Through normal operation, the OPA593 self-heats. Self-heating is a natural increase in the die junction temperature that occurs in every amplifier. The maximum allowed junction temperature sets the maximum allowed internal power dissipation (P_D) as described in the following paragraph. Design efforts should be made to prevent T_J from exceedeing the maximum temperature listed in the Absolute Maximum Ratings table.

Operating junction temperature (T_J) is determined by the ambient temperature (T_A), the internal P_D under the operating conditions, and the junction-to-ambient thermal resistance (R_θJA). This relationship is given by T_A + (P_D × R_θJA). P_D is the sum of quiescent power (P_DQ) and additional power dissipated in the output stage (P_DL) when delivering power to the load. P_DQ is 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 the P_DL is at a maximum when the output is fixed at a voltage equal to 1/2 of either supply voltage (for balanced bipolar supplies, V+ and V−). Under this condition P_DL = (V+)² / (4 × R_L), where R_L includes feedback network loading.

The power in the output stage and not into the load determines internal power dissipation.

As a worst-case example, compute the maximum T_J using the OPA593 in the circuit of Figure 8-1 operating at a maximum specified temperature of 125°C and driving a grounded 600-Ω load.

To enhance semiconductor long-term operating life, minimize T_J. Take proper measures to provide maximum heat removal through both heat conduction and radiation to help keep T_J to the lowest possible level. These proper measures include maximizing the PCB copper area to which the package thermal pad is soldered. The copper area serves as the traditional heat sink. The top layer copper is often easiest to route and is most often exposed to open air. PCB internal planes and the exposed bottom plane can also be used as heat sinks, but the connections are made with vias having higher thermal resistance. The OPA593EVM uses a board design that provides a highly effective thermal layout. The board design encompasses a large top-side copper area, and has heat conduction paths to other planes on the board. Additionally, other higher power-dissipating components are kept physically distant from the OPA593 to better accommodate heat removal by radiation.