Datasheet

OPA690
402W
175W
402W
+5V
50W
50W
C
L
R
NG
V
O
R
-5V
Power-supply
decouplingnotshown.
OPA690
www.ti.com SBOS223F –DECEMBER 2001–REVISED FEBRUARY 2010
OUTPUT CURRENT AND VOLTAGE OPA690 can be very susceptible to decreased
stability and closed-loop response peaking when a
The OPA690 provides output voltage and current
capacitive load is placed directly on the output pin.
capabilities that are unsurpassed in a low-cost
When the amplifier's open-loop output resistance is
monolithic op amp. Under no-load conditions at
considered, this capacitive load introduces an
+25°C, the output voltage typically swings closer than
additional pole in the signal path that can decrease
1V to either supply rail; the specified swing limit is
the phase margin. Several external solutions to this
within 1.2V of either rail. Into a 15Ω load (the
problem have been suggested. When the primary
minimum tested load), it will deliver more than
considerations are frequency response flatness,
±160mA.
pulse response fidelity, and/or distortion, the simplest
and most effective solution is to isolate the capacitive
The specifications described previously, though
load from the feedback loop by inserting a
familiar in the industry, consider voltage and current
series-isolation resistor between the amplifier output
limits separately. In many applications, it is the
and the capacitive load. This does not eliminate the
voltage × current, or V-I product, which is more
pole from the loop response, but rather shifts it and
relevant to circuit operation. Refer to Figure 19, the
adds a zero at a higher frequency. The additional
Output Voltage and Current Limitations plot in the
zero acts to cancel the phase lag from the capacitive
Typical Characteristics. The X- and Y-axes of this
load pole, thus increasing the phase margin and
graph show the zero-voltage output current limit and
improving stability.
the zero-current output voltage limit, respectively. The
four quadrants give a more detailed view of the
The Typical Characteristics show the recommended
OPA690 output drive capabilities, noting that the
R
S
versus capacitive load (Figure 15 for ±5V and
graph is bounded by a Safe Operating Area of 1W
Figure 30 for +5V) and the resulting frequency
maximum internal power dissipation. Superimposing
response at the load. Parasitic capacitive loads
resistor load lines onto the plot shows that the
greater than 2pF can begin to degrade the
OPA690 can drive ±2.5V into 25Ω or ±3.5V into 50Ω
performance of the OPA690. Long PCB traces,
without exceeding the output capabilities or the 1W
unmatched cables, and connections to multiple
dissipation limit. A 100Ω load line (the standard test
devices can easily exceed this value. Always
circuit load) shows the full ±3.9V output swing
consider this effect carefully, and add the
capability, as shown in the typical specifications.
recommended series resistor as close as possible to
the OPA690 output pin (see the Board Layout
The minimum specified output voltage and current
Guidelines section).
specifications over temperature are set by worst-case
simulations at the cold temperature extreme. Only at
The criterion for setting this R
S
resistor is a maximum
cold startup will the output current and voltage
bandwidth, flat frequency response at the load. For
decrease to the numbers shown in the Electrical
the OPA690 operating in a gain of +2, the frequency
Characteristic tables. As the output transistors deliver
response at the output pin is already slightly peaked
power, their junction temperatures increase,
without the capacitive load requiring relatively high
decreasing their V
BE
s (increasing the available output
values of R
S
to flatten the response at the load.
voltage swing) and increasing their current gains
Increasing the noise gain will reduce the peaking as
(increasing the available output current). In
described previously. The circuit of Figure 44
steady-state operation, the available output voltage
demonstrates this technique, allowing lower values of
and current is always greater than that shown in the
R
S
to be used for a given capacitive load.
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 OPA690. The circuit acts
to limit the maximum source or sink current to
approximately 250mA.
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
Figure 44. Capacitive Load Driving with Noise
high-speed, high open-loop gain amplifier like the
Gain Tuning
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