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LT1085IT Datasheet(PDF) 10 Page - Linear Technology |
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LT1085IT Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LT1083/LT1084/LT1085 108345fe APPLICATIONS INFORMATION Connected as shown, RP is not multiplied by the divider ratio. RP is about 0.004Ω per foot using 16-gauge wire. This translates to 4mV/ft at 1A load current, so it is important to keep the positive lead between regulator and load as short as possible and use large wire or PC board traces. Thermal Considerations The LT1083 series of regulators have internal power and thermal limiting circuitry designed to protect the device under overload conditions. For continuous normal load conditions however, maximum junction temperature rat- ings must not be exceeded. It is important to give careful consideration to all sources of thermal resistance from junction to ambient. This includes junction-to-case, case- to-heat sink interface, and heat sink resistance itself. New thermal resistance specifications have been developed to more accurately reflect device temperature and ensure safe operating temperatures. The data section for these new regulators provides a separate thermal resistance and maximum junction temperature for both the Control Sec- tion and the Power Transistor. Previous regulators, with a single junction-to-case thermal resistance specification, used an average of the two values provided here and therefore could allow excessive junction temperatures under certain conditions of ambient temperature and heat sink resistance. To avoid this possibility, calculations should be made for both sections to ensure that both thermal limits are met. Junction-to-case thermal resistance is specified from the IC junction to the bottom of the case directly below the die. This is the lowest resistance path for heat flow. Proper mounting is required to ensure the best possible thermal flow from this area of the package to the heat sink. Thermal compound at the case-to-heat sink interface is strongly recommended. If the case of the device must be electri- cally isolated, a thermally conductive spacer can be used, as long as its added contribution to thermal resistance is considered. Note that the case of all devices in this series is electrically connected to the output. For example, using an LT1083CK (TO-3, Commercial) and assuming: VIN (max continuous) = 9V, VOUT = 5V, IOUT = 6A, TA = 75°C, θHEAT SINK = 1°C/W, θCASE-TO-HEAT SINK = 0.2°C/W for K package with thermal compound. Power dissipation under these conditions is equal to: PD = (VIN – VOUT )(IOUT) = 24W Junction temperature will be equal to: TJ = TA + PD (θHEAT SINK + θCASE-TO-HEAT SINK + θJC) For the Control Section: TJ = 75°C + 24W (1°C/W + 0.2°C/W + 0.6°C/W) = 118°C 118 °C < 125°C = TJMAX (Control Section Commercial Range) For the Power Transistor: TJ = 75°C + 24W (1°C/W + 0.2°C/W + 1.6°C/W) = 142°C 142 °C < 150°C = TJMAX (Power Transistor Commercial Range) In both cases the junction temperature is below the maximum rating for the respective sections, ensuring reliable operation. |
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