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LTC3025EDC-4 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3025EDC-4 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page LTC3025-1/LTC3025-2/ LTC3025-3/LTC3025-4 9 30251234fc APPLICATIONS INFORMATION much as 75% of its intial capacitance over the operating temperature range. The X5R and X7R dielectrics result in more stable characteristics and are usually more suitable for use as the output capacitor. The X7R type has better stability across temperature, while the X5R is less expensive and is available in higher values. In all cases, the output capacitance should never drop below 0.4μF, or instability or degraded performance may occur. Thermal Considerations The power handling capability of the device will be limited by the maximum rated junction temperature (125°C). The power dissipated by the device will be the output current multiplied by the input/output voltage differential: (IOUT) (VIN – VOUT) Note that the BIAS current is less than 500μA even under heavy loads, so its power consumption can be ignored for thermal calculations. The LTC3025-X has internal thermal limiting designed to protect the device during momentary overload conditions. For continuous normal conditions, the maximum junction temperature rating of 125°C must not be exceeded. It is important to give careful consideration to all sources of thermal resistance from junction to ambient. Additional heat sources mounted nearby must also be considered. For surface mount devices, heat sinking is accomplished by using the heat-spreading capabilities of the PC board and its copper traces. Copper board stiffeners and plated through holes can also be used to spread the heat gener- ated by power devices. The LTC3025-X 2mm × 2mm DFN package is specified as having a junction-to-ambient thermal resistance of 102°C/W, which assumes a minimal heat spreading cop- per plane. The actual thermal resistance can be reduced substantially by connecting the package directly to a good heat spreading ground plane. When soldered to 2500mm2 double-sided 1 oz. copper plane, the actual junction-to- ambient thermal resistance can be less than 60°C/W. Calculating Junction Temperature Example: Given an output voltage of 1.2V, an input voltage of 1.8V to 3V, an output current range of 0mA to 100mA and a maximum ambient temperature of 50°C, what will the maximum junction temperature be? The power dissipated by the device will be equal to: IOUT(MAX) (VIN(MAX) – VOUT) where: IOUT(MAX) = 100mA VIN(MAX) = 3V So: P = 100mA(3V – 1.2V) = 0.18W Even under worst-case conditions, the LTC3025-X’s BIAS pin power dissipation is only about 1mW, thus can be ig- nored. Assuming a junction-to-ambient thermal resistance of 102°C/W, the junction temperature rise above ambient will be approximately equal to: 0.18W(102°C/W) = 18.4°C The maximum junction temperature will then be equal to the maximum junction temperature rise above ambient plus the maximum ambient temperature or: TJ = 50°C + 18.4°C = 68.4°C Short-Circuit/Thermal Protection The LTC3025-X has built-in short-circuit current limiting as well as overtemperature protection. During short-circuit conditions, internal circuitry automatically limits the output current to approximately 1130mA. At higher temperatures, or in cases where internal power dissipation causes exces- sive self heating on chip, the thermal shutdown circuitry will shut down the LDO when the junction temperature exceeds approximately 150°C. It will re enable the LDO once the junction temperature drops back to approximately 140°C. The LTC3025-X will cycle in and out of thermal |
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