VCA810_16 Datasheet, PDF (32/40 Pages) Texas Instruments – VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier

VCA810
SBOS275G – JUNE 2003 – REVISED DECEMBER 2015
11.2 Layout Example
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Figure 49. Layout Example
11.2.1 Thermal Analysis
The VCA810 will not require heatsinking or airflow in most applications. Maximum desired junction temperature
would set the maximum allowed internal power dissipation as described in this section. In no case should the
maximum junction temperature be allowed to exceed 150°C.
Operating junction temperature (T_J) is given by Equation 12:
T_J
=
T
A
+
P_D
´
q_JA
(12)
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; for a
grounded resistive load, however, it is at a maximum when the output is fixed at a voltage equal to one-half of
either supply voltage (for equal bipolar supplies). Under this worst-case condition, P_DL = V_S.²/(4 ● R_L), where R_L
is the resistive load.
Note that it is the power in the output stage and not in the load that determines internal power dissipation. As a
worst-case example, compute the maximum T_J using an VCA810ID (SO-8 package) in the circuit of Figure 34
operating at maximum gain and at the maximum specified ambient temperature of 85°C.
P_D = 10 V(24.8 mA) + 5²/(4 × 500 Ω) = 260.5 mW
(13)
Maximum T_J = 85°C + (0.260 W ×125°C/W) = 117.6°C
(14)
This maximum operating junction temperature is well below most system level targets. Most applications will be
lower since an absolute worst-case output stage power was assumed in this calculation of V_S/2 which is beyond
the output voltage range for the VCA810.
32
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