Datasheet LT1086 (Analog Devices) - 10

LT1086 Series
U U W U APPLICATIONS INFORMATION Load Regulation Thermal Considerations
Because the LT1086 is a 3-terminal device, it is not The LT1086 series of regulators have internal power and possible to provide true remote load sensing. Load regu- thermal limiting circuitry designed to protect the device lation will be limited by the resistance of the wire connect- under overload conditions. For continuous normal load ing the regulator to the load. The data sheet specification conditions however, maximum junction temperature rat- for load regulation is measured at the bottom of the ings must not be exceeded. It is important to give careful package. Negative side sensing is a true Kelvin connec- consideration to all sources of thermal resistance from tion, with the bottom of the output divider returned to the junction to ambient. This includes junction-to-case, case- negative side of the load. Although it may not be immedi- to-heat sink interface and heat sink resistance itself. New ately obvious, best load regulation is obtained when the thermal resistance specifications have been developed to top of the resistor divider R1 is connected directly to the more accurately reflect device temperature and ensure case not to the load, as illustrated in Figure 2. If R1 were safe operating temperatures. The data section for these connected to the load, the effective resistance between the new regulators provides a separate thermal resistance and regulator and the load would be: maximum junction temperature for both the Control Sec- tion and the Power Transistor. Previous regulators, with a single junction-to-case thermal resistance specification, R R2 + R1 ( ) P , RP = Parasitic Line Resistance used an average of the two values provided here and R1 therefore could allow excessive junction temperatures under certain conditions of ambient temperature and heat RP PARASITIC sink resistance. To avoid this possibility, calculations LINE RESISTANCE V IN LT1086 OUT should be made for both sections to ensure that both IN ADJ thermal limits are met. R1* For example, using a LT1086CK (TO-3, Commercial) and RL assuming: R2* VIN(max continuous) = 9V, VOUT = 5V, IOUT = 1A, T *CONNECT R1 TO CASE A = 75°C, θHEAT SINK = 3°C/W, CONNECT R2 TO LOAD 1086 • F02 θCASE-TO-HEAT SINK = 0.2°C/W for T package with
Figure 2. Connections for Best Load Regulation
thermal compound. Power dissipation under these conditions is equal to: Connected as shown, RP is not multiplied by the divider PD = (VIN – VOUT)(IOUT) = 4W ratio. RP is about 0.004Ω per foot using 16-gauge wire. Junction temperature will be equal to: This translates to 4mV/ft at 1A load current, so it is important to keep the positive lead between regulator and TJ = TA + PD (θHEAT SINK + θ CASE-TO-HEAT SINK + θJC) load as short as possible and use large wire or PC board For the Control Section: traces. TJ = 75°C + 4W(3°C/W + 0.2°C/ W + 1.5°C/W) = 94.6°C Note that the resistance of the package leads for the H 95°C < 125°C = TJMAX (Control Section package ≈ 0.06Ω/inch. While it is usually not possible to Commercial Range) connect the load directly to the package, it is possible to For the Power Transistor: connect larger wire or PC traces close to the case to avoid T voltage drops that will degrade load regulation. J = 75°C + 4W(3°C/ W + 0.2°C/ W + 4°C/ W) = 103.8°C 103.8°C < 150°C = TJMAX (Power Transistor For fixed voltage devices the top of R1 is internally Kelvin Commercial Range) connected and the ground pin can be used for negative side sensing. sn1086 1086ffs 10