LPV321 Datasheet, PDF (12/22 Pages) National Semiconductor (TI) – General Purpose, Low Voltage, Low Power, Rail-to-Rail Output Operational Amplifiers

Application Information
BENEFITS OF THE LPV321/358/324
Size
The small footprints of the LPV321/358/324 packages save
space on printed circuit boards, and enable the design of
smaller electronic products, such as cellular phones, pagers,
or other portable systems. The low profile of the LPV321/
358/324 make them possible to use in PCMCIA type III
cards.
Signal Integrity
Signals can pick up noise between the signal source and the
amplifier. By using a physically smaller amplifier package,
the LPV321/358/324 can be placed closer to the signal
source, reducing noise pickup and increasing signal integrity.
Simplified Board Layout
These products help you to avoid using long pc traces in
your pc board layout. This means that no additional compo-
nents, such as capacitors and resistors, are needed to filter
out the unwanted signals due to the interference between
the long pc traces.
Low Supply Current
These devices will help you to maximize battery life. They
are ideal for battery powered systems.
Low Supply Voltage
National provides guaranteed performance at 2.7V and 5V.
These guarantees ensure operation throughout the battery
lifetime.
Rail-to-Rail Output
Rail-to-rail output swing provides maximum possible dy-
namic range at the output. This is particularly important
when operating on low supply voltages.
Input Includes Ground
Allows direct sensing near GND in single supply operation.
The differential input voltage may be larger than V⁺ without
damaging the device. Protection should be provided to pre-
vent the input voltages from going negative more than −0.3V
(at 25˚C). An input clamp diode with a resistor to the IC input
terminal can be used.
CAPACITIVE LOAD TOLERANCE
The LPV321/358/324 can directly drive 200 pF in unity-gain
without oscillation. The unity-gain follower is the most sensi-
tive configuration to capacitive loading. Direct capacitive
loading reduces the phase margin of amplifiers. The combi-
nation of the amplifier’s output impedance and the capacitive
load induces phase lag. This results in either an under-
damped pulse response or oscillation. To drive a heavier
capacitive load, circuit in Figure 1 can be used.
10092004
FIGURE 1. Indirectly Driving A Capacitive Load Using
Resistive Isolation
In Figure 1, the isolation resistor R_ISO and the load capacitor
C_L form a pole to increase stability by adding more phase
margin to the overall system. The desired performance de-
pends on the value of R_ISO. The bigger the R_ISO resistor
value, the more stable V_OUT will be. Figure 2 is an output
waveform of Figure 1 using 100 kΩ for R_ISO and 1000 pF for
C_L.
10092075
FIGURE 2. Pulse Response of the LPV324 Circuit in
Figure 1
The circuit in Figure 3 is an improvement to the one in Figure
1 because it provides DC accuracy as well as AC stability. If
there were a load resistor in Figure 1, the output would be
voltage divided by R_ISO and the load resistor. Instead, in
Figure 3, R_F provides the DC accuracy by using feed-
forward techniques to connect V_IN to R_L. Caution is needed
in choosing the value of R_F due to the input bias current of
the LPV321/358/324. C_F and R_ISO serve to counteract the
loss of phase margin by feeding the high frequency compo-
nent of the output signal back to the amplifier’s inverting
input, thereby preserving phase margin in the overall feed-
back loop. Increased capacitive drive is possible by increas-
ing the value of C_F. This in turn will slow down the pulse
response.
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