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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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LTM4634 датащи(PDF) 19 Page - Linear Technology |
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LTM4634 датащи(HTML) 19 Page - Linear Technology |
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19 / 32 page  LTM4634 19 4634f For more information www.linear.com/LTM4634 applicaTions inForMaTion The derating curves are plotted with the output current starting at 15A (5A/CH) and the ambient temperature at ~40°C. The 15A comes from each of the three channels operating at 5A each. This simplifies the loading for this thermal testing. The output voltages are 3.3V, and 5V when all three channels are loaded together in parallel. Channel 1 and Channel 2 are designed to operate with outputs up to 5V, and Channel 3 is designed for 12V. The power loss curve values at a particular output voltage and output current for each output are taken and multiplied by 1.4 for increased power loss at 120°C junction. 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 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 specifies how much module temperature rise can be allowed. As an example, in Figure 17 the 5.0V load current is derated to ~12.6A at ~71°C with no air and with no heat sink. In Figure 10, the 12V to 5.0V power loss at 4.2A per chan- nel is 1.25W. The total loss would be 3 times 1.25W for 3.75W total power loss. The 3.75W is then multiplied by the 1.4 multiplier for 120°C junction. This 5.25W value is used with the total temperature rise of 120°C minus the 71°C ambient to calculate θJA thermal resistance. If the 71°C ambient temperature is subtracted from the 120°C junction temperature, then the difference of 49°C divided 5.25W equals a 9.3°C/W θJA thermal resistance. Table 2 specifies a 9.0°C/W value which is very close. Tables 2 and 3 provide equivalent thermal resistances for 3.3V and 5V outputs with and without air flow and heat sinking. The derivedthermalresistancesinTables 2and3forthevarious conditions can be multiplied by the calculated power loss as a function of the 120°C maximum junction tempera- ture to determine if the temperature rise plus ambient is belowthe120°Cmaximumjunctiontemperature.Thermal measurements or infrared analysis should be performed to validate the values. Ambient temperature power loss can be derived from the power loss curves in Figures 8 to 13 and adjusted with the 1.4 multiplier. The printed circuit board is a 1.6mm thick four-layer board with two ounce copper for the two outer layers and 1 ounce copper for the two inner layers. The PCB dimensions are 95mm × 76mm. The BGA heat sinks are listed in Table 3. Temperature Monitoring (TEMP1 and TEMP2) Diode connected PNP transistors are used for the TEMP1, TEMP2monitoringfunctionsincethediodeforwardvoltage varies with temperature. The temperature dependence of the diodes can be understood in the equation: VD =nVT ln ID IS where VT is the thermal voltage (kT/q), and n, the ideality factor, is 1 for the two diode connected PNPs being used in the LTM4634. IS is expressed by the typical empirical equation: IS =I0 exp –VG0 VT where I0 is a process and geometry-dependent current (I0 is typically around 20 orders of magnitude larger than IS at room temperature), and VG0 is the band gap voltage of 1.2V extrapolated to absolute zero or –273°C. If we take the IS equation and substitute into the VD equa- tion, then we get: VD = VG0 – kT q ln I0 ID , VT = kT q The expression shows that the diode voltage decreases (linearly if I0 were constant) with increasing temperature and constant diode current. Figure 5 shows a plot of VD vs Temperature over the operating temperature range of the LTM4634. |
Аналогичный номер детали - LTM4634 |
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Аналогичное описание - LTM4634 |
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