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LTM4636-1 데이터시트(PDF) 23 Page - Linear Technology |
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LTM4636-1 데이터시트(HTML) 23 Page - Linear Technology |
23 / 38 page LTM4636-1 23 46361fa For more information www.linear.com/LTM4636-1 APPLICATIONS INFORMATION Within the LTM4636-1, 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 LTM4636-1 and the specified PCB with all of the correct material coefficients along with accurate power loss source 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 is used to evaluate the LTM4636-1 with heat sink and airflow; (4) having solved for and analyzed these thermal resistance 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 operating 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 Figure 10 to Figure 12 can be used in coordination with the load current derating curves in Figure 13 to Figure 18 for calculating an approximate JA thermal resistance for the LTM4636-1 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 moduletemperaturerisecanbeallowed.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 JA 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 ambient, thus maximum junction temperature. Room temperature power loss curves are provided in Figure 10 through Figure 12. The printed circuit board is a 1.6mm thick six layer board with two ounce copper for all layers and one ounce copper for the two inner layers. The PCB dimensions are 95mm × 76mm. Safety Considerations The LTM4636-1 does not provide galvanic isolation from VIN to VOUT. There is no internal fuse. If required, a slow blow fuse with a rating twice the maximum input current needs to be provided to protect each unit from catastrophic failure. The fuse or circuit breaker should be selected to limit the current to the regulator during overvoltage in case of an internal top MOSFET fault. If the internal top MOSFET fails, then turning it off will not resolve the overvoltage, thus the internal bottom MOSFET will turn on indefinitely trying to protect the load. Under this fault condition, the |
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