LM1875-MWC National Semiconductor Corporation, LM1875-MWC Datasheet - Page 7

• Manufacturer Part Number: LM1875-MWC
• Description: 20w Audio Power Amplifier
• Manufacturer: National Semiconductor Corporation

Application Hints
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 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
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
JC
=2˚C/W, the sum of the case-to-heat-sink interface
(Continued)
7
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
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 8Ω and a phase angle of 60˚. The real part of
this load will then be 4Ω, and the amplifier power dissipation
will roughly follow the curve of power dissipation with a 4Ω
load.
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.
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