OPA3690ID Datasheet, PDF (22/39 Pages) Texas Instruments – Triple, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA3690
SBOS237G – MARCH 2002 – REVISED MARCH 2010
OUTPUT CURRENT AND VOLTAGE
The OPA3690 provides output voltage and current
capabilities that are unsurpassed in a low-cost
monolithic op amp. Under no-load conditions at
+25°C, the output voltage typically swings closer than
1V to either supply rail; the specified swing limit is
within 1.2V of either rail. Into a 15Ω load (the
minimum tested load), it will deliver more than
±160mA.
The specifications described previously, though
familiar in the industry, consider voltage and current
limits separately. In many applications, it is the
voltage × current, or V-I product, that is more relevant
to circuit operation. Refer to Figure 19, the Output
Voltage and Current Limitations plot in the Typical
Characteristics. The X- and Y-axes of this graph
show the zero-voltage output current limit and the
zero-current output voltage limit, respectively. The
four quadrants give a more detailed view of the
OPA3690 output drive capabilities, noting that the
graph is bounded by a Safe Operating Area of 1W
maximum internal power dissipation for a single
channel. Superimposing resistor load lines onto the
plot shows that the OPA3690 can drive ±2.5V into
25Ω or ±3.5V into 50Ω without exceeding the output
capabilities or the 1W dissipation limit. A 100Ω load
line (the standard test circuit load) shows the full
±3.9V output swing capability (see the Electrical
Characteristics).
The minimum specified output voltage and current
specifications over temperature are set by worst-case
simulations at the cold temperature extreme. Only at
cold startup will the output current and voltage
decrease to the numbers shown in the Electrical
Characteristic tables. As the output transistors deliver
power, their junction temperatures increase,
decreasing their V_BEs (increasing the available output
voltage swing) and increasing their current gains
(increasing the available output current). In
steady-state operation, the available output voltage
and current is always greater than that shown in the
over-temperature specifications because the output
stage junction temperatures will be higher than the
minimum specified operating ambient.
To protect the output stage from accidental shorts to
ground and the power supplies, output short-circuit
protection is included in the OPA3690. The circuit
acts to limit the maximum source or sink current to
approximately 250mA.
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DRIVING CAPACITIVE LOADS
One of the most demanding and yet very common
load conditions for an op amp is capacitive loading.
Often, the capacitive load is the input of an
ADC—including additional external capacitance which
may be recommended to improve ADC linearity. A
high-speed, high open-loop gain amplifier like the
OPA3690 can be very susceptible to decreased
stability and closed-loop response peaking when a
capacitive load is placed directly on the output pin.
When the amplifier's open-loop output resistance is
considered, this capacitive load introduces an
additional pole in the signal path that can decrease
the phase margin. Several external solutions to this
problem have been suggested. When the primary
considerations are frequency response flatness,
pulse response fidelity, and/or distortion, the simplest
and most effective solution is to isolate the capacitive
load from the feedback loop by inserting a
series-isolation resistor between the amplifier output
and the capacitive load. This does not eliminate the
pole from the loop response, but rather shifts it and
adds a zero at a higher frequency. The additional
zero acts to cancel the phase lag from the capacitive
load pole, thus increasing the phase margin and
improving stability.
The Typical Characteristics show the recommended
R_S versus capacitive load (Figure 15 for ±5V and
Figure 30 for +5V) and the resulting frequency
response at the load. Parasitic capacitive loads
greater than 2pF can begin to degrade the
performance of the OPA3690. Long PCB traces,
unmatched cables, and connections to multiple
devices can easily exceed this value. Always
consider this effect carefully, and add the
recommended series resistor as close as possible to
the OPA3690 output pin (see the Board Layout
Guidelines section).
The criterion for setting this R_S resistor is a maximum
bandwidth, flat frequency response at the load. For
the OPA3690 operating in a gain of +2, the frequency
response at the output pin is already slightly peaked
without the capacitive load requiring relatively high
values of R_S to flatten the response at the load.
Increasing the noise gain will reduce the peaking as
described previously. The circuit of Figure 48
demonstrates this technique, allowing lower values of
R_S to be used for a given capacitive load. This was
used to generate the Recommended R_S vs
Capacitive Load plots (Figure 15 for ±5V and
Figure 30 for +5V).
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