OPA3695_14 Datasheet, PDF (20/39 Pages) Texas Instruments – Triple, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3695
SBOS355A – APRIL 2008 – REVISED SEPTEMBER 2008 ............................................................................................................................................... www.ti.com
Turn-on time is very quick from the shutdown
condition (typically < 25ns). Turn-off time strongly
depends on the selected gain configuration and load,
but is typically 1µs for the circuit of Figure 35. To shut
down, the control pin must be asserted low. This logic
control is referenced to the positive supply, as the
simplified circuit of Figure 44 shows.
+V_S
15kW
Q1
25kW
110kW
V_DIS
I_S
-V_S
Control
Figure 44. Simplified Disable Control Circuit
In normal operation, base current to Q1 is provided
through the 110kΩ resistor while the emitter current
through the 15kΩ 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 15kΩ resistor, eventually turning
on these two diodes (≈ 80µA). 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 sequence shuts off the
collector current out of Q1, turning the amplifier off.
The supply current in the shutdown mode is only that
required to operate the circuit of Figure 44.
The shutdown feature for the OPA3695 is a
positive-supply-referenced,
current-controlled
interface. Open-collector (or drain) interfaces are
most effective, as long as the controlling logic can
sustain the resulting voltage (in the open mode) that
appears at the V_DIS pin. That voltage is one diode
below the positive supply voltage applied to the
OPA3695. For voltage output logic interfaces, the
on/off voltage levels described in the Electrical
Characteristics apply only for a +5V positive supply
on the OPA3695. An open-drain interface is
recommended for shutdown operation using a higher
positive supply for the OPA3695 and/or logic families
with inadequate high-level voltage swings.
THERMAL ANALYSIS
The OPA3695 does not require heatsinking or airflow
in most applications. Maximum desired junction
temperature sets the maximum allowed internal
power dissipation as described here. 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 but would, for a grounded resistive
load, be at a maximum when the output is fixed at a
voltage equal to 1/2 either supply voltage (for equal
bipolar supplies). Under this worst-case condition,
P_DL = V_S2/(4 × R_L) where R_L includes feedback
network loading. This value is the absolute highest
power that can be dissipated for a given R_L. All actual
applications dissipate less power in the output stage.
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 OPA3695IDBQ (SSOP-16 package) in the
circuit of Figure 35 operating at the maximum
specified ambient temperature of +85°C and driving a
grounded 100Ω load at V_S/2. Maximum internal
power is:
P_D = 10V ´ 42.6mA + 3 ´ 5²/(4 ´ (100W || 1.2kW)) = 629mW
Maximum T_J = +85°C + (0.629W ´ 80°C/W) = 135°C
Actual applications operate at a lower junction
temperature than the +135°C computed above. This
condition is because the RMS voltage of the output
signals vary, along with the fact that part of the
quiescent current is steered to the output, thus
reducing the 10V × 42.6mA dominant term. Compute
the actual output stage power to get an accurate
estimate of maximum junction temperature, or use
the results shown here as an absolute worst case
maximum scenario.
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