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LX8117A-28 Datasheet(PDF) 8 Page - Microsemi Corporation |
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LX8117A-28 Datasheet(HTML) 8 Page - Microsemi Corporation |
8 / 9 page 0.8, 1 & 1.2A L OW DROPOUT POSITIVE REGULATORS LX8117-xx/8117A-xx/8117B-xx PRODUCT DA T ABOOK 1996/1997 Copyright © 1999 Rev. 1.4 3/99 8 P RODUCTION D ATA 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 LX8117 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. 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 LX8117 junction temperature within specified limits. T J T C T S T A Rq JT Rq CS Rq SA T J - TA P D LOAD REGULATION (continued) Even when the circuit is optimally configured, parasitic resistance can be a significant source of error. A 100 mil (2.54 mm) wide PC trace built from 1 oz. copper-clad circuit board material has a parasitic resistance of about 5 milliohms per inch of its length at room temperature. If a 3-terminal regulator used to supply 2.50 volts is connected by 2 inches of this trace to a load which draws 5 amps of current, a 50 millivolt drop will appear between the regulator and the load. Even when the regulator output voltage is precisely 2.50 volts, the load will only see 2.45 volts, which is a 2% error. It is important to keep the connection between the regulator output pin and the load as short as possible, and to use wide traces or heavy-gauge wire. The minimum specified output capacitance for the regulator should be located near the reglator package. If several capacitors are used in parallel to construct the power system output capaci- tance, any capacitors beyond the minimum needed to meet the specified requirements of the regulator should be located near the sections of the load that require rapidly-changing amounts of current. Placing capacitors near the sources of load transients will help ensure that power system transient response is not impaired by the effects of trace impedance. To maintain good load regulation, wide traces should be used on the input side of the regulator, especially between the input capacitors and the regulator. Input capacitor ESR must be small enough that the voltage at the input pin does not drop below V IN (MIN) during transients. V IN (MIN) = VOUT + VDROPOUT (MAX) where: V IN (MIN) ≡ the lowest allowable instantaneous voltage at the input pin. V OUT ≡ the designed output voltage for the power supply system. V DROPOUT (MAX) ≡ the specified dropout voltage for the installed regulator. THERMAL CONSIDERATIONS The LX8117 regulators have internal power and thermal limiting circuitry designed to protect each device under overload conditions. For continuous normal load conditions, however, maximum junc- tion temperature ratings must not be exceeded. It is important to give careful consideration to all sources of thermal resistance from junction to ambient. This includes junction to case, case to heat sink interface, and heat sink thermal resistance itself. 3.1°C/W Rθ SA - 22.3°C/W |
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