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LTM4634 Scheda tecnica(PDF) 17 Page - Linear Technology |
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LTM4634 Scheda tecnica(HTML) 17 Page - Linear Technology |
17 / 32 page LTM4634 17 4634f For more information www.linear.com/LTM4634 applicaTions inForMaTion ceramic capacitor to ground at EXTVCC and ensure the voltage at CNTL_PWR is always greater than the voltage at EXTVCC at all times during start-up and shutdown. Connecting VOUT3 to EXTVCC may present a convenient way to meet the sequencing requirement. Otherwise float EXTVCC if not used. Thermal Considerations and Output Current Derating The thermal resistances reported in the Pin Configuration section of the data sheet are consistent with those param- eters defined by JESD51-12 and are intended for use with finite element analysis (FEA) software modeling tools that leverage the outcome of thermal modeling, simulation, and correlation to hardware evaluation performed on a µModule package mounted to a hardware test board. The motivation for providing these thermal coefficients is found in JESD51-12 (“Guidelines for Reporting and Using Electronic Package Thermal Information”). Many designers may opt to use laboratory equipment and a test vehicle such as the demo board to predict the µModule regulator’s thermal performance in their appli- cation at various electrical and environmental operating conditions to compliment any FEA activities. Without FEA software, the thermal resistances reported in the Pin Con- figuration section are, in and of themselves, not relevant to providing guidance of thermal performance; instead, the derating curves provided in the data sheet can be used in a manner that yields insight and guidance pertaining to one’s application usage, and can be adapted to correlate thermal performance to one’s own application. The Pin Configuration section gives four thermal coeffi- cients explicitly defined in JESD51-12. These coefficients are quoted or paraphrased as follows: 1. θJA:Thethermalresistancefromjunctiontoambient,is the natural convection junction-to-ambient air thermal resistance measured in a one cubic foot sealed enclo- sure. This environment is sometimes referred to as “still air” although natural convection causes the air to move. This value is determined with the part mounted to a 95mm × 76mm PCB with four layers. 2. θJCbottom: The thermal resistance from the junction to the bottom of the product case, is determined with all of the internal power dissipation flowing through the bottom of the package. In a typical µModule regulator, the bulk of the heat flows out the bottom of the pack- age, but there is always heat flow out into the ambient environment. As a result, this thermal resistance value may be useful for comparing packages but the test conditions don’t generally match the user’s application. 3. θJCtop: The thermal resistance from junction to top of the product case, is determined with nearly all of the componentpowerdissipationflowingthroughthetopof the package. As the electrical connections of the typical µModule regulator are on the bottom of the package, it is rare for an application to operate such that most of the heat flows from the junction to the top of the part. As in the case of θJCbottom, this value may be useful for comparing packages but the test conditions don’t generally match the user’s application. 4. θJB: The thermal resistance from junction to the printed circuitboard,isthejunction-to-boardthermalresistance where almost all of the heat flows through the bottom of the µModule package and into the board, and is really the sum of the θJCbottom and the thermal resistance of the bottom of the part through the solder joints and through a portion of the board. The board temperature is measured at a specified distance from the package. A graphical representation of the aforementioned ther- mal resistances is given in Figure 4; blue resistances are contained within the μModule regulator, whereas green resistances are external to the µModule package. As a practical matter, it should be clear to the reader that no individual or sub-group of the four thermal resistance parameters defined by JESD51-12 or provided in the Pin Configuration section replicates or conveys normal oper- ating conditions of a μModule regulator. For example, in normal board-mounted applications, never does 100% of the device’s total power loss (heat) thermally con- duct exclusively through the top or exclusively through |
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