The internal power dissipation of any amplifier causes the internal (junction) temperature to rise. This phenomenon is called self heating. The absolute maximum junction temperature of the OPAx197-Q1 is 150°C and exceeding this maximum temperature causes damage to the device. The OPAx197-Q1 have a thermal protection feature that prevents damage from self heating. The protection works by monitoring the temperature of the device and turning off the op amp output drive for temperatures above 140°C. Figure 7-6 shows an application example for the OPAx197-Q1 that has significant self heating (159°C) because of the power dissipation (0.81 W). Thermal calculations indicate that for an ambient temperature of 65°C, the device junction temperature must reach 187°C. The actual device, however, turns off the output drive to maintain a safe junction temperature. Figure 7-6 shows how the circuit behaves during thermal protection. During normal operation, the device acts as a buffer so the output is 3 V. When self heating causes the device junction temperature to increase above 140°C, the thermal protection forces the output to a high-impedance state and the output is pulled to ground through resistor RL.

Figure 7-6 Thermal Protection