Datasheet

LM2876
SNAS088C –AUGUST 1995–REVISED MARCH 2013 www.ti.com
THERMAL CONSIDERATIONS
Heat Sinking
The choice of a heat sink for a high-power audio amplifier is made entirely to keep the die temperature at a level
such that the thermal protection circuitry does not operate under normal circumstances. The heat sink should be
chosen to dissipate the maximum IC power for a given supply voltage and rated load.
With high-power pulses of longer duration than 100 ms, the case temperature will heat up drastically without the
use of a heat sink. Therefore the case temperature, as measured at the center of the package bottom, is entirely
dependent on heat sink design and the mounting of the IC to the heat sink. For the design of a heat sink for your
audio amplifier application refer to the Determining the Correct Heat Sink section.
Since a semiconductor manufacturer has no control over which heat sink is used in a particular amplifier design,
we can only inform the system designer of the parameters and the method needed in the determination of a heat
sink. With this in mind, the system designer must choose his supply voltages, a rated load, a desired output
power level, and know the ambient temperature surrounding the device. These parameters are in addition to
knowing the maximum junction temperature and the thermal resistance of the IC, both of which are provided by
Texas Instruments.
As a benefit to the system designer we have provided Maximum Power Dissipation vs Supply Voltages curves
for various loads in the TYPICAL PERFORMANCE CHARACTERISTICS section, giving an accurate figure for
the maximum thermal resistance required for a particular amplifier design. This data was based on θ
JC
= 1°C/W
and θ
CS
= 0.2°C/W. We also provide a section regarding heat sink determination for any audio amplifier design
where θ
CS
may be a different value. It should be noted that the idea behind dissipating the maximum power
within the IC is to provide the device with a low resistance to convection heat transfer such as a heat sink.
Therefore, it is necessary for the system designer to be conservative in his heat sink calculations. As a rule, the
lower the thermal resistance of the heat sink the higher the amount of power that may be dissipated. This is of
course guided by the cost and size requirements of the system. Convection cooling heat sinks are available
commercially, and their manufacturers should be consulted for ratings.
Proper mounting of the IC is required to minimize the thermal drop between the package and the heat sink. The
heat sink must also have enough metal under the package to conduct heat from the center of the package
bottom to the fins without excessive temperature drop.
A thermal grease such as Wakefield type 120 or Thermalloy Thermacote should be used when mounting the
package to the heat sink. Without this compound, thermal resistance will be no better than 0.5°C/W, and
probably much worse. With the compound, thermal resistance will be 0.2°C/W or less, assuming under 0.005
inch combined flatness runout for the package and heat sink. Proper torquing of the mounting bolts is important
and can be determined from heat sink manufacturer's specification sheets.
Should it be necessary to isolate V
−
from the heat sink, an insulating washer is required. Hard washers like
beryluum oxide, anodized aluminum and mica require the use of thermal compound on both faces. Two-mil mica
washers are most common, giving about 0.4°C/W interface resistance with the compound.
Silicone-rubber washers are also available. A 0.5°C/W thermal resistance is claimed without thermal compound.
Experience has shown that these rubber washers deteriorate and must be replaced should the IC be
dismounted.
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