Datasheet

LM675

SNOSBP3E –MAY 1999–REVISED MARCH 2013

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POWER DISSIPATION AND HEAT SINKING

The LM675 should always be operated with a heat sink, even though at idle worst case power dissipation will be

only 1.8W (30 mA × 60V) which corresponds to a rise in die temperature of 97°C above ambient assuming θ

54°C/W for a TO-220 package. This in itself will not cause the thermal protection circuitry to shut down the

amplifier when operating at room temperature, but a mere 0.9W of additional power dissipation will shut the

amplifier down since T

will then increase from 122°C (97°C + 25°C) to 170°C.

In order to determine the appropriate heat sink for a given application, the power dissipation of the LM675 in that

application must be known. When the load is resistive, the maximum average power that the IC will be required

to dissipate is approximately:

where

• V

is the total power supply voltage across the LM675

• R

is the load resistance

• P

is the quiescent power dissipation of the amplifier

The above equation is only an approximation which assumes an “ideal” class B output stage and constant power

dissipation in all other parts of the circuit. As an example, if the LM675 is operated on a 50V power supply with a

resistive load of 8Ω, 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

= 2°C/W, the sum of the case-to-heat sink interface thermal resistance and the heat-sink-to-ambient

thermal resistance 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 LM675 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. (1)

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. The maximum ambient operating temperature can be

restricted 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.

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 will be roughly

that of the same amplifier driving the resistive part of that load. For example, some reactive loads may at some

frequency have an impedance with a magnitude of 8Ω and a phase angle of 60°. The real part of this load will

then be 8Ω × cos 60° or 4Ω, and the amplifier power dissipation will roughly follow the curve of power dissipation

with a 4Ω load.

Product Folder Links: LM675