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LT1086CH Datasheet(PDF) 10 Page - Linear Technology |
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LT1086CH Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page 10 LT1086 Series APPLICATIONS INFORMATION Load Regulation Because the LT1086 is a 3-terminal device, it is not possible to provide true remote load sensing. Load regu- lation will be limited by the resistance of the wire connect- ing the regulator to the load. The data sheet specification for load regulation is measured at the bottom of the package. Negative side sensing is a true Kelvin connec- tion, with the bottom of the output divider returned to the negative side of the load. Although it may not be immedi- ately obvious, best load regulation is obtained when the top of the resistor divider R1 is connected directly to the case not to the load. This is illustrated in Figure 2. If R1 were connected to the load, the effective resistance be- tween the regulator and the load would be: RP , RP = Parasitic Line Resistance R2 + R1 R1 () Thermal Considerations The LT1086 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. For example, using a LT1086CK (TO-3, Commercial) and assuming: VIN(max continuous) = 9V, VOUT = 5V, IOUT = 1A, TA = 75°C, θHEAT SINK = 3°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) = 4W Junction temperature will be equal to: TJ = TA + PD (θHEAT SINK + θCASE-TO-HEAT SINK + θJC) For the Control Section: TJ = 75°C + 4W(3°C/W + 0.2°C/ W + 0.7°C/W) = 95°C 95 °C < 125°C = TJMAX (Control Section Commercial Range) For the Power Transistor: TJ = 75°C + 4W(3°C/ W + 0.2°C/W + 4°C/ W) = 103.8°C 103.8 °C < 150°C = TJMAX (Power Transistor Commercial Range) LT1086 OUT IN VIN ADJ RP PARASITIC LINE RESISTANCE R1* *CONNECT R1 TO CASE CONNECT R2 TO LOAD 1086 • F02 RL R2* Figure 2. Connections for Best Load Regulation 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. Note that the resistance of the package leads for the H package ≈0.06Ω/inch. While it is usually not possible to connect the load directly to the package, it is possible to connect larger wire or PC traces close to the case to avoid voltage drops that will degrade load regulation. For fixed voltage devices the top of R1 is internally Kelvin connected and the ground pin can be used for negative side sensing. |
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