As a result of the high output power capability of the OPA690, sinking heat or forced airflow can be required under extreme operating conditions. Maximum desired junction temperature sets the maximum allowed internal power dissipation. In no case can 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 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 but, for a grounded resistive load, be at a maximum when the output is fixed at a voltage equal to 1/2 of either supply voltage (for equal bipolar supplies) under the condition in Equation 3.

where

• R_L includes feedback network loading

As a worst-case example, compute the maximum T_J using an OPA690-DBV (6-pin SOT-23 package) in the circuit of Figure 7-1 operating at the maximum specified ambient temperature of 85°C and driving a grounded
20-Ω load.

Although this result is still much less than the specified maximum junction temperature, system reliability considerations can require lower tested junction temperatures. The highest possible internal dissipation occurs if the load requires current to be forced into the output for positive output voltages or sourced from the output for negative output voltages. This puts a high current through a large internal voltage drop in the output transistors. Figure 6-55, the output V-I plot shown in Section 6.11, include a boundary for 1-W maximum internal power dissipation under these conditions.