Texas Instruments APA100 manual 1. TAS5111 Thermal Table

Thermal

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. RθJA is a system thermal resistance from junction to ambient air. As such, it is a system parameter with roughly the following components:

-RθJC (the thermal resistance from junction to case, or in this case the metal pad)

-Thermal grease thermal resistance

-Heatsink thermal resistance

RθJC 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 in2. 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.
RqJA + RqJC ) 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-Ωloads 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 RθJA	13.2°C/W
RθJA ⋅ power dissipation	85°C

Technical Information 4-11