LMV715MF NSC [National Semiconductor], LMV715MF Datasheet - Page 11

• Manufacturer Part Number: LMV715MF
• Description: Low Power, RRIO Operational Amplifiers with High Output Current Drive and Shutdown Option
• Manufacturer: NSC [National Semiconductor]

Application Note
1.0 SUPPLY BYPASSING
The application circuits in this datasheet do not show the
power supply connections and the associated bypass ca-
pacitors for simplification. When the circuits are built, it is
always required to have bypass capacitors. Ceramic disc
capacitors (0.1µF) or solid tantalum (1µF) with short leads,
and located close to the IC are usually necessary to prevent
interstage coupling through the power supply internal imped-
ance. Inadequate bypassing will manifest itself by a low
frequency oscillation or by high frequency instabilities.
Sometimes, a 10µF (or larger) capacitor is used to absorb
low frequency variations and a smaller 0.1µF disc is paral-
leled across it to prevent any high frequency feedback
through the power supply lines.
2.0 SHUTDOWN MODE
The LMV711 and LMV715 have a shutdown pin. To conserve
battery life in portable applications, they can be disabled
when the shutdown pin voltage is pulled low. For LMV711
during shutdown mode, the output stays at about 50mV from
the lower rail, and the current drawn from the power supply
is 0.2µA (typical). This makes the LMV711 an ideal solution
for power sensitive applications. For the LMV715 during
shutdown mode, the output will be “Tri-stated”.
The shutdown pin should never be left unconnected. In
applications where shutdown operation is not needed and
the LMV711 or LMV715 is used, the shutdown pin should be
connected to V
in an undefined operation mode and the device may oscillate
between shutdown and active modes.
3.0 RAIL-TO-RAIL INPUT
The rail-to-rail input is achieved by using paralleled PMOS
and NMOS differential input stages. (See Simplified Sche-
matics in this datasheet). When the common mode input
voltage changes from ground to the positive rail, the input
stage goes through three modes. First, the NMOS pair is
cutoff and the PMOS pair is active. At around 1.4V, both
PMOS and NMOS pairs operate, and finally the PMOS pair
is cutoff and NMOS pair is active. Since both input stages
have their own offset voltage (V
becomes a function of the common-mode input voltage. See
curves for V
As shown in the curve, the V
above V
coupled applications to avoid problems. For large input sig-
nals that include the V
range, it will cause distortion in the output signal. One way to
avoid such distortion is to keep the signal away from the
crossover point. For example, in a unity gain buffer configu-
ration and with V
2.5V will contain input-crossover distortion. To avoid this, the
input signal should be centered at 3.5V instead. 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. See Figure 1. In this circuit, the common mode
DC voltage (V
crossover point.
−
. Proper design must be done in both DC and AC
OS
CM
+
vs. V
. Leaving the shutdown pin floating will result
S
) can be set at a level away from the V
= 5V, a 3V peak-to-peak signal center at
CM
OS
in curve section.
crossover point in their dynamic
OS
OS
has a crossover point at 1.4V
), the offset of the amplifier
OS
11
When the input is a small signal and this small signal falls
inside the V
other parameters will be degraded. To resolve this problem,
the small signal should be placed such that it avoids the V
crossover point.
To achieve maximum output swing, the output should be
biased at mid-supply. This is normally done by biasing the
input at mid-supply. But with supply voltage range from 2V to
3.4V, the input of the op amp should not be biased at
mid-supply because of the transition of the V
shows an example of how to get away from the V
over point and maintain a maximum swing with a 2.7V
supply. Figure 3 shows the waveforms of V
The inputs can be driven 300mV beyond the supply rails
without causing phase reversal at the output. However, the
inputs should not be allowed to exceed the maximum rat-
ings.
OS
transition range, the gain, CMRR and some
FIGURE 1.
FIGURE 2.
FIGURE 3.
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OS
www.national.com
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. Figure 2
OS
OUT
cross-
.
OS