Datasheet LT1585-1.5, LT1585A-1.5 (Analog Devices) - 6

LT1585-1.5/LT1585A-1.5
U U W U APPLICATIONS INFORMATION Load Regulation
Junction-to-case thermal resistance is specified from the IC junction to the bottom of the case directly below the die. It is not possible to provide true remote load sensing This is the lowest resistance path for heat flow. Proper because the LT1585-1.5/LT1585A-1.5 are 3-terminal de- mounting ensures the best thermal flow from this area of vices. Load regulation is limited by the resistance of the the package to the heat sink. Linear Technology strongly wire connecting the regulators to the load. Load regulation recommends thermal compound at the case-to-heat sink per the data sheet specification is measured at the bottom interface. Use a thermally conductive spacer if the case of of the package. the device must be electrically isolated and include its For fixed voltage devices, negative side sensing is a true contribution to the total thermal resistance. Please consult Kelvin connection with the GND pin of the device returned “Mounting Considerations for Power Semiconductors” to the negative side of the load. This is illustrated in 1990 Linear Applications Handbook, Volume I, Pages Figure 3. RR3-1 to RR3-20. The output connects to the case of the R device in the LT1585-1.5/LT1585A-1.5. P PARASITIC LT1585-1.5 LINE RESISTANCE For example, using an LT1585ACT-1.5 (TO-220, commer- VIN IN OUT cial) and assuming: GND RL VIN (Max Continuous) = 3.465V (3.3V + 5%), VOUT = 1.5V IOUT = 5A 1585-1.5 F03 TA = 70°C, θHEAT SINK = 3°C/W
Figure 3. Connection for Best Load Regulation
θCASE-TO-HEAT SINK = 1°C/W (with Thermal Compound) Power dissipation under these conditions is equal to:
Thermal Considerations
PD = (VIN – VOUT)(IOUT) = (3.465 – 1.5)(5A) = 9.825W The LT1585-1.5/LT1585A-1.5 protect the device under Junction temperature will be equal to: overload conditions with internal power and thermal limit- T ing circuitry. However, for normal continuous load condi- J = TA + PD(θHEAT SINK + θCASE-TO-HEAT SINK + θJC) tions, do not exceed maximum junction temperature rat- For the Control Section: ings. It is important to consider all sources of thermal TJ = 70°C + 9.825W (3°C/W + 1°C/W + 0.7°C/W) = 116.2°C resistance from junction-to-ambient. These sources in- 116.2°C < 125°C = TJMAX (Control Section Commercial clude the junction-to-case resistance, the case-to-heat Range) sink interface resistance, and the heat sink resistance. Thermal resistance specifications have been developed to For the Power Transistor: more accurately reflect device temperature and ensure safe TJ = 70°C + 9.825W (3°C/W + 1°C/W + 3°C/W) = 138.8°C operating temperatures. The electrical characteristics sec- 138.8°C < 150°C = TJMAX (Power Transistor Commercial tion provides a separate thermal resistance and maximum Range) junction temperature for both the control circuitry and the power transistor. Older regulators with a single junction- In both cases the junction temperature is below the maxi- to-case thermal resistance specification, use an average of mum rating for the respective sections, ensuring reliable the two values provided here and allow excessive junction operation. temperatures under certain conditions of ambient tem- perature and heat sink resistance. Calculate the maximum junction temperature for both sections to ensure that both thermal limits are met. 6