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LMC7221 Datasheet(PDF) 7 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LMC7221
Description  Tiny CMOS Comparator with Rail-To-Rail Input and Open Drain Output
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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LMC7221 Datasheet(HTML) 7 Page - National Semiconductor (TI)

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Application Information (Continued)
Zero Crossing Detector. Since the LMC7221’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 LMC7221 to be used to sense high
impedance signals from sensors. They also make it possible
to use the LMC7221 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 LMC7221 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 LMC7221
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 LMC7221 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.
At V
+ = 2.7V propagation delays are t
PLH = 4 µs and tPHL =
4 µs with overdrives of 100 mV.
Please refer to the performance curves for more extensive
characterization.
3.0 Open Drain Output
Figure 2 shows the difference between push-pull output and
open drain output.
Push pull outputs will have a conventional high or low digital
output, the same as a logic gate. Low will be the negative
supply rail (usually ground) and high will be the positive sup-
ply rail.
This is useful if the chips you are interfacing to run on the
same supply voltage as the comparator. An example would
be an all +5V system.
Open drain outputs will only pull low — for the high output
they depend on an external pull-up resistor. This can pull up
to a voltage higher or lower than the comparator supply volt-
age. This voltage can be as high as 15V. This makes the
open drain parts useful in mixed voltage systems. An ex-
ample would be where the comparator runs at 5V and the
logic circuits are at 3.3V. The pull-up resistor would go to the
3.3V supply.
Open drain outputs are the CMOS equivalent of open collec-
tor outputs.
4.0 Output Short Circuit Current
The LMC7221 has short circuit protection of 40 mA. How-
ever, it is not designed to withstand continuous short circuits,
transient voltage or current spikes, or shorts to any voltage
beyond the supplies. A resistor in series with the output
DS012346-16
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
Output Stage
DS012346-17
DS012346-21
FIGURE 2.
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