LMC7211 Datasheet, PDF (7/13 Pages) National Semiconductor (TI) – Tiny CMOS Comparator with Rail-to-Rail Input and Push-Pull Output

Application Information
1.0 Benefits of the LMC7211 Tiny Comparator
Size. The small footprint of the SOT 23-5 packaged Tiny
Comparator, (0.120 x 0.118 inches, 3.05 x 3.00 mm) saves
space on printed circuit boards, and enable the design of
smaller electronic products. Because they are easier to
carry, many customers prefer smaller and lighter products.
Height. The height (0.056 inches, 1.43 mm) of the Tiny
Comparator makes it possible to use it in PCMCIA type III
cards.
Simplified Board Layout. The Tiny Comparator can sim-
plify board layout in several ways. First, by placing a com-
parator where comparators are needed, instead of routing
signals to a dual or quad device, long pc traces may be
avoided.
By using multiple Tiny Comparators instead of duals or
quads, complex signal routing and possibly crosstalk can be
reduced.
Low Supply Current. The typical 7 µA supply current of the
LMC7211 extends battery life in portable applications, and
may allow the reduction of the size of batteries in some ap-
plications.
Wide Voltage Range. The LMC7211 is characterized at
15V, 5V and 2.7V. Performance data is provided at these
popular voltages. This wide voltage range makes the
LMC7211 a good choice for devices where the voltage may
vary over the life of the batteries.
Digital Outputs Representing Signal Level. Comparators
provide a high or low digital output depending on the voltage
levels of the (+) and (−) inputs. This makes comparators use-
ful for interfacing analog signals to microprocessors and
other digital circuits. The LMC7211 can be thought of as a
one-bit a/d converter.
Push-Pull Output. The push-pull output of the LMC7211 is
capable of both sourcing and sinking milliamp level currents
even at a 2.7 volt supply. This can allow the LMC7211 to
drive multiple logic gates.
Driving LEDs (Light Emitting Diodes). With a 5 volt power
supply, the LMC7211’s output sinking current can drive
small, high efficiency LEDs for indicator and test point cir-
cuits. The small size of the Tiny package makes it easy to
find space to add this feature to even compact designs.
Input range to Beyond Rail to Rail. The input common
mode range of the LMC7211 is slightly larger than the actual
power supply range. This wide input range means that the
comparator can be used to sense signals close to the power
supply rails. This wide input range can make design easier
by eliminating voltage dividers, amplifiers, and other front
end circuits previously used to match signals to the limited
input range of earlier comparators. This is useful to power
supply monitoring circuits which need to sense their own
power supply, and compare it to a reference voltage which is
close to the power supply voltage. The wide input range can
also be useful for sensing the voltage drop across a current
sense resistor for battery chargers.
Zero Crossing Detector. Since the LMC7211’s common
mode input range extends below ground even when pow-
ered by a single positive supply, it can be used with large in-
put resistors as a zero crossing detector.
Low Input Currents and High Input Impedance. These
characteristics allow the LMC7211 to be used to sense high
impedance signals from sensors. They also make it possible
to use the LMC7211 in timing circuits built with large value
resistors. This can reduce the power dissipation of timing cir-
cuits. For very long timing circuits, using high value resistors
can reduce the size and cost of large value capacitors for the
same R-C time constant.
Direct Sensor Interfacing. The wide input voltage range
and high impedance of the LMC7211 may make it possible
to directly interface to a sensor without the use of amplifiers
or bias circuits. In circuits with sensors which can produce
outputs in the tens to hundreds of millivolts, the LMC7211
can compare the sensor signal with an appropriately small
reference voltage. This may be done close to ground or the
positive supply rail. Direct sensor interfacing may eliminate
the need for an amplifier for the sensor signal. Eliminating
the amplifier can save cost, space, and design time.
2.0 Low Voltage Operation
Comparators are the common devices by which analog sig-
nals interface with digital circuits. The LMC7211 has been
designed to operate at supply voltages of 2.7V without sac-
rificing performance to meet the demands of 3V digital sys-
tems.
At supply voltages of 2.7V, the common-mode voltage range
extends 200 mV (guaranteed) below the negative supply.
This feature, in addition to the comparator being able to
sense signals near the positive rail, is extremely useful in low
voltage applications.
DS012337-5
FIGURE 1. Even at Low-Supply Voltage of 2.7V, an
Input Signal which Exceeds the Supply Voltages
Produces No Phase Inversion at the Output
At V⁺ = 2.7V propagation delays are t_PLH = 4 µs and t_PHL =
4 µs with overdrives of 100 mV.
Please refer to the performance curves for more extensive
characterization.
3.0 Shoot-Through Current
The shoot-through current is defined as the current surge,
above the quiescent supply current, between the positive
and negative supplies of a device. The current surge occurs
when the output of the device switches states. The
shoot-through current results in glitches in the supply volt-
ages. Usually, glitches in the supply lines are prevented by
bypass capacitors. When the glitches are minimal, the value
of the bypass capacitors can be reduced.
7
www.national.com