National Semiconductor Application Note 1629 Chris Richardson May 2007 Introduction ible PSOP-8 package with an exposed thermal pad (also called a die-attach paddle, or DAP) will be compared to help the user estimate the die temperature under various conditions, and determine which package is best for their application. The second part of this application note uses the power dissipation calculated in the first section to estimate the LM3404HV’s die temperature under two typical configurations of the LEDs relative to the LED driver. The first case is for the LM3404HV mounted on a separate PCB that is connected to the LEDs by a wiring harness. This case assumes an ambient temperature influenced by the heating of the LEDs, but no direct heating of the PCB by the LEDs themselves. The second case assumes the LM3404HV is mounted to the same MCPCB as the LEDs themselves. In this configuration the temperature of the MCPCB has a much stronger influence over the LM3404HV die temperature than the ambient temperature, and the tests assume a fixed MCPCB temperature instead of a fixed ambient temperature. The LM3404/04HV is a buck-regulator designed for driving high powered LEDs at forward currents of up to 1.0A. LED drivers often experience thermal conditions that are extreme even by the standards of switching converters. For example, LED drivers are often placed on the same metal-core PCB (MCPCB) as the LEDs themselves. At 1.0A, a single-die white LED can dissipate more than 3W. The temperature of the MCPCB can easily reach 60°C or more. Even when the driver is placed on a separate PCB, the combination of high power dissipation, small, enclosed spaces, and little-to-no air flow create high ambient temperatures and even higher junction temperatures. Thermal conditions for integrated (MOSFET on-board) LED drivers are made worse by the high duty cycles of LED drivers. For applications that use multiple LEDs, as many LEDs as possible are placed in series to match the current and voltage limitations of the regulator regulator. The result is that output voltage is just below the input voltage. A voltage regulator that provides a 5V output from a 24V input has a duty cycle of 21%, meaning that the internal MOSFET is on for 21% of the time. In contrast, a 24V input is often used to drive five series-connected white LEDs, and at 3.5V each this gives an output voltage of 17.5V, forcing the MOSFET to conduct for 73% of the time. The first part of this application note will explore the performance of the LM3404HV in a high current, high input voltage, high duty cycle application typical of many LED drivers, using lab-tested thermal performance results and simulations. The industry standard SO-8 package and the pin-for-pin compat- Test Circuit The test circuit uses the LM3404HV to drive ten series-connected 3W white LEDs from an input voltage of 48V ±5%. The total forward current, IF, is 1A ±5% at a typical forward drop of 36V (in thermal equilibrium). Output current ripple is 70 mAP-P or less. Switching frequency is 550 kHz ±10% and the circuit is surge protected up to 60V. A complete BOM is listed at the back of this document, and performance waveforms are given in Application Note AN-1585. The schematic is shown in Figure 1. Thermal Performance of the LM3404/04HV in SO-8 and PSOP-8 Packages Thermal Performance of the LM3404/04HV in SO-8 and PSOP-8 Packages 30019201 FIGURE 1. AN-1629 © 2007 National Semiconductor Corporation 300192 www.national.com AN-1629 PS = 0.5 x VIN x IF x (tR + tF) x fSW PS = 0.5 x 48 x 1.0 x (40 x 10-9) x 550000 = 528 mW Test PCB The test PCB for this application note is the LM3402/04 PSOP-8 Evaluation board. The board measures 1.95” by 1.25”, with 2 layers of 1oz copper and 62mil FR4. To obtain best thermal performance, most of the top layer and the entire bottom layer are composed of large copper areas (shapes). These areas act as heatsinks for the LM3404HV. Of special importance is the pad and thermal via arrangement that connects the DAP of the PSOP-8 package to the ground plane on the bottom layer. Figure 2 shows a detail of the pad, which is the recommended layout for best thermal performance. The LM3404HV in PSOP-8 can be used on a standard SO-8 footprint, and the LM3404HV in SO-8 can be used on the PSOP-8 Evaluation Board, however neither of these options take advantage of the enhanced thermal performance of the