LP2902 Datasheet, PDF (7/13 Pages) National Semiconductor (TI) – Micropower Quad Operational Amplifier

Typical Performance Curves
(Continued)
Current Limiting
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Application Hints
The LP324 series is a micro-power pin-for-pin equivalent to
the LM324 op amps. Power supply current, input bias cur-
rent, and input offset current have all been reduced by a
factor of 10 over the LM324. Like its predecessor, the LP324
series op amps can operate on single supply, have
true-differential inputs, and remain in the linear mode with an
input common-mode voltage of 0 V_DC.
The pinouts of the package have been designed to simplify
PC board layouts. Inverting inputs are adjacent to outputs for
all of the amplifiers and the outputs have also been placed at
the corners of the package (pins 1, 7, 8, and 14).
Precautions should be taken to insure that the power supply
for the integrated circuit never becomes reversed in polarity
or the unit is not inadvertently installed backwards in the test
socket as an unlimited current surge through the resulting
forward diode within the IC could destroy the unit.
Large differential input voltages can be easily accommo-
dated and, as input differential voltage protection diodes are
not needed, no large input currents result from large differ-
ential input voltages. The differential input voltage may be
larger than V⁺ without damaging the device. Protection
should be provided to prevent the input voltages from going
negative more than −0.3 V_DC (at 25˚C). An input clamp diode
with a resistor to the IC input terminal can be used.
The amplifiers have a class B output stage which allows the
amplifiers to both source and sink output currents. In appli-
cations where crossover distortion is undesirable, a resistor
should be used from the output of the amplifier to ground.
The resistor biases the output into class A operation.
The LP324 has improved stability margin for driving capaci-
tive loads. No special precautions are needed to drive loads
in the 50 pF to 1000 pF range. It should be noted however
that since the power supply current has been reduced by a
factor of 10, so also has the slew rate and gain bandwidth
product. This reduction can cause reduced performance in
AC applications where the LM324 is being replaced by an
LP324. Such situations usually occur when the LM324 has
been operated near its power bandwidth.
Output short circuits either to ground or to the positive power
supply should be of short time duration. Units can be de-
stroyed, not as a result of the short circuit current causing
metal fusing, but rather due to the large increase in IC chip
dissipation which will cause eventual failure due to exces-
sive junction temperatures. For example: If all four amplifiers
were simultaneously shorted to ground on a 10V supply the
junction temperature would rise by 110˚C.
Exceeding the negative common-mode limit on either input
will cause a reversal of phase to the output and force the
amplifier to the corresponding high or low state. Exceeding
the negative common-mode limit on both inputs will force the
amplifier output to a high state. Exceeding the positive
common-mode limit on a single input will not change the
phase of the output. However, if both inputs exceed the limit,
the output of the amplifier will be forced to a low state. In
neither case does a latch occur since returning the input
within the common mode range puts the input stage and
thus the amplifier in a normal operating mode.
The circuits presented in the section on typical applications
emphasize operation on only a single power supply voltage.
If complementary power supplies are available, all of the
standard op amp circuits can be used. In general, introduc-
ing a pseudo-ground (a bias voltage reference to V⁺/2) will
allow operation above and below this value in single power
supply systems. Many application circuits are shown which
take advantage of the wide input common-mode voltage
range which includes ground. In most cases, input biasing is
not required and input voltages which range to ground can
easily be accommodated.
Driving CMOS
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