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LTM4644 数据表(PDF) 22 Page - Linear Technology |
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LTM4644 数据表(HTML) 22 Page - Linear Technology |
22 / 38 page LTM4650A 22 4650afb For more information www.linear.com/LTM4650A APPLICATIONS INFORMATION 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 JESD51-9 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 µModulepackagemountedtoahardwaretestboard—also defined by JESD51-9 (“Test Boards for Area Array Surface MountPackageThermalMeasurements”).Themotivation for providing these thermal coefficients is found in JESD 51-12 (“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 anticipate the µModule regulator’s thermal performance in their ap- plication 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 typically gives four thermal coefficients explicitly defined in JESD 51-12; these coef- ficients are quoted or paraphrased below: 1. θJA, the thermal resistance from junction to ambi- ent, 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 “still air” although natural convection causes the air to move. This value is determined with the part mounted to a JESD 51-9 defined test board, which does not reflect an actual application or viable operating condition. 2. θJCbottom, the thermal resistance from junction to the bottom of the product case, is the junction-to-board thermal resistance with all of the component power dissipationflowingthroughthebottomofthepackage. In the typical µModule, the bulk of the heat flows out the bottom of the package, but there is always heat flowoutintotheambientenvironment.Asaresult,this thermal resistance value may be useful for comparing packagesbutthetestconditionsdon’tgenerallymatch the user’s application. 3. θJCTOP, the thermal resistance from junction to top of the product case, is determined with nearly all of the component power dissipation flowing through the top of the package. As the electrical connections of the typical µModule 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 circuit board, is the junction-to-board thermal resistance where almost all of the heat flows through the bottom of the µModule and into the board, and is really the sum of the θJCbottom and the thermal re- sistance of the bottom of the part through the solder joints and through a portion of the board. The board temperature is measured a specified distance from the package, using a two sided, two layer board. This board is described in JESD 51-9. A graphical representation of the aforementioned thermal resistances is given in Figure 10; blue resistances are contained within the µModule regulator, whereas green resistances are external to the µModule. 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 JESD 51-12 or provided in the Pin Configuration section replicates or conveys normal operating conditions of a µModule. For example, in normal board-mounted applications, never does 100% of the device’s total power loss (heat) thermally conduct exclu- sively through the top or exclusively through bottom of the µModule—asthestandarddefinesforθJCtopandθJCbottom, respectively.Inpractice,powerlossisthermallydissipated in both directions away from the package—granted, in the absence of a heat sink and airflow, a majority of the heat flow is into the board. |
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类似说明 - LTM4644 |
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