OPA2694ID Datasheet, PDF (20/28 Pages) Texas Instruments – Dual, Wideband, Low-Power, Current Feedback Operational Amplifier

OPA2694
SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013
THERMAL ANALYSIS
Due to the high output power capability of the OPA2694,
heatsinking or forced airflow may be required under
extreme operating conditions. Maximum desired junction
temperature will set 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 will depend 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 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 in the
load that determines internal power dissipation.
As a worst-case example, compute the maximum T_J using
an OPA2694ID (SO-8 package) in the circuit of Figure 1,
with both amplifiers operating at the maximum specified
ambient temperature of +85°C and driving a grounded
20Ω load to +2.5V DC:
P_D = 10V × 12.7mA + 2 × [5²/(4 × (20Ω || 804Ω))] = 768mW
Maximum T_J = +85°C + (0.45W × (125°C/W)) = 180°C
This absolute worst-case condition exceeds the specified
maximum junction temperature. Remember, this is a
worst-case internal power dissipation—use your actual
signal and load to compute P_DL. The highest possible
internal dissipation will occur if the load requires current to
be forced into the output for positive output voltages or
sourced from the output for negative output voltages. This
puts a high current through a large internal voltage drop in
the output transistors. The Output Voltage and Current
Limitations plot shown in the Typical Characteristics
includes a boundary for 1W maximum internal power
dissipation under these conditions.
BOARD LAYOUT GUIDELINES
Achieving optimum performance with a high-frequency
amplifier like the OPA2694 requires careful attention to
board layout parasitics and external component types.
Recommendations that will optimize performance include:
a) Minimize parasitic capacitance to any AC ground for
all of the signal I/O pins. Parasitic capacitance on the
output and inverting input pins can cause instability: on the
noninverting input, it can react with the source impedance
to cause unintentional bandlimiting. To reduce unwanted
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capacitance, a window around the signal I/O pins should
be opened in all of the ground and power planes around
those pins. Otherwise, ground and power planes should
be unbroken elsewhere on the board.
b) Minimize the distance (< 0.25”) from the power-supply
pins to high-frequency 0.1μF decoupling capacitors. At the
device pins, the ground and power plane layout should not
be in close proximity to the signal I/O pins. Avoid narrow
power and ground traces to minimize inductance between
the pins and the decoupling capacitors. The power-supply
connections (on pins 4 and 7) should always be decoupled
with these capacitors. An optional supply decoupling
capacitor across the two power supplies (for bipolar
operation) will improve 2nd-harmonic distortion
performance. Larger (2.2μF to 6.8μF) decoupling
capacitors, effective at lower frequencies, should also be
used on the main supply pins. These may be placed
somewhat farther from the device and may be shared
among several devices in the same area of the PCB.
c) Careful selection and placement of external
components will preserve the high-frequency
performance of the OPA2694. Resistors should be a very
low reactance type. Surface-mount resistors work best
and allow a tighter overall layout. Metal-film and carbon
composition, axially-leaded resistors can also provide
good high-frequency performance. Again, keep their leads
and PC-board trace length as short as possible. Never use
wirewound type resistors in a high-frequency application.
Since the output pin and inverting input pin are the most
sensitive to parasitic capacitance, always position the
feedback and series output resistor, if any, as close as
possible to the output pin. Other network components,
such as noninverting input termination resistors, should
also be placed close to the package. Where double-side
component mounting is allowed, place the feedback
resistor directly under the package on the other side of the
board between the output and inverting input pins. The
frequency response is primarily determined by the
feedback resistor value, as described previously.
Increasing its value will reduce the bandwidth, while
decreasing it will give a more peaked frequency response.
The 402Ω feedback resistor used in the Electrical
Characteristic tables at a gain of +2 on ±5V supplies is a
good starting point for design. Note that a 430Ω feedback
resistor, rather than a direct short, is recommended for the
unity-gain follower application. A current-feedback op amp
requires a feedback resistor even in the unity-gain follower
configuration to control stability.
d) Connections to other wideband devices on the board
may be made with short, direct traces or through onboard
transmission lines. For short connections, consider the
trace and the input to the next device as a lumped
capacitive load. Relatively wide traces (50mils to 100mils)
should be used, preferably with ground and power planes
opened up around them. Estimate the total capacitive load