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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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LTM8049 데이터시트(PDF) 14 Page - Linear Technology |
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LTM8049 데이터시트(HTML) 14 Page - Linear Technology |
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14 / 20 page  LTM8049 14 8049fa For more information www.linear.com/LTM8049 APPLICATIONS INFORMATION 4. Place the CIN and COUT capacitors such that their ground current flow directly adjacent or underneath the LTM8049. 5. Connect all of the GND connections to as large a copper pour or plane area as possible on the top layer. Avoid breaking the ground connection between the external components and the LTM8049. 6. Use vias to connect the GND copper area to the board’s internal ground planes. Liberally distribute these GND vias to provide both a good ground connection and thermal path to the internal planes of the printed circuit board. Pay attention to the location and density of the thermal vias in Figure 2. The LTM8049 can benefit from theheat-sinkingaffordedbyviasthatconnecttointernal GND planes at these locations, due to their proximity to internal power handling components. The optimum number of thermal vias depends upon the printed circuit board design. For example, a board might use very small via holes. It should employ more thermal vias than a board that uses larger holes. Hot-Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LTM8049. However, these capacitors can cause problems if the LTM8049 is plugged into a live input supply (see Application Note 88 for a complete dis- cussion). The low loss ceramic capacitor combined with stray inductance in series with the power source forms an underdamped tank circuit, and the voltage at the VIN pin of the LTM8049 can ring to more than twice the nominal inputvoltage,possiblyexceedingtheLTM8049’sratingand damaging the part. If the input supply is poorly controlled or the user will be plugging the LTM8049 into an energized supply,theinputnetworkshouldbedesignedtopreventthis overshoot. This can be accomplished by installing a small resistor in series to VIN, but the most popular method of controlling input voltage overshoot is to add an electrolytic bulk capacitor to the VINnet.Thiscapacitor’srelativelyhigh equivalentseriesresistancedampsthecircuitandeliminates the voltage overshoot. The extra capacitor improves low frequency ripple filtering and can slightly improve the ef- ficiency of the circuit, though it is physically large. Thermal Considerations The LTM8049 output current may need to be derated if it is required to operate in a high ambient temperature or deliver a large amount of continuous power. The amount of current derating is dependent upon the input voltage, output power and ambient temperature. The temperature rise curves given in the Typical Performance Character- istics section can be used as a guide. These curves were generated by a LTM8049 mounted to a 58cm2 4-layer FR4 printed circuit board. Boards of other sizes and layer count can exhibit different thermal behavior, so it is incumbent upon the user to verify proper operation over the intended system’sline,loadandenvironmentaloperatingconditions. The thermal resistance numbers listed in Page 2 of the data sheet are based on modeling the µModule package mounted on a test board specified per JESD51-9 (Test Boards for Area Array Surface Mount Package Thermal Measurements). The thermal coefficients provided in this page are based on JESD 51-12 (Guidelines for Reporting and Using Electronic Package Thermal Information). Forincreasedaccuracyandfidelitytotheactualapplication, many designers use FEA to predict thermal performance. To that end, Page 2 of the data sheet typically gives four thermal coefficients: θJA: Thermal resistance from junction to ambient θJCbottom: Thermal resistance from junction to the bottom of the product case θJCtop: Thermal resistance from junction to top of the product case θJB:Thermalresistancefromjunctiontotheprintedcircuit board. While the meaning of each of these coefficients may seem to be intuitive, JEDEC has defined each to avoid confusion and inconsistency. These definitions are given in JESD 51-12, and are quoted or paraphrased below: θJA is the natural convection junction-to-ambient air thermal resistance measured in a one cubic foot sealed enclosure. This environment is sometimes referred to as |
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