Application Information
(Continued)
should reduce the effect of shorts. For outputs which send
signals off PC boards additional protection devices, such as
diodes to the supply rails, and varistors may be used.
5.0 Input Protection
If input signals are likely to exceed the common mode range
of the LMC7221, or it is likely that signals may be present
when power is off, damage to the LMC7221 may occur.
Large value (100 kΩ to MΩ) input resistors may reduce the
likelihood of damage by limiting the input currents. Since the
LMC7221 has very low input leakage currents, the effect on
accuracy will be small. Additional protection may require the
use of diodes, as shown in
Figure 3.
Note that diode leakage
current may affect accuracy during normal operation.
The R-C time constant of R
IN
and the diode capacitance may
also slow response time.
8.0 Additional SOT23-5 Tiny Devices
National Semiconductor has additional parts available in the
space saving SOT23 Tiny package, including amplifiers,
voltage references, and voltage regulators. These devices
include —
LMC7101
1 MHz gain-bandwidth rail-to-rail input and out-
put amplifier — high input impedance and high
gain 700 µA typical current 2.7V, 3V, 5V and 15V
specifications.
LMC7111
Low power 50 kHz gain-bandwidth rail-to-rail in-
put and output amplifier with 25 µA typical cur-
rent specified at 2.7V, 3.0V, 3.3V, 5V and 10V.
LM7131
Tiny Video amp with 70 MHz gain bandwidth 3V,
5V and
±
5V specifications.
LP2980
Micropower SOT 50 mA Ultra Low-Dropout
Regulator.
LM4040
Precision micropower shunt voltage reference.
Fixed voltages of 2.500V, 4.096V, 5.000V,
8.192V and 10.000V.
Precision micropower shut voltage reference
1.225V and adjustable.
LM385
Low current voltage reference. Fixed Voltages of
1.2V and 2.5V.
Contact your National Semiconductor representative for the
latest information.
9.0 Spice Macromodel
A Spice Macromodel is available for the LMC7221 compara-
tor on the National Semiconductor Amplifier Macromodel
disk. Contact your National Semiconductor representative to
obtain the latest version.
LM4041
DS012346-18
FIGURE 3.
6.0 Layout Considerations
The LMC7221 is not an especially fast comparator, so high
speed design practices are not required. The LMC7221 is
capable of operating with very high impedance inputs, so
precautions should be taken to reduce noise pickup with
high impedance (∼ 100 kΩ and greater) designs and in elec-
trically noisy environments.
Keeping high value resistors close to the LMC7221 and mini-
mizing the size of the input nodes is a good practice. With
multilayer designs, try to avoid long loops which could act as
inductors (coils). Sensors which are not close to the com-
parator may need twisted pair or shielded connections to re-
duce noise.
7.0 Push-Pull Outputs, Dual Versions
The LMC7211 is a comparator similar to the LMC7221, but
with push-pull outputs which can source current.
The performance of the LMC7221 is available in a dual de-
vice. Please see the LMC6772 datasheet. For a dual device
with push-pull outputs, please see the LMC6762 datasheet.
Rail-to-Rail Input Low Power Comparators —
Push-Pull Output
LMC7221
LMC6762
SOT23-5, SO-8
SO-8, DIP
Single
Dual
Open Drain Output
LMC7221
LMC6772
SOT23-5, SO-8
SO-8, DIP
Single
Dual
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