Datasheet

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SBOS365D − JUNE 2006 − REVISED JUNE 2009
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10
A simplified schematic illustrating the rail-to-rail input
circuitry is shown in Figure 5.
CAPACITIVE LOADS
The OPA365 may be used in applications where driving
a capacitive load is required. As with all op amps, there
may be specific instances where the OPA365 can be-
come unstable, leading to oscillation. The particular op
amp circuit configuration, layout, gain and output load-
ing are some of the factors to consider when establish-
ing whether an amplifier will be stable in operation. An
op amp in the unity-gain (+1V/V) buffer configuration
and driving a capacitive load exhibits a greater tenden-
cy to be unstable than an amplifier operated at a higher
noise gain. The capacitive load, in conjunction with 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 ca-
pacitive loading increases.
When operating in the unity-gain configuration, the
OPA365 remains stable with a pure capacitive load up
to approximately 1nF. The equivalent series resistance
(ESR) of some very large capacitors (C
L
> 1µF) is suffi-
cient to alter the phase characteristics in the feedback
loop such that the amplifier remains stable. Increasing
the amplifier closed-loop gain allows the amplifier to
drive increasingly larger capacitance. This increased
capability is evident when observing the overshoot re-
sponse of the amplifier at higher voltage gains. See the
typical characteristic graph, Small-Signal Overshoot
vs. Capacitive Load.
One technique for increasing the capacitive load drive
capability of the amplifier operating in unity gain is to in-
sert a small resistor, typically 10Ω to 20Ω, in series with
the output; see Figure 6. This resistor significantly re-
duces the overshoot and ringing associated with large
capacitive loads. A possible problem with this technique
is that a voltage divider is created with the added series
resistor and any resistor connected in parallel with the
capacitive load. The voltage divider introduces a gain
error at the output that reduces the output swing. The
error contributed by the voltage divider may be insignifi-
cant. For instance, with a load resistance, R
L
= 10kΩ,
and R
S
= 20Ω, the gain error is only about 0.2%. Howev-
er, when R
L
is decreased to 600Ω, which the OPA365
is able to drive, the error increases to 7.5%.
Regulated
Charge Pump
V
OUT
=V
CC
+1.8V
Patent Pending
Very Low Ripple
Topology
I
BIAS
V
CC
+1.8V
I
BIAS
I
BIAS
I
BIAS
V
S
I
BIAS
V
OUT
V
IN
− V
IN
+
Figure 5. Simplified Schematic
10
Ω
to
20
Ω
V+
V
IN
V
OUT
R
S
R
L
C
L
OPA365
Figure 6. Improving Capacitive Load Drive