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LX8587-XXCDT-TR Datasheet(PDF) 7 Page - Microsemi Corporation |
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LX8587-XXCDT-TR Datasheet(HTML) 7 Page - Microsemi Corporation |
7 / 11 page Microsemi Integrated Products 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 7 Copyright © 2000 Rev. 2.0a, 2005-10-25 LX8587x-xx 3A Low Dropout Positive Regulators PRODUCTION DATA SHEET TM ® APPLICATION NOTES ( C ONTIN UE D) LOAD REGULATION (continued) Even when the circuit is configured optimally, parasitic resistance can be a significant source of error. A 100 mil. 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 regulator package. If several capacitors are used in parallel to construct the power system output capacitance, 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 VIN(MIN) during transients. X) DROPOUT(MA OUT ΙΝ(ΜΙΝ) V V V + = where: VIN(MIN) ≡ the lowest allowable instantaneous voltage at the input pin. VOUT ≡ the designed output voltage for the power supply system. VDROPOUT(MAX)≡ the specified dropout voltage for the installed regulator. THERMAL CONSIDERATIONS The LX8587/87A regulators have internal power and thermal limiting circuitry designed to protect each device under overload conditions. For continuous normal load conditions, however, maximum junction 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. Junction-to-case thermal resistance is specified from the IC junction to the back surface of the case directly opposite the die. This is the lowest resistance path for heat flow. Proper mounting is required to ensure the best possible thermal flow from this area of the package to the heat sink. Thermal compound at the case to heat sink interface is strongly recommended. If the case of the device must be electrically isolated, a thermally conductive spacer can be used, as long as its added contribution to thermal resistance is considered. Note that the case of all devices in this series is electrically connected to the output. Example Given: VIN = 5V VOUT = 2.8V IOUT = 5.0A TA = 50°C RθJT = 2.7°C/W for TO-220 300 ft/min airflow available Find: Proper Heat Sink to keep IC’s junction temperature below 125°C.** Solution: The junction temperature is: A θSA θCS θJT J T ) R R (R T + + + = D P where: P D ≡ Dissipated power. RθJT ≡ Thermal resistance from the junction to the mounting tab of the package. RθCS ≡ Thermal resistance through the interface between the IC and the surface on which it is mounted. (1.0°C/W at 6 in-lbs mounting screw torque). RθSA ≡ Thermal resistance from the mounting surface to ambient (thermal resistance of the heat sink). TS ≡ Heat Sink Temperature. T J T C T S T A R θJT R θCS R θSA First, find the maximum allowable thermal resistance of the heat sink: () CS θ θJT D A J SA θ R R P T T R + − − = 11.0W A 5 2.8V 5.0V = × − = − = D D P P 0 . ) ( I ) V (V OUT OUT IN(MAX) C/W C/W) 1.0 C/W (2.7 5.0A * 2.8V) (5.0V C 50 C 125 ° = ° + ° − − ° − ° = 1 . 3 R R θSA θSA Next, select a suitable heat sink. The selected heat sink must have RθSA < 3.1°C/W. Thermalloy heatsink 6296B has RθSA = 3.0°C/W with 3000ft/min air flow. Finally, verify that junction temperature remains within speci- fication using the selected heat sink: C 124 C 50 C/W) 3.0 C/W 1.0 C/W 11W(2.7 ° = ° + ° + ° + ° = J J T T ** Although the device can operate up to 150°C junction, it is recommended for long term reliability to keep the junction temperature below 125°C whenever possible. |
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