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LT3012BEDE Datasheet(PDF) 10 Page - Linear Technology |
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LT3012BEDE Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LT3012B 3012bf The LT3012B has internal thermal limiting designed to protect the device during overload conditions. For con- tinuous normal conditions the maximum junction tem- perature rating of 125 °C must not be exceeded. It is important to give careful consideration to all sources of thermal resistance from junction to ambient. Additional heat sources mounted nearby must also be considered. For surface mount devices, heat sinking is accomplished by using the heat spreading capabilities of the PC board and its copper traces. Copper board stiffeners and plated through-holes can also be used to spread the heat gener- ated by power devices. The following tables list thermal resistance for several different board sizes and copper areas. All measurements were taken in still air on 3/32" FR-4 board with one ounce copper. Table 1. DFN Measured Thermal Resistance COPPER AREA THERMAL RESISTANCE TOPSIDE BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT) 2500 sq mm 2500 sq mm 2500 sq mm 40 °C/W 1000 sq mm 2500 sq mm 2500 sq mm 45 °C/W 225 sq mm 2500 sq mm 2500 sq mm 50 °C/W 100 sq mm 2500 sq mm 2500 sq mm 62 °C/W Table 2. TSSOP Measured Thermal Resistance COPPER AREA THERMAL RESISTANCE TOPSIDE BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT) 2500 sq mm 2500 sq mm 2500 sq mm 40 °C/W 1000 sq mm 2500 sq mm 2500 sq mm 45 °C/W 225 sq mm 2500 sq mm 2500 sq mm 50 °C/W 100 sq mm 2500 sq mm 2500 sq mm 62 °C/W The thermal resistance junction-to-case ( θJC), measured at the exposed pad on the back of the die, is 16 °C/W. Continuous operation at large input/output voltage differ- entials and maximum load current is not practical due to thermal limitations. Transient operation at high input/ output differentials is possible. The approximate thermal time constant for a 2500sq mm 3/32" FR-4 board with maximum topside and backside area for one ounce copper is 3 seconds. This time constant will increase as more thermal mass is added (i.e. vias, larger board, and other components). For an application with transient high power peaks, aver- age power dissipation can be used for junction tempera- ture calculations as long as the pulse period is significantly less than the thermal time constant of the device and board. Calculating Junction Temperature Example 1: Given an output voltage of 5V, an input voltage range of 24V to 30V, an output current range of 0mA to 50mA, and a maximum ambient temperature of 50 °C, what will the maximum junction temperature be? The power dissipated by the device will be equal to: IOUT(MAX) • (VIN(MAX) – VOUT) + (IGND • VIN(MAX)) where: IOUT(MAX) = 50mA VIN(MAX) = 30V IGND at (IOUT = 50mA, VIN = 30V) = 1mA So: P = 50mA • (30V – 5V) + (1mA • 30V) = 1.28W The thermal resistance will be in the range of 40 °C/W to 62 °C/W depending on the copper area. So the junction temperature rise above ambient will be approximately equal to: 1.31W • 50 °C/W = 65.5°C APPLICATIO S I FOR ATIO |
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