current may affect accuracy during normal operation. The
R-C time constant of RIN and the diode capacitance may
also slow response time.
Application Information (Continued)
DS012337-9
@
@
R
R
R
R
and
F
1
F
2
FIGURE 4. Positive Feedback for Hysteresis
DS012337-12
FIGURE 7.
Without Positive Feedback
(No Hysteresis)
7.0 Layout Considerations
The LMC7211 is not an especially fast comparator, so high
speed design practices are not required. The LMC7211 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 LMC7211 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.
8.0 Open Drain Output, Dual Versions
DS012337-10
The LMC7221 is a comparator similar to the LMC7211, but
with an open drain output which allows the output voltage to
be different (higher or lower) than the supply voltage. The
open drain output is like the open collector output of a logic
gate. This makes the LMC7221 very useful for mixed voltage
systems. Many systems will have different voltages for the
analog and microprocessor sections. Please see the
LMC7221 datasheet for details.
FIGURE 5.
With Positive Feedback
(Hysteresis or Memory)
The performance of the LMC7211 is available in dual de-
vices. Please see the LMC6762 datasheet for details on a
dual push-pull output device. For a dual device with open
drain outputs, please see the LMC6772 datasheet.
Rail-to-Rail Input Low Power Comparators —
Push-Pull Output
LMC7211
LMC6762
SOT23-5, SO-8
SO-8, DIP
Single
Dual
Open Drain Output
DS012337-11
LMC7221
LMC6772
SOT23-5, SO-8
SO-8, DIP
Single
Dual
FIGURE 6.
6.0 Input Protection
If input signals are like to exceed the common mode range of
the LMC7211, or it is likely that signals may be present when
power is off, damage to the LMC7211 may occur. Large
value (100 kΩ to MΩ) input resistors may reduce the likeli-
hood of damage by limiting the input currents. Since the
LMC7211 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 7. Note that diode leakage
9
www.national.com
NSC [ National Semiconductor ]
