The very low internal quiescent power dissipation for the THS4551, combined with the excellent thermal impedance of the 16-pin VQFN package (RGT), limits the possibility of excessively high internal junction temperatures. A more detailed analysis may be warranted because the 10-pin WQFN (RUN) package has a much higher junction-to-ambient thermal impedance (θ_JA = 163°C/W).

To estimate the internal T_J, an estimate of the maximum internal power dissipation is first required. There are two pieces to the internal power dissipation: quiescent current power and the power used in the output stage to deliver load current. To simplify the latter, the worst-case output stage power drives a dc differential voltage across a load using half the total supply voltage. Also assume a maximum ambient temperature of 125°C, giving the maximum quiescent current as shown in Figure 11-1. As an example:

• Assume a maximum operating supply voltage of 5.4 V. This 5.4-V supply with a maximum I_CC of 1.75 mA gives a quiescent power term of 9.45 mW.
• Assume a 200-Ω differential load with a static 2.7-V differential voltage established across the load. The 1.35 mA of dc load current generates a maximum output stage power of (5.4 V – 2.7 V) × 1.35 mA = 3.65 mW.
• From the worst-case total internal P_D of 13.1 mW, multiplying the internal P_D with a 163°C/W thermal impedance times the 163°C/W thermal impedance for the very small 10-pin WQFN package results in a 2.1°C rise from ambient.

Even for this extreme condition and the maximum-rated ambient of 125°C, the junction temperature is a maximum of 127°C, which is less than the rated absolute maximum of 150°C. Follow this same calculation sequence for the exact application and package selected to predict the maximum T_J.