LMV931MFX National Semiconductor Corporation, LMV931MFX Datasheet - Page 13

• Manufacturer Part Number: LMV931MFX
• Description: LMV931 - Single 1.8V, Rrio Operational Amplifiers, Package: SOT-23, Pin Nb=5
• Manufacturer: National Semiconductor Corporation

Application Note
1.0 INPUT AND OUTPUT STAGE
The rail-to-rail input stage of this family provides more flex-
ibility for the designer. The LMV931/LMV932/LMV934 use a
complimentary PNP and NPN input stage in which the PNP
stage senses common mode voltage near V
stage senses common mode voltage near V
from the PNP stage to NPN stage occurs 1V below V
both input stages have their own offset voltage, the offset of
the amplifier becomes a function of the input common mode
voltage and has a crossover point at 1V below V
This V
and AC coupled signals if proper care is not taken. Large
input signals that include the V
distortion in the output signal. One way to avoid such distor-
tion is to keep the signal away from the crossover. For
example, in a unity gain buffer configuration and with V
5V, a 5V peak-to-peak signal will contain input-crossover
distortion while a 3V peak-to-peak signal centered at 1.5V
will not contain input-crossover distortion as it avoids the
crossover point. Another way to avoid large signal distortion
is to use a gain of −1 circuit which avoids any voltage
excursions at the input terminals of the amplifier. In that
circuit, the common mode DC voltage can be set at a level
away from the V
transition in V
signal in series with the input signal and can effectively
degrade small signal parameters such as gain and common
mode rejection ratio. To resolve this problem, the small
signal should be placed such that it avoids the V
over point. In addition to the rail-to-rail performance, the
output stage can provide enough output current to drive
600Ω loads. Because of the high current capability, care
should be taken not to exceed the 150˚C maximum junction
temperature specification.
2.0 INPUT BIAS CURRENT CONSIDERATION
The LMV931/LMV932/LMV934 family has a complementary
bipolar input stage. The typical input bias current (I
15nA. The input bias current can develop a significant offset
voltage. This offset is primarily due to I
negative feedback resistor, R
and R
(V
shown in Figure 1, cancels this effect. But the input offset
current (I
same manner.
OS
F
= I
OS
is 100kΩ, then an offset voltage of 5mV will develop
B
OS
crossover point can create problems for both DC
x R
) will still contribute to an offset voltage in the
OS
F
). Using a compensation resistor (R
OS
shows up as a V
cross-over point. For small signals, this
F
OS
. For example, if I
crossover point will cause
CM
B
dependent spurious
flowing through the
+
−
. The transition
and the NPN
+
B
OS
.
+
is 50nA
. Since
cross-
C
B
), as
S
) is
=
13
Typical Applications
3.0 HIGH SIDE CURRENT SENSING
The high side current sensing circuit (Figure 2) is commonly
used in a battery charger to monitor charging current to
prevent over charging. A sense resistor R
to the battery directly. This system requires an op amp with
rail-to-rail input. The LMV931/LMV932/LMV934 are ideal for
this application because its common mode input range goes
up to the rail.
FIGURE 1. Canceling the Offset Voltage due to Input
FIGURE 2. High Side Current Sensing
Bias Current
SENSE
is connected
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