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LTM4639 Scheda tecnica(PDF) 17 Page - Linear Technology |
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LTM4639 Scheda tecnica(HTML) 17 Page - Linear Technology |
17 / 28 page LTM4632 17 4632fc For more information www.linear.com/LTM4632 APPLICATIONS INFORMATION power loss as that which was simulated. The outcome of this process and due diligence yields the set of derating curves provided in other sections of this data sheet. After these laboratory test have been performed and correlated to the µModule model, then the θJB and θBA are summed together to correlate quite well with the µModule model with no airflow or heat sinking in a properly define cham- ber. This θJB + θBA value is shown in the Pin Configuration section and should accurately equal the θJA value because approximately 100% of power loss flows from the junc- tion through the board into ambient with no airflow or top mounted heat sink. The 1.0V, 1.5V, and 2.5V power loss curves in Figures 9 to 11 can be used in coordination with the load current derat- ing curves in Figures 12 to 17 for calculating an approxi- mate θJA thermal resistance for the LTM4632 with no heat sinking and various airflow conditions. The power loss curves are taken at room temperature, and are increased with multiplicative factors of 1.35 assuming junction temperature at 120°C. The derating curves are plotted with the output current starting at 6A by putting LTM4632 into two phase single output setup (Figure 20) and the ambient temperature at 40°C. These output voltages are chosen to include the lower and higher output voltage ranges for correlating the thermal resistance. Thermal models are derived from several temperature measure- ments in a controlled temperature chamber along with thermal modeling analysis. The junction temperatures are monitored while ambient temperature is increased with and without airflow. The power loss increase with ambient temperature change is factored into the derating Figure 8. Graphical Representation of JESD51-12 Thermal Coefficients curves. The junctions are maintained at 120°C maximum while lowering output current or power with increasing ambient temperature. The decreased output current will decrease the internal module loss as ambient tempera- ture is increased. The monitored junction temperature of 120°C minus the ambient operating temperature speci- fies how much module temperature rise can be allowed. As an example in Figure 12 the load current is derated to ~3A at ~100°C with no air or heat sink and the power loss for the 5V to 1V at 3A output is about 0.95W. The 0.95W loss is calculated with the ~0.7W room tempera- ture loss from the 5V to 1V power loss curve at 3A, and the 1.35 multiplying factor at 120°C measured junction temperature. If the 100°C ambient temperature is sub- tracted from the 120°C junction temperature, then the difference of 20°C divided by 0.95W equals a 20°C/W θJA thermal resistance. Table 2 specifies a 19°C~20°C/W value which is very close. Table 2 to 4 provide equivalent thermal resistances for 1.0V, 1.5V, and 2.5V outputs with and without airflow. The derived thermal resistances in Table 2 to 4 for the various conditions can be multiplied by the calculated power loss as a function of ambient temperature to derive temperature rise above ambient, thus maximum junction temperature. Room temperature power loss can be derived from the efficiency curves in the Typical Performance Characteristics section and adjusted with the above ambient temperature multiplica- tive factors. The printed circuit board is a 1.6mm thick four layer board with two ounce copper for the two outer layers and one ounce copper for the two inner layers. The PCB dimensions are 95mm × 76mm. 4638 F08 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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