Datasheet LTC3026-1 (Analog Devices) - 10

LTC3026-1
OPERATION
more stable characteristics and are more suitable for use A junction-to-ambient thermal coefficient of 40°C/W is as the output capacitor. The X7R type has better stability achieved by connecting the exposed pad of the MSOP or across temperature, while the X5R is less expensive and DFN package directly to a ground plane of about 2500mm2. is available in higher values.
Calculating Junction Temperature
A minimum capacitance of 5μF must be maintained at all times on the LTC3026-1 LDO output. Example: Given an output voltage of 1.2V, an input voltage of 1.8V ±4%, an output current range of 0mA to 1A and
Thermal Considerations
a maximum ambient temperature of 50°C, what will the maximum junction temperature be? The power handling capability of the device will be limited by the maximum rated junction temperature (125°C). The The power dissipated by the device will be approximately: majority of the power dissipated in the device will be the I output current multiplied by the input/output voltage dif- OUT(MAX)(VIN(MAX) – VOUT) ferential: (I where: OUT)(VIN – VOUT). Note that the BIAS current is less than 200μA even under heavy loads, so its power IOUT(MAX) = 1A consumption can be ignored for thermal calculations. VIN(MAX) = 1.87V The LTC3026-1 has internal thermal limiting designed to so: protect the device during momentary overload conditions. For continuous normal conditions, the maximum junction P = 1A(1.87V – 1.2V) = 0.67W temperature rating of 125°C must not be exceeded. It is Even under worst-case conditions LTC3026-1’s BIAS pin important to give careful consideration to all sources of power dissipation is only about 1mW, thus can be ignored. thermal resistance from junction to ambient. Additional The junction to ambient thermal resistance will be on the heat sources mounted nearby must also be considered. order of 40°C/W. The junction temperature rise above For surface mount devices, heat sinking is accomplished ambient will be approximately equal to: by using the heat-spreading capabilities of the PC board 0.67W(40°C/W) = 26.8°C and its copper traces. Copper board stiffeners and plated The maximum junction temperature will then be equal to through holes can also be used to spread the heat gener- the maximum junction temperature rise above ambient ated by power devices. plus the maximum ambient temperature or: TA = 26.8°C + 50°C = 76.8°C 30261f 10