Datasheet

LM1875
SNAS524A –MAY 2004–REVISED MAY 2004 www.ti.com
THERMAL PROTECTION
The LM1875 has a sophisticated thermal protection scheme to prevent long-term thermal stress to the device.
When the temperature on the die reaches 170°C, the LM1875 shuts down. It starts operating again when the die
temperature drops to about 145°C, but if the temperature again begins to rise, shutdown will occur at only 150°C.
Therefore, the device is allowed to heat up to a relatively high temperature if the fault condition is temporary, but
a sustained fault will limit the maximum die temperature to a lower value. This greatly reduces the stresses
imposed on the IC by thermal cycling, which in turn improves its reliability under sustained fault conditions.
Since the die temperature is directly dependent upon the heat sink, the heat sink should be chosen for thermal
resistance low enough that thermal shutdown will not be reached during normal operation. Using the best heat
sink possible within the cost and space constraints of the system will improve the long-term reliability of any
power semiconductor device.
POWER DISSIPATION AND HEAT SINKING
The LM1875 must always be operated with a heat sink, even when it is not required to drive a load. The
maximum idling current of the device is 100 mA, so that on a 60V power supply an unloaded LM1875 must
dissipate 6W of power. The 54°C/W junction-to-ambient thermal resistance of a TO-220 package would cause
the die temperature to rise 324°C above ambient, so the thermal protection circuitry will shut the amplifier down if
operation without a heat sink is attempted.
In order to determine the appropriate heat sink for a given application, the power dissipation of the LM1875 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
S
is the total power supply voltage across the LM1875
• R
L
is the load resistance
• P
Q
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. 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
JC
=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 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.
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