The THS3491 is designed for high-speed, high-output-power applications that require paying special attention to the thermal considerations when designing the PCB. The amplifier includes automatic thermal shutdown protection circuitry that shuts down the device when the internal junction temperature (T_J) exceeds approximately 160°C, and turns on the device when the device cools down to approximately 145°C. When delivering high output power into capacitive loads, the internal junction temperature (T_J) can exceed the 160°C limit because of internal power dissipation, resulting in thermal shutdown of the device. The occasional T_J(MAX) operation of 160°C is allowed. However, for long-term reliability of the device, TI recommends keeping the maximum internal device junction temperature (T_J) below 150°C. Any increase in the device junction temperature is the result of an increased internal power dissipation for a given ambient temperature (T_A). As a result, the internal power dissipation must be limited to prevent the amplifier from continuously running into thermal shutdown.

The following subsections discuss internal amplifier power dissipation (both DC and AC) for a purely resistive output load, as well as the average power dissipation for a simple resistive-capacitive (RC) output load. It is important note that this analysis is not equivalent to the total power drawn from the supplies, but instead is used to determine the linear safe operating area (SOA) of the system.