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Thermal
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Technical Information
4.6 Thermal
The APA100 thermal issues lie with the TAS5111. The following thermal
calculations and tables are taken from the TAS5111 data sheet. The TAS5111
is designed to be interfaced directly to a heatsink using a thermal interface
compound (for example, Wakefield Engineering type 126 thermal grease.)
The heatsink then absorbs heat from the ICs and couples it to the local air. If
the heatsink is carefully designed, this process can reach equilibrium and heat
can be continually removed from the ICs. Because of the efficiency of the
TAS5111, heatsinks are smaller than those required for linear amplifiers of
equivalent performance. R
θ
JA
is a system thermal resistance from junction to
ambient air. As such, it is a system parameter with roughly the following
components:
- R
θ
JC
(the thermal resistance from junction to case, or in this case the metal
pad)
- Thermal grease thermal resistance
- Heatsink thermal resistance
R
θ
JC
has been provided in the General Information section.
The thermal grease thermal resistance can be calculated from the exposed
pad area and the thermal grease manufacturer’s area thermal resistance
(expressed in °C−in
2
/W). The area thermal resistance of the example thermal
grease with a 0.001-inch thick layer is about 0.054°C−in
2
/W. The approximate
exposed pad area is 0.0164 in
2
. Dividing the example thermal grease area
resistance by the area of the pad gives the actual resistance through the
thermal grease, 3.3°C/W.
Heatsink thermal resistance is generally predicted by the heatsink vendor,
modeled using a continuous flow dynamics (CFD) model, or measured.
Thus, for a single monaural IC, the system is defined by Equation 9.
R
q
JA
+ R
q
JC
) thermal greasse resistance ) heatsink resistance.
(9)
The following table indicates modeled parameters for one TAS5111 IC on a
heatsink. The junction temperature is set at 110°C in both cases while
delivering 70 W RMS into 4-Ω loads with no clipping. It is assumed that the
thermal grease is about 0.001 inch thick (this is critical).
Table 4−1.TAS5111 Thermal Table
32-Pin TSSOP
Ambient temperature 25°C
Power to load 70 W
Delta T inside package 12.3°C
Delta T through thermal grease 21.1°C
Required heatsink thermal resistance 8.2°C/W
Junction temperature 110°C
System R
θ
JA
13.2°C/W
R
θ
JA
× power dissipation 85°C