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LTM8068 Scheda tecnica(PDF) 15 Page - Linear Technology |
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LTM8068 Scheda tecnica(HTML) 15 Page - Linear Technology |
15 / 20 page LTM8067 15 8067fc For more information www.linear.com/LTM8067 APPLICATIONS INFORMATION confusion andinconsistency.Thesedefinitionsaregivenin JESD 51-12, and are quoted or paraphrased as follows: 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 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. θJCbottom is the junction-to-board thermal resistance with allofthecomponentpowerdissipationflowingthroughthe bottom of the package. In the typical µModule converter, the bulk of the heat flows out the bottom of the package, but there is always heat flow out into the ambient envi- ronment. 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. θJCtopisdeterminedwithnearlyallofthecomponentpower dissipation flowing through the top of the package. As the electricalconnectionsofthetypicalµModuleconverterare on the bottom of the package, it is rare for an application to operate such that most of the heat flows from the junc- tion 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. θJCboardisthejunction-to-boardthermalresistancewhere almost all of the heat flows through the bottom of the µModule converter 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 a specified distance from the package, using a two-sided, two-layer board. This board is described in JESD 51-9. Giventhesedefinitions,itshouldnowbeapparentthatnone of these thermal coefficients reflects an actual physical operating condition of a µModule converter. Thus, none of them can be individually used to accurately predict the thermal performance of the product. Likewise, it would be inappropriate to attempt to use any one coefficient to correlate to the junction temperature vs load graphs given in the product’s data sheet. The only appropriate way to use the coefficients is when running a detailed thermal analysis, such as FEA, which considers all of the thermal resistances simultaneously. A graphical representation of these thermal resistances is given in Figure 3. The blue resistances are contained within the µModule converter, and the green are outside. The die temperature of the LTM8067 must be lower than the maximum rating of 125°C, so care should be taken in the layout of the circuit to ensure good heat sinking of the LTM8067. The bulk of the heat flow out of the LTM8067 is through the bottom of the module and the BGA pads into the printed circuit board. Consequently a poor printed circuit board design can cause excessive heating, result- ing in impaired performance or reliability. Please refer to the PCB Layout section for printed circuit board design suggestions. Figure 3. Approximate Thermal Model of LTM8067 8067 F03 µMODULE DEVICE JUNCTION-TO-CASE (TOP) RESISTANCE JUNCTION-TO-BOARD RESISTANCE JUNCTION-TO-AMBIENT RESISTANCE (JESD 51-9 DEFINED BOARD) CASE (TOP)-TO-AMBIENT RESISTANCE BOARD-TO-AMBIENT RESISTANCE JUNCTION-TO-CASE (BOTTOM) RESISTANCE JUNCTION AMBIENT CASE (BOTTOM)-TO-BOARD RESISTANCE |
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