PSOP-8 when properly soldered to a thermal pad connected to a large (1 square inch or more) copper area. Plots of the PCB layers are shown at the end of this document. The total power dissipation inside the LM3404HV is then: PD = PC + PG + PS = 1.14W Thermal Calculations The LM3404HV has a maximum operating junction temperature (TJ) of 125°C. Calibrated testing of the LM3404HV in both the SO-8 package (NSID LM3404HVMA) and PSOP-8 (NSID LM3404HVMR) was performed using the actual PSOP-8 evaluation PCB. The results for junction-to-ambient thermal resistance (θJA) in °C/W are summarized below: Package 1.0W 1.5W SO-8 102 99 N/A PSOP-8 50.9 49.6 48.4 To match the expected application conditions, all tests were performed with no air flow. Data for the SO-8 package at 1.5W is not available because the final TJ exceeded 125°C. θJA is as much a property of the PCB as it is of the semiconductor chip. The top layer of the PSOP-8 Evaluation board is approximately 75% copper, and the bottom (accounting for holes and traces) is approximately 90%. The estimated total copper area is therefore (0.75 + 0.9) x (1.25” x 1.95”) = 4 square inches. With the power dissipation and thermal resistance data the maximum ambient operating temperature can be predicted or, given the ambient operating temperature, a decision can be made as to the proper package for the LM3404HV. Maximum ambient operating temperature, TA-MAX, can be determined with the following equation: 30019202 FIGURE 2. PSOP-8 Pad and Thermal Via Layout TA-MAX = TJ-MAX – PD x θJA TA-MAX (SO-8) = 125 – 1.14 x 99 = 12°C TA-MAX (PSOP-8) = 125 – 1.14 x 50 = 68°C Power Dissipation It is clear from the calculations that the PSOP-8 package must be used in this high dissipation application. As an alternative, if the ambient temperature is known, then the die temperature of the LM3404HV can be predicted by rearranging the previous equation: Power dissipation inside the LM3404HV can be divided into three types: conduction (I2R) loss, gate charge loss, and switching loss. For each calculation the maximum, worstcase values have been used. Duty cycle, D, is 0.75. The MOSFET RDSON is 0.75Ω, gate charge, QG is 6 nC, and the rise are fall times, tR and tF, are 20 ns each. Conduction loss, PC, in the internal MOSFET TJ = TA + PD x θJA For example, if the ambient temperature inside an enclosure with high power LEDs reaches 60°C, then the two package options can again be evaluated: PC = (IF x D)2 x RDSON = (1.0 x 0.75)2 x 0.75 = 420 mW Gate charging and VCC loss, PG, in the gate drive and linear regulator: TJ (SO-8) = 60 + 1.14 x 99 = 173°C TJ (PSOP-8) = 60 + 1.14 x 50 = 117°C PG = (IIN-OP + fSW x QG) x VIN PG = (675 x 10-6 + 550000 x 6 x 10-9) x 48 = 191 mW Again, the results show that the SO-8 package will not be able to keep the junction temperature within specification limits. Switching loss, PS, in the internal NFET: www.national.com 0.5W 2 AN-1629 Predicting Thermals with FR4 30019203 FIGURE 3. System Diagram 1 Using finite element analysis, the thermal performance of the LM3404HV in PSOP-8 and SO-8 was simulated with three different sized PCBs. These simulations assumed an ambient temperature of 22°C, a power dissipation of 1W, and a 2-layer PCB comprised of 1oz copper on top and bottom, separated by 62mil of FR4. The results are listed below: Material Thickness (mil) Thermal Conductivity (W/m-K) Metal Core (6061-T6 aluminum) 40 167 θJA (°C/W) PSOP-8 SO-8 Insulator Material 3 2.4 1.25" x 1.95" (Eval Board) 55.1 104.8 1.4 377 1" x 1" (Typical Case) 73.0 120.8 Copper Traces (75% coverage) 0.5" x 0.5" (Worst Case) 240.2 263.7 To attain typical operating conditions, the 1” square section containing the LM3404HV was assumed to be part of a larger MCPCB measuring 1” x 6”. The other 5 square inches contained 5 high power LEDs, and it was assumed that they heated the entire board to a uniform temperature, TBD, of 50° C (representing a large heatsink) and 75°C (representing a smaller heatsink.) The results for thermal impedance from the , are listed below: board to the LM3404HV die, There is good correlation between the test results and simulation results for the Evaluation Board Case. The strong influence of the PCB size can be seen by comparing the results from the demo board to those of the Typical and Worst Cases. Without enough copper area to spread and dissipate heat, the advantages of the exposed