The OPA817 will not require heatsinking or airflow in most applications. Maximum allowed junction temperature will set the maximum allowed internal power dissipation as described in the following paragraph. In no case should the maximum junction temperature be allowed to exceed 150°C.

Operating junction temperature (T_J) is given by T_A + P_D × R_θJA. The total internal power dissipation (P_D) is the sum of quiescent power (P_DQ) and additional power dissipated in the output stage (P_DL) to deliver load power. Quiescent power is the specified no-load supply current times the total supply voltage across the part. P_DL will depend on the required output signal and load, but for a grounded resistive load the P_DL will be at a maximum when the output is fixed at a voltage equal to 1/2 of either supply voltage (for balanced bipolar supplies). Under this condition P_DL = V_S²/(4 × R_L) where R_L includes feedback network loading.

Note that it is the power in the output stage and not into the load that determines internal power dissipation.

As a worst-case example, compute the maximum T_J using OPA817 in the circuit of Figure 9-1 operating at the maximum specified ambient temperature of +105°C and driving a grounded 100-Ω load.

P_D = 10 V × 23.5 mA + 5² /(4 × (100 Ω || 500 Ω)) ≅ 310 mW

Maximum T_J = 105°C + (0.310 W × 64.9°C/W) = 125.1°C.

All actual applications will be operating at lower internal power and junction temperature.