25˚C.) (Continued)

Application Section

The LMV111 integrates a rail-to-rail op amp and a V

circuit into one ultra tiny package. With its small footprint and

reduced component count for bias network, it enables the

design of smaller portable electronic products, such as cellu-

lar phones, pagers, PDAs, PCMCIA cards, etc. In addition,

the integration solution minimizes printed circuit board stray

capacitance, and reduces the complexity of circuit design.

The core op amp of this family is National’s LMV321.

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 al-

ways required to have bypass capacitors. Ceramic disc ca-

pacitors (0.1µF) or solid tantalum (1µF) with short leads, and

located close to the IC are usually necessary to prevent in-

terstage coupling through the power supply internal imped-

ance. Inadequate bypassing will manifest itself by a low fre-

quency

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 Input Voltage Range

The input voltage should be within the supply rails. The ESD

protection circuitry at the input of the device includes a diode

between the input pin and the negative supply pin. Driving

the input more than 0.6V (at 25˚C) beyond the negative sup-

ply will turn on the diode and cause signal distortions.

3.0 Capacitive Load Tolerance

The LMV111 can directly drive 200pF capacitive load with

unity gain without oscillation. The unity-gain follower is the

most sensitive configuration to capacitive loading. Direct ca-

pacitive loading reduces the phase margin of amplifiers. The

combination of the amplifier’s output impedance and the ca-

pacitive load induces phase lag. This results in either an un-

derdamped pulse or oscillation. To drive a heavier capacitive

load, a resistive isolation can be used as shown in

Figure 1.

The isolation resistor R

stability by adding more phase margin to the overall system.

The desired performance depends on the value of R

50

Ω to 100Ω isolation resistor is recommended for initial

evaluation. The bigger the R

stable V

Output Voltage Swing vs.

Supply Voltage

DS101262-22

DS101262-23

FIGURE 1. Resistive Isolation of a Heavy Capacitive

Load

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