OPA2889 Datasheet, PDF (26/35 Pages) Burr-Brown (TI) – Dual, Low-Power, Wideband, Voltage Feedback OPERATIONAL AMPLIFIER with Disable

OPA2889
SBOS373 – JUNE 2007
In normal operation, base current to Q1 is provided
through the 4MΩ resistor, while the emitter current
through the 100kΩ 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 100kΩ resistor, eventually turning
on those two diodes (≈18μA). 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 63.
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 OPA2889 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 will be very poor as a result.
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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.
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_S2/(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 OPA2889ID (SO-8 package) in the circuit of
Figure 50 operating at the maximum specified
ambient temperature of +85°C and with both outputs
driving a grounded 75Ω load to +2.5V.
P_D = 10V ´ 2.5mA + 2[5²/(4 ´ (75W || 1.5kW))] = 200mW
Maximum T_J = +85°C + (200mW ´ 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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