Datasheet

1/2
OPA2690
402W
175W
402W
+5V
50W
50W
C
LOAD
R
NG
V
O
R
-5V
Power-supplydecoupling
notshown.
OPA2690
www.ti.com SBOS238G –JUNE 2002–REVISED MARCH 2010
OUTPUT CURRENT AND VOLTAGE OPA2690 can be very susceptible to decreased
stability and closed-loop response peaking when a
The OPA2690 provides output voltage and current
capacitive load is placed directly on the output pin.
capabilities in a low-cost monolithic op amp. Under
When the amplifier's open-loop output resistance is
no-load conditions at +25°C, the output voltage
considered, this capacitive load introduces an
typically swings closer than 1V to either supply rail;
additional pole in the signal path that can decrease
the specified swing limit is within 1.2V of either rail.
the phase margin. Several external solutions to this
Into a 15Ω load (the minimum tested load), it will
problem have been suggested. When the primary
deliver more than ±160mA.
considerations are frequency response flatness,
pulse response fidelity, and/or distortion, the simplest
The specifications described previously, though
and most effective solution is to isolate the capacitive
familiar in the industry, consider voltage and current
load from the feedback loop by inserting a
limits separately. In many applications, it is the
series-isolation resistor between the amplifier output
voltage × current, or V-I product, that is more relevant
and the capacitive load. This does not eliminate the
to circuit operation. Refer to Figure 19, the Output
pole from the loop response, but rather shifts it and
Voltage and Current Limitations plot in the Typical
adds a zero at a higher frequency. The additional
Characteristics. The X- and Y-axes of this graph
zero acts to cancel the phase lag from the capacitive
show the zero-voltage output current limit and the
load pole, thus increasing the phase margin and
zero-current output voltage limit, respectively. The
improving stability.
four quadrants give a more detailed view of the
OPA2690 output drive capabilities, noting that the
The Typical Characteristics show the recommended
graph is bounded by a Safe Operating Area of 1W
R
S
versus capacitive load (Figure 15 for ±5V and
maximum internal power dissipation for each channel
Figure 30 for +5V) and the resulting frequency
separately. Superimposing resistor load lines onto the
response at the load. Parasitic capacitive loads
plot shows that the OPA2690 can drive ±2.5V into
greater than 2pF can begin to degrade the
25Ω or ±3.5V into 50Ω without exceeding the output
performance of the OPA2690. Long PCB traces,
capabilities or the 1W dissipation limit. A 100Ω load
unmatched cables, and connections to multiple
line (the standard test circuit load) shows the full
devices can easily exceed this value. Always
±3.9V output swing capability (see the Electrical
consider this effect carefully, and add the
Characteristics).
recommended series resistor as close as possible to
the OPA2690 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 OPA2690 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 48
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 OPA2690. 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 48. Capacitive Load Driving with Noise
high-speed, high open-loop gain amplifier like the
Gain Tuning
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