ZHCSJO8 May 2019 OPA818
ADVANCE INFORMATION for pre-production products; subject to change without notice.
The OPA818 will not require heatsinking or airflow in most applications. Maximum allowed junction temperature will set the maximum allowed internal power dissipation as described below. In no case should the maximum junction temperature be allowed to exceed 105°C.
Operating junction temperature (TJ) is given by TA + PD × RθJA. The total internal power dissipation (PD) is the sum of quiescent power (PDQ) and additional power dissipated in the output stage (PDL) to deliver load power. Quiescent power is simply the specified no-load supply current times the total supply voltage across the part. PDL will depend on the required output signal and load but would, 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 balanced bipolar supplies). Under this condition PDL = VS2/(4 × RL) where RL 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 TJ using OPA818 in the circuit of Figure 12 operating at the maximum specified ambient temperature of +85°C and driving a grounded 100-Ω load.
PD = 10 V × 27.7 mA + 52 /(4 × (100 Ω || 350.9 Ω)) ≈ 357 mW
Maximum TJ = 85°C + (0.357 W × 54.6°C/W) = 104.5°C.
All actual applications will be operating at lower internal power and junction temperature.