LM3492HCEVM

User's Guide SNVA639A – May 2012 – Revised April 2013 AN-2231 LM3492HC Evaluation Board Reference Design 1 Introduction The LM3492HC 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 ProjectedOn-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 10000:1 at a dimming frequency of 200Hz. The LED current is programmable from 50 mA to 250 mA by a single resistor. To maximize the efficiency, Dynamic Headroom Control (DHC) automatically adjusts the output voltage to a minimum. To increase the contrast ratio, the DHC can be over-ridded when the contrast ratio is high. 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 LM3492HC 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 LM3492HC include precise enable, VCC under-voltage lock-out, current regulator Over-Power protection, and thermal shutdown protection. The LM3492HC is available in the thermally enhanced HTSSOP-20 package. This application note details the design of a LM3492HC evaluation board which drives two 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. All trademarks are the property of their respective owners. SNVA639A – May 2012 – Revised April 2013 Submit Documentation Feedback AN-2231 LM3492HC Evaluation Board Reference Design Copyright © 2012–2013, Texas Instruments Incorporated 1 Evaluation Board Schematic and PCB Layout 2 www.ti.com Evaluation Board Schematic and PCB Layout Figure 1. LM3492HC Evaluation Board Schematic Figure 2. LM3492HC Evaluation Board Top Overlay 2 AN-2231 LM3492HC Evaluation Board Reference Design Copyright © 2012–2013, Texas Instruments Incorporated SNVA639A – May 2012 – Revised April 2013 Submit Documentation Feedback Evaluation Board Schematic and PCB Layout www.ti.com Figure 3. LM3492HC Evaluation Board Top View Figure 4. LM3492HC Evaluation Board Bottom View Table 1. 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 Table 2. Evaluation Board Performance Characteristic Description Symbol Input Voltage VIN Condition Min Typ Max Unit 9 12 16 V Rail Voltage VOUT 39 V LED Current ILED 150 mA LED Current Regulationt ΔILED Efficiency SNVA639A – May 2012 – Revised April 2013 Submit Documentation Feedback ALL VIN conditions -3 +3 % VIN = 9V 86.3 % VIN = 12V 88.9 % VIN = 16V 89.8 % AN-2231 LM3492HC Evaluation Board Reference Design Copyright © 2012–2013, Texas Instruments Incorporated 3 Design Procedure 3 www.ti.com Design Procedure The following procedures detail the design of the LM3492HC 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. We design VOUT to be 50V when VFB is 2.5V. Then at VOUT(NOM), VFB(NOM) is about 1.95V, which is within the suggested dynamic range of VFB under DHC (from 1.05V to 2V). By designing RFB2 to be 16.2 kΩ, RFB1 is calculated as follows: RFB1 = RFB2((VOUT(MAX)/2.5V) – 1) (1) RFB1 is calculated to be 307.8 kΩ, and a standard resistor value of 309 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) (2) ton = (1 – VIN/VOUT) / fSW (3) Also To ensure a CCM operation, L1 = (VIN(MAX) x ton) / 2IIN(MIN) (4) 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 ILR = (VIN x ton) / L1 (5) From (5), ILR is 0.53A. The steady state peak inductor current IL1(PEAK) is IL1(PEAK) = IL1 + ILR / 2 (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. 4 AN-2231 LM3492HC Evaluation Board Reference Design Copyright © 2012–2013, Texas Instruments Incorporated SNVA639A – May 2012 – Revised April 2013 Submit Documentation Feedback PC Board Layout www.ti.com CVCC: The capacitor on the VCC pin provides noise filtering and stabilizes the LDO regulator. It also prevents false triggering 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: tSS = CCDHC x 2.25V 120 PA (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 LM3492HC datasheet, RRT is selected to be 274 kΩ if fSW is 500 kHz (Figure 3), and RIREF is selected to be 8.25 kΩ if ILED is 150 mA (Figure 4 ). 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. 4 PC Board Layout The layout of the printed circuit board is critical to optimize the performance of the LM3492HC application circuit. In general, external components should be placed as close to the LM3492HC 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 LM3492HC 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 LM3492HC 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 LM3492HC. The ground plane should be used to connect the exposed pad of the LM3492HC, which is internally connected to the LM3492HC die substrate. The area of the ground plane should be extended as much as possible on the same copper layer around the LM3492HC. Using numerous vias beneath the exposed pad to dissipate heat of the LM3492HC to another copper layer is also a good practice. SNVA639A – May 2012 – Revised April 2013 Submit Documentation Feedback AN-2231 LM3492HC Evaluation Board Reference Design Copyright © 2012–2013, Texas Instruments Incorporated 5 Bill of Materials 5 6 www.ti.com Bill of Materials Item Part Number Mfg name Qty Ref Designator(s) Size 1 GRM31CR61E106KA12L muRata Cap 10 µF 25V X5R Part Description 2 CIN1, CIN2 1206 2 GRM188R71C474KA88D muRata 0603/X7R/0.47 µF/16V 1 CCDHC 0603 3 GRM1885C2A100JA01D 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 CRCW0603309KFKEA Vishay Resistor Chip 309 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 0603 12 CDRH10D68/ANP270MC Sumida Inductor 27 µH 