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LM1875T Folha de dados(PDF) 7 Page - National Semiconductor (TI) |
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LM1875T Folha de dados(HTML) 7 Page - National Semiconductor (TI) |
7 / 11 page Application Hints (Continued) output stage and constant power dissipation in all other parts of the circuit. The curves of “Power Dissipation vs Power Output” give a better representation of the behavior of the LM1875 with various power supply voltages and resistive loads. As an example, if the LM1875 is operated on a 50V power supply with a resistive load of 8 Ω, it can develop up to 19W of internal power dissipation. If the die temperature is to remain below 150˚C for ambient temperatures up to 70˚C, the total junction-to-ambient thermal resistance must be less than Using θ JC=2˚C/W, the sum of the case-to-heat-sink interface thermal resistance and the heat-sink-to-ambient thermal re- sistance must be less than 2.2˚C/W. The case-to-heat-sink thermal resistance of the TO-220 package varies with the mounting method used. A metal-to-metal interface will be about 1˚C/W if lubricated, and about 1.2˚C/W if dry. If a mica insulator is used, the thermal resistance will be about 1.6˚C/W lubricated and 3.4˚C/W dry. For this example, we assume a lubricated mica insulator between the LM1875 and the heat sink. The heat sink thermal resistance must then be less than 4.2˚C/W−2˚C/W−1.6˚C/W=0.6˚C/W. This is a rather large heat sink and may not be practical in some applications. If a smaller heat sink is required for reasons of size or cost, there are two alternatives. [EM00001]The maximum ambient operating temperature can be reduced to 50˚C (122˚F), resulting in a 1.6˚C/W heat sink, or the heat sink can be isolated from the chassis so the mica washer is not needed. This will change the required heat sink to a 1.2˚C/W unit if the case-to-heat-sink interface is lubricated. Note: When using a single supply, maximum transfer of heat away from the LM1875 can be achieved by mounting the device directly to the heat sink (tab is at ground potential); this avoids the use of a mica or other type insulator. The thermal requirements can become more difficult when an amplifier is driving a reactive load. For a given magnitude of load impedance, a higher degree of reactance will cause a higher level of power dissipation within the amplifier. As a general rule, the power dissipation of an amplifier driving a 60˚ reactive load (usually considered to be a worst-case loudspeaker load) will be roughly that of the same amplifier driving the resistive part of that load. For example, a loud- speaker may at some frequency have an impedance with a magnitude of 8 Ω and a phase angle of 60˚. The real part of this load will then be 4 Ω, and the amplifier power dissipation will roughly follow the curve of power dissipation with a 4 Ω load. www.national.com 7 |
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Descrição semelhante - LM1875T |
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