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LTM4636 Datasheet(PDF) 21 Page - Linear Technology |
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LTM4636 Datasheet(HTML) 21 Page - Linear Technology |
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21 / 36 page  LTM4636 21 4636f For more information www.linear.com/LTM4636 applicaTions inForMaTion 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 op- erating 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 the bottom of the µModule package—as the standard defines for θJCtop and θJCbottom, respectively. In practice, power loss is thermally dissipated 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. Within the LTM4636, be aware there are multiple power devices and components dissipating power, with a con- sequence that the thermal resistances relative to different junctions of components or die are not exactly linear with respect to total package power loss. To reconcile this complicationwithoutsacrificingmodelingsimplicity—but alsonotignoringpracticalrealities—anapproachhasbeen taken using FEA software modeling along with laboratory testing in a controlled-environment chamber to reason- ably define and correlate the thermal resistance values supplied in this data sheet: (1) Initially, FEA software is used to accurately build the mechanical geometry of the LTM4636 and the specified PCB with all of the correct materialcoefficientsalongwithaccuratepowerlosssource definitions; (2) this model simulates a software-defined JEDEC environment consistent with JESD51-12 to predict power loss heat flow and temperature readings at different interfaces that enable the calculation of the JEDEC-defined thermal resistance values; (3) the model and FEA software isusedtoevaluatetheLTM4636withheatsinkandairflow; (4) having solved for and analyzed these thermal resis- tance values and simulated various operating conditions in the software model, a thorough laboratory evaluation replicates the simulated conditions with thermocouples within a controlled-environment chamber while operat- ing the device at the same power loss as that which was simulated. The outcome of this process and due diligence yields the set of derating curves shown in this data sheet. The power loss curves in Figures 10 to 12 can be used in coordination with the load current derating curves in Figures 13 to 18 for calculating an approximate θJA thermal resistance for the LTM4636 with various airflow conditions. The power loss curves are taken at room temperature and can be increased with a multiplicative factor according to the junction temperature, which is ~1.4 for 120°C. The derating curves are plotted with the output current starting at 40A and the ambient temperature increased. The output voltages are 1V, 2.5V and 3.3V. These are chosen to include the lower, middle and higher output voltage ranges for correlating the thermal resistance. Thermal models are derived from several temperature measurements in a controlled temperature chamber along with thermal modeling analysis. The junction temperatures are monitored while ambienttemperatureisincreasedwithandwithoutairflow. Thepowerlossincreasewithambienttemperaturechange is factored into the derating curves. The junctions are maintained at ~125°C maximum while lowering output current or power with increasing ambient temperature. The decreased output current will decrease the internal module loss as ambient temperature is increased. The monitored junction temperature of 125°C minus the ambient operating temperature specifies how much moduletemperature risecan beallowed.Asanexample,in Figure 14 the load current is derated to ~30A at ~94°C with no air flow and the power loss for the 12V to 1.0V at 30A output is about 4.2W. The 4.2W loss is calculated with the ~3W room temperature loss from the 12V to 1.0V power loss curve at 30A, and the 1.4 multiplying factor at 125°C junction. If the 94°C ambient temperature is subtracted from the 125°C junction temperature, then the difference of 31°C divided by 4.2W equals a 7.4°C/W θJA thermal resistance. Table 2 specifies a 7.2°C/W value which is very close. Tables 2, 3, and 4 provide equivalent thermal resistances for 1V, 1.5V and 3.3V outputs with and without airflow and heat sinking. The derived thermal resistances in Tables 2 thru 4 for the various conditions canbemultipliedbythecalculatedpowerlossasafunction of ambient temperature to derive temperature rise above |
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