pad are reduced, and even the PSOP-8 package will be limited to low power, low ambient temperature applications. (°C/W) Predicting Thermals with MetalCore PCB TBD = 75°C TBD = 75°C TBD = 50°C TBD = 50°C TA = 60°C TA = 45°C TA = 40°C TA = 30°C SO-8 70.5 69.2 71.4 70.5 PSOP8 9.8 9.2 10.5 10.1 The parameter can be used to calculate the LM3404HV junction temperature using the following equation: Alternatively, can be used to determine the maximum tolerable board temperature by working back from a TJ-MAX of 125°C: 30019204 FIGURE 4. System Diagram 2 Finite element analysis was again used to simulate the performance of the LM3404HV when dissipating 1W in PSOP-8 and SO-8 packages on a 1” square section of MCPCB with the following composition: Two main conclusions can be drawn from the results. First, the thermal impedance has little dependence on the board temperature or the ambient temperature, and instead depends highly on the package used. Second, the PSOP-8 package is again far superior to the SO-8 package when the DAP is soldered to a mass with low thermal impedance, such as an MCPCB. 3 www.national.com AN-1629 The results presented in this application note are intended to guide the user in selecting the correct package and configuration for a high powered LED driver such as the LM3404HV. The general trends observed are valid for other applications, but will not correlate directly to other circuits or applications. Conclusion The higher the ambient temperature, the board temperature, and the power dissipation of the LM3404HV, the more likely that the PSOP-8 package will be required in order to maintain the junction temperature below TJ-MAX. In addition, as the thermal stress becomes greater and the PCB area becomes smaller, the more likely that the LM3404HV will need to be mounted on an MCPCB. Bill of Materials ID Part Number Type Size Parameters Qty Vendor U1 LM3404HVMR LED Driver PSOP-8 75V 1A 1 NSC L1 SLF10145T-470M1R4 Inductor 10.0 x 10.0 x 4.5mm 47µH, 1.4A, 0.1Ω 1 TDK D2 CMSH2-60 Schottky Diode SMB 60V 2A 1 Central Semi Cf VJ0603Y104KXXAT Capacitor 0603 100nF 10% 1 Vishay Cb VJ0603Y103KXXAT Capacitor 0603 10nF 10% 1 Vishay Cin1, Cin2 C4532X7R2A105M Capacitor 1812 2.2µF 100V 2 TDK Co C3216X7R2A474M Capacitor 1206 0.47µF 100V 1 TDK Rsns ERJ8BQFR20V Resistor 1206 0.2Ω 1% 1 Panasonic Ron CRCW06035363F Resistor 0603 536kΩ1% 1 Vishay PCB Layout 30019205 Top Layer www.national.com 4 AN-1629 30019206 Bottom Layer 5 www.national.com Thermal Performance of the LM3404/04HV in SO-8 and PSOP-8 Packages Notes THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS, IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT. TESTING AND OTHER QUALITY CONTROLS ARE USED TO THE EXTENT NATIONAL DEEMS NECESSARY TO SUPPORT NATIONAL’S PRODUCT WARRANTY. EXCEPT WHERE MANDATED BY GOVERNMENT REQUIREMENTS, TESTING OF ALL PARAMETERS OF EACH PRODUCT IS NOT NECESSARILY PERFORMED. NATIONAL ASSUMES NO LIABILITY FOR APPLICATIONS ASSISTANCE OR BUYER PRODUCT DESIGN. BUYERS ARE RESPONSIBLE FOR THEIR PRODUCTS AND APPLICATIONS USING NATIONAL COMPONENTS. PRIOR TO USING OR DISTRIBUTING ANY PRODUCTS THAT INCLUDE NATIONAL COMPONENTS, BUYERS SHOULD PROVIDE ADEQUATE DESIGN, TESTING AND OPERATING SAFEGUARDS. EXCEPT AS PROVIDED IN NATIONAL’S TERMS AND CONDITIONS OF SALE FOR SUCH PRODUCTS, NATIONAL ASSUMES NO LIABILITY WHATSOEVER, AND NATIONAL DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY RELATING TO THE SALE AND/OR USE OF NATIONAL PRODUCTS INCLUDING LIABILITY OR WARRANTIES RELATING TO FITNESS FOR A PARTICULAR PURPOSE, MERCHANTABILITY, OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE CHIEF EXECUTIVE OFFICER AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b) support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness. National Semiconductor and the National Semiconductor logo are registered trademarks of National Semiconductor Corporation. All other brand or product names may be trademarks or registered trademarks of their respective holders. Copyright© 2007 National Semiconductor Corporation AN-1629 For the most current product information visit us at www.national.com National Semiconductor Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530-85-86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +49 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: [email protected] National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560