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

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.

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