OPA657 Datasheet, PDF (27/37 Pages) Texas Instruments – 1.6GHz, Low-Noise, FET-Input OPERATIONAL AMPLIFIER

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OPA657
SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015
Thermal Considerations (continued)
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 but will—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 this
condition P_DL = V_S 2 / (4 × R_L) where R_L includes feedback network loading.
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 T_J using an OPA657N (SOT23-5 package) in the circuit of
Figure 29 operating at the maximum specified ambient temperature of +85°C and driving a grounded 100-Ω load.
P_D = 10V ´ 16.1mA + 5²/(4 ´ (100W||500W)) = 236mW
(10)
Maximum T_J = +85°C + (0.25W ´ 150°C/W) = 121°C
(11)
All actual applications are operating at lower internal power and junction temperature.
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