Datasheet

+
-
V
IN
R
ISO
V
OUT
LPV511
SNOSAG7C –AUGUST 2005–REVISED MARCH 2013 www.ti.com
APPLICATION NOTES
The LPV511 is fabricated with Texas Instrument's state-of-the-art VIP50C process.
INPUT STAGE
The LPV511 has a rail-to-rail input which provides more flexibility for the system designer. As can be seen from
the simplified schematic, rail-to-rail input is achieved by using in parallel, one PNP differential pair and one NPN
differential pair. When the common mode input voltage (V
CM
) is near V
+
, the NPN pair is on and the PNP pair is
off. When V
CM
is near V
−
, the NPN pair is off and the PNP pair is on. When V
CM
is between V
+
and V
−
, internal
logic decides how much current each differential pair will get. This special logic ensures stable and low distortion
amplifier operation within the entire common mode voltage range.
Because both input stages have their own offset voltage (V
OS
) characteristic, the offset voltage of the LPV511
becomes a function of V
CM
. V
OS
has a crossover point at 1.0V below V
+
. Refer to the ’V
OS
vs. V
CM
’ curve in the
Typical Performance Characteristics section. Caution should be taken in situations where the input signal
amplitude is comparable to the V
OS
value and/or the design requires high accuracy. In these situations, it is
necessary for the input signal to avoid the crossover point.
The input bias current, I
B
will change in value and polarity as the input crosses the transition region. In addition,
parameters such as PSRR and CMRR which involve the input offset voltage will also be affected by changes in
V
CM
across the differential pair transition region.
Differential input voltage is the difference in voltage between the non-inverting (+) input and the inverting input (−)
of the op amp. Due to the three series diodes across the two inputs, the absolute maximum differential input
voltage is ±2.1V. This may not be a problem to most conventional op amp designs; however, designers should
avoid using the LPV511 as a comparator.
OUTPUT STAGE
The LPV511 output voltage swing 100 mV from rails @ 3V supply, which provides the maximum possible
dynamic range at the output. This is particularly important when operating on low supply voltages.
The LPV511 Maximum Output Voltage Swing defines the maximum swing possible under a particular output
load. The LPV511 output swings 110 mV from the rail @ 5V supply with an output load of 100 kΩ.
DRIVING CAPACITIVE LOAD
The LPV511 is internally compensated for stable unity gain operation, with a 27 kHz typical gain bandwidth.
However, the unity gain follower is the most sensitive configuration to capacitive load. Direct capacitive loading
reduces the phase margin of the op amp. When the output is required to drive a large capacitive load, greater
than 100 pF, a small series resistor at the output of the amplifier improves the phase margin (see Figure 29).
In Figure 29, 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 depends on the value of R
ISO
. The bigger the R
ISO
resistor value, the more stable V
OUT
will be. But the DC accuracy is degraded when the R
ISO
gets bigger. If there
were a load resistor in Figure 29, the output voltage would be divided by R
ISO
and the load resistor.
Figure 29. Resistive Isolation of Capacitive Load
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