Maximum desired junction temperature sets the maximum allowed internal power dissipation, as described below. In no case, the maximum junction temperature can be allowed to exceed +150°C.

Operating junction temperature (T_J) is given by T_A + P_D × θ_JA. 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 simply the specified no-load supply current times the total supply voltage across the part. P_DL depends on the required output signal and load; though, for resistive loads connected to mid-supply (V_S/2), P_DL is at a maximum when the output is fixed at a voltage equal to V_S/4 or 3V_S/4. Under this condition, P_DL = V_S²/(16 × R_L), where R_L includes feedback network loading.

This 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 an OPA4830 (TSSOP-14 package) in the circuit of Figure 8-1 operating at the maximum specified ambient temperature of +85°C and driving a 150Ω load at mid-supply.

P_D = 5V × 19mA +4 * [5² / (4 × (150Ω || 750Ω))] = 295mW

Maximum T_J = +85°C + (0.295W × 109.6°C/W) = 117.4°C.

Although this value is still well below the specified maximum junction temperature, system reliability considerations can require lower mandatory junction temperatures. The highest possible internal dissipation occurs if the load requires current to be forced into the output at high output voltages or sourced from the output at low output voltages. This puts a high current through a large internal voltage drop in the output transistors.