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LTM4646 数据表(PDF) 23 Page - Linear Technology |
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LTM4646 数据表(HTML) 23 Page - Linear Technology |
23 / 34 page LTM4646 23 4646f For more information www.linear.com/LTM4646 for JCtop and JCbottom, 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 LTM4646, be aware there are multiple power devices and components dissipating power, with a con- sequence that the thermal resistances relative to differ- ent junctions of components or die are not exactly linear with respect to total package power loss. To reconcile this complication without sacrificing modeling simplicity— but also, not ignoring practical realities—an approach has been taken using FEA software modeling along with laboratory testing in a controlled-environment chamber to reasonably define and correlate the thermal resistance values supplied in this data sheet: (1) Initially, FEA soft- ware is used to accurately build the mechanical geometry of the LTM4646 and the specified PCB with all of the cor- rect 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 LTM4646 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 a set of derating curves provided in other sections of this data sheet. After these laboratory tests have been performed, then the JB and BA are summed together to correlate quite well with the LTM4646 model with no airflow or heat sinking in a define chamber. This JB + BA value should accurately equal the JA value because approximately 100% of power loss flows from the junction through the board into ambient with no airflow or top mounted heat sink. Each system has its own thermal characteristics, therefore thermal analysis must be performed by the user in a particular system. The LTM4646 has been designed to effectively remove heat from both the top and bottom of the package. The APPLICATIONS INFORMATION bottom substrate material has very low thermal resistance to the printed circuit board. An external heat sink can be applied to the top of the device for excellent heat sinking with airflow. Safety Considerations The LTM4646 modules do not provide 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 input voltage will source very large cur- rents to ground through the failed internal top MOSFET and enabled internal bottom MOSFET. This can cause excessive heat and board damage depending on how much power the input voltage can deliver to this system. A fuse or circuit breaker can be used as a secondary fault protector in this situation. The device does support over current protection. Temperature diodes are provided for monitoring inter- nal temperature, and can be used to detect the need for thermal shutdown that can be done by controlling the RUN pin. Power Derating The 5V, 8V and 12V power loss curves in Figures 9 through 11 can be used in coordination with the load current derating curves in Figures 12 to 16 for calculating an approximate JA thermal resistance for the LTM4646 with airflow conditions. The power loss curves are taken at room temperature, and are increased with a 1.35 to 1.4 multiplicative factor at 125°C. These factors come from the fact that the power loss of the regulator increases about 45% from 25°C to 150°C, thus a 50% spread |
类似零件编号 - LTM4646 |
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类似说明 - LTM4646 |
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