Datasheet

5kW
OPA377
10mAmax
V+
V
IN
V
OUT
I
OVERLOAD
10 toW
20W
V+
V
IN
V
OUT
R
S
R
L
C
L
OPA377
OPA377
OPA2377
OPA4377
SBOS504B –FEBRUARY 2010–REVISED JANUARY 2011 www.ti.com
INPUT AND ESD PROTECTION CAPACITIVE LOAD AND STABILITY
The OPA377 family incorporates internal electrostatic The OPA377 series of amplifiers may be used in
discharge (ESD) protection circuits on all pins. In the applications where driving a capacitive load is
case of input and output pins, this protection primarily required. As with all op amps, there may be specific
consists of current steering diodes connected instances where the OPAx377 can become unstable,
between the input and power-supply pins. These ESD leading to oscillation. The particular op amp circuit
protection diodes also provide in-circuit, input configuration, layout, gain, and output loading are
overdrive protection, as long as the current is limited some of the factors to consider when establishing
to 10mA as stated in the Absolute Maximum Ratings. whether an amplifier will be stable in operation. An op
Figure 23 shows how a series input resistor may be amp in the unity-gain (+1V/V) buffer configuration and
added to the driven input to limit the input current. driving a capacitive load exhibits a greater tendency
The added resistor contributes thermal noise at the to be unstable than an amplifier operated at a higher
amplifier input and its value should be kept to a noise gain. The capacitive load, in conjunction with
minimum in noise-sensitive applications. the op amp output resistance, creates a pole within
the feedback loop that degrades the phase margin.
The degradation of the phase margin increases as
the capacitive loading increases.
The OPAx377 in a unity-gain configuration can
directly drive up to 250pF pure capacitive load.
Increasing the gain enhances the ability of the
amplifier to drive greater capacitive loads; see the
typical characteristic plot, Small-Signal Overshoot vs
Capacitive Load. In unity-gain configurations,
Figure 23. Input Current Protection
capacitive load drive can be improved by inserting a
small (10Ω to 20Ω) resistor, R
S
, in series with the
output, as shown in Figure 24. This resistor
EMI SUSCEPTIBILITY AND INPUT FILTERING
significantly reduces ringing while maintaining dc
performance for purely capacitive loads. However, if
Operational amplifiers vary in susceptibility to
there is a resistive load in parallel with the capacitive
electromagnetic interference (EMI). If conducted EMI
load, a voltage divider is created, introducing a gain
enters the operational amplifier, the dc offset
error at the output and slightly reducing the output
observed at the amplifier output may shift from the
swing. The error introduced is proportional to the ratio
nominal value while the EMI is present. This shift is a
R
S
/R
L
, and is generally negligible at low output
result of signal rectification associated with the
current levels.
internal semiconductor junctions. While all amplifier
pin functions can be affected by EMI, the input pins
are likely to be the most susceptible. The OPA377
operational amplifier family incorporates an internal
input low-pass filter that reduces the amplifier
response to EMI. Both common-mode and differential
mode filtering are provided by the input filter. The
filter is designed for a cutoff frequency of
approximately 75MHz (–3dB), with a roll-off of 20dB
per decade.
Figure 24. Improving Capacitive Load Drive
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