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LX8415-25CST Datasheet(PDF) 6 Page - Microsemi Corporation |
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LX8415-25CST Datasheet(HTML) 6 Page - Microsemi Corporation |
6 / 7 page 0.5A L OW D ROPOUT P OSITIVE R EGULA TORS LX8415-xx PRODUCT DA T ABOOK 1996/1997 Copyright © 1999 Rev. 0.4 1/99 6 P RELIMINAR Y D AT A S HEET APPLICA TION NOTES THERMAL CONSIDERATIONS (continued) Example Given: V IN = 5.0V ±5%, VOUT = 2.5V ±3% I OUT = 0.5A, TA = 55°C, TJ = 125°C Rθ JT = 15°C/W, R θTS = 5°C/W Find: The size of a square area of 1oz. copper circuit- board trace-foil that will serve as a heatsink, adequate to maintain the junction temperature of the LX8415 in the ST (SOT-223) package within specified limits. Solution: The junction temperature is: T J = PD (R θJT + RθCS + RθSA) + TA where: P D ≡ Dissipated power. Rθ JT ≡ Thermal resistance from the junction to the mounting tab of the package. Rθ TS ≡ Thermal resistance through the interface between the IC and the surface on which it is mounted. Rθ SA ≡ Thermal resistance from the mounting surface of the heatsink to ambient. T S ≡ Heat sink temperature. T J T C T S T A Rq JT Rq CS Rq SA T J - TA P D 3.1°C/W Rθ SA - 22.3°C/W First, find the maximum allowable thermal resistance of the heat sink: P D = [[V IN * (1 + TolVIN )] - [VOUT * (1 - TolVOUT)]] * IOUT P D = 1.4W Rθ SA = - (Rθ JT + R θTS) , Rθ SA = 29.6°C/W A test was conducted to determine the thermal characteristics of 1 oz. copper circuit-board trace material. The following equation describes the observed relationship between the area of a square copper pad, and the thermal resistance from the tab of a SOT-223 package soldered at the center of the pad to ambient. Area SINK = in2 Substituting the value for Rθ SA calculated above, we find that a square pad with area: Area SINK = 0.43 in 2 (0.66" x 0.66"), 280mm2 (17 x 17 mm) will be required to maintain the LX8415 junction temperature within specified limits. |
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