Datasheet LTC3025 (Analog Devices) - 8

LTC3025
APPLICATIONS INFORMATION
temperature range. The X5R and X7R dielectrics result in
Calculating Junction Temperature
more stable characteristics and are usually more suitable Example: Given an output voltage of 1.2V, an input voltage for use as the output capacitor. The X7R type has better of 1.8V to 3V, an output current range of 0mA to 100mA stability across temperature, while the X5R is less expensive and a maximum ambient temperature of 50°C, what will and is available in higher values. In all cases, the output the maximum junction temperature be? capacitance should never drop below 0.4μF, or instability or degraded performance may occur. The power dissipated by the device will be equal to:
Thermal Considerations
IOUT(MAX) (VIN(MAX) – VOUT) The power handling capability of the device will be limited where: by the maximum rated junction temperature (125°C). The IOUT(MAX) = 100mA power dissipated by the device will be the output current multiplied by the input/output voltage differential: VIN(MAX) = 3V (I So: OUT) (VIN – VOUT) Note that the BIAS current is less than 300μA even under P = 100mA(3V – 1.2V) = 0.18W heavy loads, so its power consumption can be ignored Even under worst-case conditions, the LTC3025’s BIAS pin for thermal calculations. power dissipation is only about 1mW, thus can be ignored. The LTC3025 has internal thermal limiting designed to Assuming a junction-to-ambient thermal resistance of protect the device during momentary overload conditions. 102°C/W, the junction temperature rise above ambient For continuous normal conditions, the maximum junction will be approximately equal to: temperature rating of 125°C must not be exceeded. It is 0.18W(102°C/W) = 18.4°C important to give careful consideration to all sources of The maximum junction temperature will then be equal to thermal resistance from junction to ambient. Additional the maximum junction temperature rise above ambient heat sources mounted nearby must also be considered. plus the maximum ambient temperature or: For surface mount devices, heat sinking is accomplished by using the heat-spreading capabilities of the PC board T = 50°C + 18.4°C = 68.4°C and its copper traces. Copper board stiffeners and plated through holes can also be used to spread the heat gener-
Short-Circuit/Thermal Protection
ated by power devices. The LTC3025 has built-in short-circuit current limiting as The LTC3025 2mm × 2mm DFN package is specifi ed as hav- well as overtemperature protection. During short-circuit ing a junction-to-ambient thermal resistance of 102°C/W, conditions, internal circuitry automatically limits the output which assumes a minimal heat spreading copper plane. The current to approximately 600mA. At higher temperatures, or actual thermal resistance can be reduced substantially by in cases where internal power dissipation causes excessive connecting the package directly to a good heat spreading self heating on chip, the thermal shutdown circuitry will ground plane. When soldered to 2500mm2 double-sided shut down the LDO when the junction temperature exceeds 1 oz. copper plane, the actual junction-to-ambient thermal approximately 150°C. It will re enable the LDO once the resistance can be less than 60°C/W. junction temperature drops back to approximately 140°C. 3025fd 8