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LT3070 Datasheet(PDF) 23 Page - Linear Technology |
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LT3070 Datasheet(HTML) 23 Page - Linear Technology |
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23 / 30 page  LT3070 23 3070fc For more information www.linear.com/LT3070 APPLICATIONS INFORMATION The LT3070’s internal power and thermal limiting circuitry protect it under overload conditions. For continuous nor- mal load conditions, do not exceed the maximum junction temperature of 125°C. Give careful consideration to all sources of thermal resistance from junction to ambient. This includes junction to case, case-to-heat sink interface, heat sink resistance or circuit board to ambient as the applicationdictates.Also,consideradditionalheatsources mounted in proximity to the LT3070. The LT3070 is a surface mount device and as such, heat sinking is ac- complished by using the heat spreading capabilities of the PC board and its copper traces. Surface mount heat sinks and plated through-holes can also be used to spread the heatgeneratedbypowerdevices.Junction-to-casethermal resistance is specified from the IC junction to the bottom of the case directly below the die. This is the lowest resis- tance path for heat flow. Proper mounting is required to ensure the best possible thermal flow from this area of the packagetotheheatsinkingmaterial.Notethattheexposed pad is electrically connected to GND. Table 3 lists thermal resistance as a function of copper area in a fixed board size. All measurements were taken in still air on a 4-layer FR-4 board with 1 oz solid internal planes and 2 oz top/bottom external trace planes with a totalboardthicknessof1.6mm.PCBlayers,copperweight, board layout and thermal vias affect the resultant thermal resistance. For further information on thermal resistance and high thermal conductivity test boards, refer to JEDEC standard JESD51, notably JESD51-12 and JESD51-7. Achieving low thermal resistance necessitates attention to detail and careful PCB layout. Table 3, UFD Plastic Package, 28-Lead QFN COPPER AREA BOARD AREA THERMAL RESISTANCE (JUNCTION-TO-AMBIENT) TOPSIDE* BACK SIDE 2500mm2 2500mm2 2500mm2 30°C/W 1000mm2 2500mm2 2500mm2 32°C/W 225mm2 2500mm2 2500mm2 33°C/W 100mm2 2500mm2 2500mm2 35°C/W *Device is mounted on topside Calculating Junction Temperature Example: Given an output voltage of 0.9V, an input voltage range of 1.2V ± 5%, a BIAS voltage of 2.5V, a maximum output current of 4A and a maximum ambient temperature of 50°C, what will the maximum junction temperature be? The power dissipated by the device equals: IOUT(MAX) • (VIN(MAX) – VOUT) + (IBIAS – IGND) • VOUT + IGND • VBIAS where: IOUT(MAX) = 4A VIN(MAX) = 1.26V IBIAS at (IOUT = 4A, VBIAS = 2.5V) = 6.91mA IGND at (IOUT = 4A, VBIAS = 2.5V) = 0.87mA thus: P = 4A(1.26V – 0.9V) + (6.91mA – 0.87mA)0.9V + 0.87mA(2.5V) = 1.448W With the QFN package soldered to maximum copper area, the thermal resistance is 30°C/W. So the junction temperature rise above ambient equals: 1.448W at 30°C/W = 43.44°C The maximum junction temperature equals the maximum ambienttemperatureplusthemaximumjunctiontempera- ture rise above ambient or: TJMAX = 50°C + 43.44°C = 93.44°C Applications that cannot support extensive PCB space for heat sinking the LT3070 require a derating of output current or increased airflow. Paralleling Devices for Higher IOUT MultipleLT3070smaybeparalleledtoobtainhigheroutput current.Thisparallelingconceptborrowsfromthescheme employed by the LT3080. To accomplish this paralleling, tie the REF/BYP pins of the paralleled regulators together. This effectively gives an averaged value of multiple 600mV reference voltage sources. Tie the OUT pins of the paralleled regulators to |
Similar Part No. - LT3070_15 |
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Similar Description - LT3070_15 |
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