 µF/16V 1 CVCC 0603 5 GRM32ER72A225KA35L muRata Cap 2.2uF 100V X7R 2 CO1, CO2 1210 6 CRCW060352K3FKEA Vishay Resistor Chip 52.3 kΩ 1% 1 RCOMM 0603 7 CRCW0603274KFKEA Vishay Resistor Chip 274 kΩ 1% 1 RRT 0603 8 CRCW0603402KFKEA Vishay Resistor Chip 402 kΩ 1% 1 RFB1 0603 9 CRCW060316K2FKEA Vishay Resistor Chip 16.2 kΩ 1% 1 RFB2 0603 10 CRCW06038K25FKEA Vishay Resistor Chip 8.25 kΩ 1% 1 RIREF 0603 11 CRCW06030000Z0EA Vishay Resistor Chip 0Ω 1% 1 RILIM0 12 CDRH10D68/ANP-270MC Sumida Inductor 27 µH 1.9A 1 L1 10×10×6.8 13 SK310A-TP SMA 14 1502-2k-ND 15 16 Micro Commercial Schottky 100V 3A 1 D1 KEYSTONE Terminal DBL Turret 0.109”L Brass 11 VIN, GND, PGND, VLED1, VLED2, IOUT1, IOUT2, DIM1, DIM2, COMM, EN LM3492EVAL NSC LM3492 demo board 1 PCB LM3492MH NSC IC LM3492 1 U1 5 0603 eTSSOP-20 www.national.com AN-2056 Bill of Materials AN-2056 Typical Performance and Waveforms All curves and waveforms taken at VIN = 12V with the evaluation board and TA = 25°C unless otherwise specified. Efficiency vs Input Voltage (ILED = 150 mA) ILED Regulation vs Input Voltage (ILED = 150 mA) 30123306 30123307 Steady State Operation (VIN = 12V, ILED = 150 mA) LED 50% Dimming (VIN = 12V, ILED = 150 mA) 30123308 30123309 Power Up (VIN = 12V, ILED = 150 mA) Enable Transient (VIN = 12V, ILED = 150 mA) 30123310 www.national.com 30123311 6 AN-2056 7 www.national.com LM3492 Evaluation Board Reference Design 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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