OPA2890 Datasheet, PDF (26/38 Pages) Texas Instruments – Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2890
SBOS364C – DECEMBER 2007 – REVISED DECEMBER 2009
In normal operation, base current to Q1 is provided
through the 2MΩ resistor, while the emitter current
through the 50kΩ resistor sets up a voltage drop that
is inadequate to turn on the two diodes in the Q1
emitter. As V _DIS is pulled LOW, additional current is
pulled through the 50kΩ resistor, eventually turning
on those two diodes ( 30μA). At this point, any
further current pulled out of V _DIS goes through those
diodes holding the emitter-base voltage of Q1 at
approximately 0V. This current shuts off the collector
current out of Q1, turning the amplifier off. The supply
currents in the disable mode are only those required
to operate the circuit of Figure 62. Additional circuitry
ensures that turn-on time occurs faster than turn-off
time (make-before-break).
When disabled, the output and input nodes go to a
high-impedance state. If the OPA2890 is operating at
a gain of +1V/V, the device shows a very high
impedance at the output and exceptional signal
isolation. If operating at a gain greater than +1V/V,
the total feedback network resistance (R_F + R_G)
appears as the impedance looking back into the
output, but the circuit still shows very high forward
and reverse isolation. If configured as an inverting
amplifier, the input and output are connected through
the feedback network resistance (R_F + R_G) and the
isolation is very poor as a result.
THERMAL ANALYSIS
Maximum desired junction temperature sets the
maximum allowed internal power dissipation as
described below. In no case should the maximum
junction temperature be allowed to exceed +150°C.
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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; for a grounded resistive load, P_DL is
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.
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 T_J
using an OPA2890ID (SO-8 package) in the circuit of
Figure 49 operating at the maximum specified
ambient temperature of +85°C and with both outputs
driving a grounded 20Ω load to +2.5V.
P_D = 10V ´ 2.5mA + 2[5²/(4 ´ (75W || 1.5kW))] = 200mW
Maximum T_J = +85°C + (0.2W ´ 125°C/W) = 110°C
This absolute worst-case condition does not exceed
the specified maximum junction temperature. Actual
P_DL is normally less than that considered here.
Carefully consider maximum T_J in your application.
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