1.9A 1 L1 10×10×6.8 13 SK310A-TP 1 D1 SMA Terminal DBL Turret 0.109”L Brass 11 VIN, GND, PGND, VLED1, VLED2, IOUT1, IOUT2, DIM1, DIM2, COMM, EN Micro Commercial Schottky 100V 3A 14 1502-2k-ND KEYSTONE 15 LM3492HC TI LM3492HC Evaluation Board 1 PCB 16 LM3492HC TI IC LM3492HC 1 U1 AN-2231 LM3492HC Evaluation Board Reference Design Copyright © 2012–2013, Texas Instruments Incorporated HTSSOP-20 SNVA639A – May 2012 – Revised April 2013 Submit Documentation Feedback Typical Performance and Waveforms www.ti.com 6 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 Regulation vs Input Voltage 100 1.00 125°C 0.50 90 ûILED(%) EFFICIENCY (%) 0.75 -40°C 95 85 0.00 25°C -0.25 125°C 80 0.25 -0.50 25°C 75 -0.75 70 -40°C -1.00 9 10 11 12 13 14 INPUT VOLTAGE (V) Steady State Operation SNVA639A – May 2012 – Revised April 2013 Submit Documentation Feedback 15 16 9 10 11 12 13 14 INPUT VOLTAGE (V) 15 16 LED Dimming 10000:1 (Dimming frequency 200Hz) AN-2231 LM3492HC Evaluation Board Reference Design Copyright © 2012–2013, Texas Instruments Incorporated 7 Typical Performance and Waveforms www.ti.com Power Up 8 AN-2231 LM3492HC Evaluation Board Reference Design Copyright © 2012–2013, Texas Instruments Incorporated Enable Transient SNVA639A – May 2012 – Revised April 2013 Submit Documentation Feedback IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s terms and conditions of sale of semiconductor products. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where mandated by applicable law, testing of all parameters of each component is not necessarily performed. TI assumes no liability for applications assistance or the design of Buyers’ products. Buyers are responsible for their products and applications using TI components. To minimize the risks associated with Buyers’ products and applications, Buyers should provide adequate design and operating safeguards. TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other intellectual property right relating to any combination, machine, or process in which TI components or services are used. Information published by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the third party, or a license from TI under the patents or other intellectual property of TI. Reproduction of significant portions of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied by all associated warranties, conditions, limitations, and notices. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional restrictions. Resale of TI components or services with statements different from or beyond the parameters stated by TI for that component or service voids all express and any implied warranties for the associated TI component or service and is an unfair and deceptive business practice. TI is not responsible or liable for any such statements. Buyer acknowledges and agrees that it is solely responsible for compliance with all legal, regulatory and safety-related requirements concerning its products, and any use of TI components in its applications, notwithstanding any applications-related information or support that may be provided by TI. Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards which anticipate dangerous consequences of failures, monitor failures and their consequences, lessen the likelihood of failures that might cause harm and take appropriate remedial actions. Buyer will fully indemnify TI and its representatives against any damages arising out of the use of any TI components in safety-critical applications. In some cases, TI components may be promoted specifically to facilitate safety-related applications. With such components, TI’s goal is to help enable customers to design and create their own end-product solutions that meet applicable functional safety standards and requirements. Nonetheless, such components are subject to these terms. No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the parties have executed a special agreement specifically governing such use. Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use in military/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI components which have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal and regulatory requirements in connection with such use. TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use of non-designated products, TI will not be responsible for any failure to meet ISO/TS16949. Products Applications Audio www.ti.com/audio Automotive and Transportation www.ti.com/automotive Amplifiers amplifier.ti.com Communications and Telecom www.ti.com/communications Data Converters dataconverter.ti.com Computers and Peripherals www.ti.com/computers DLP® Products www.dlp.com Consumer Electronics www.ti.com/consumer-apps DSP dsp.ti.com Energy and Lighting www.ti.com/energy Clocks and Timers www.ti.com/clocks Industrial www.ti.com/industrial Interface interface.ti.com Medical www.ti.com/medical Logic logic.ti.com Security www.ti.com/security Power Mgmt power.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defense Microcontrollers microcontroller.ti.com Video and Imaging www.ti.com/video RFID www.ti-rfid.com OMAP Applications Processors www.ti.com/omap TI E2E Community e2e.ti.com Wireless Connectivity www.ti.com/wirelessconnectivity Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265 Copyright © 2013, Texas Instruments Incorporated