Datasheet LT1083, LT1084, LT1085 (Analog Devices) - 13

LT1083/LT1084/LT1085
applicaTions inForMaTion
Connected as shown, RP is not multiplied by the divider compound at the case-to-heat sink interface is strongly ratio. RP is about 0.004Ω per foot using 16-gauge wire. This recommended. If the case of the device must be electri- translates to 4mV/ft at 1A load current, so it is important cally isolated, a thermally conductive spacer can be used, to keep the positive lead between regulator and load as as long as its added contribution to thermal resistance is short as possible and use large wire or PC board traces. considered. Note that the case of all devices in this series is electrically connected to the output.
Thermal Considerations
For example, using an LT1083CK (TO-3, Commercial) The LT1083 series of regulators have internal power and and assuming: thermal limiting circuitry designed to protect the device under overload conditions. For continuous normal load VIN (Max Continuous) = 9V, VOUT = 5V, IOUT = 6A, conditions however, maximum junction temperature rat- TA = 75°C, θHEAT SINK = 1°C/W, ings must not be exceeded. It is important to give careful consideration to all sources of thermal resistance from θCASE-TO-HEAT SINK = 0.2°C/W for K package with thermal compound. junction to ambient. This includes junction-to-case, case- to-heat sink interface, and heat sink resistance itself. New Power dissipation under these conditions is equal to: thermal resistance specifications have been developed to P more accurately reflect device temperature and ensure D = (VIN – VOUT )(IOUT) = 24W safe operating temperatures. The data section for these Junction temperature will be equal to: new regulators provides a separate thermal resistance and TJ = TA + PD (θHEAT SINK + θCASE-TO-HEAT SINK + θJC) maximum junction temperature for both the Control Section and the Power Transistor. Previous regulators, with a single For the Control Section: junction-to-case thermal resistance specification, used TJ = 75°C + 24W (1°C/W + 0.2°C/W + 0.6°C/W) = 118°C an average of the two values provided here and therefore 118°C < 125°C = TJMAX (Control Section could allow excessive junction temperatures under certain Commercial Range) conditions of ambient temperature and heat sink resistance. For the Power Transistor: To avoid this possibility, calculations should be made for both sections to ensure that both thermal limits are met. TJ = 75°C + 24W (1°C/W + 0.2°C/W + 1.6°C/W) = 142°C 142°C < 150°C = T Junction-to-case thermal resistance is specified from the JMAX (Power Transistor Commercial Range) IC junction to the bottom of the case directly below the die. This is the lowest resistance path for heat flow. Proper In both cases the junction temperature is below the maxi- mounting is required to ensure the best possible thermal mum rating for the respective sections, ensuring reliable flow from this area of the package to the heat sink. Thermal operation. 108345fh For more information www.linear.com/LT1083 13 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Block Diagram Applications Information Typical Application Package Description Revision History Typical Applications Related Parts