lmc7211 National Semiconductor Corporation, lmc7211 Datasheet - Page 10

• Manufacturer Part Number: lmc7211
• Description: Tiny Cmos Comparator With Rail-to-rail Input And Push-pull Output
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
Note that the switching current of the comparator can spread
to other parts of the board as noise. The bypass capacitor
reduces this noise. For low noise systems this may be
reason to make the capacitor larger.
For non-precision circuits, such as using a comparator to
determine if a push-button switch is on or off, it is often
cheaper and easier to use a larger value of hysteresis and a
small value or bypass capacitance. The low shoot-through
current of the LMC7211 can allow the use of smaller and less
expensive bypass capacitors in non-critical circuits.
4.0 Output Short Circuit Current
The LMC7211 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
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 Hysteresis
If the input signal is very slow or very noisy, the comparator
output might trip several times as the input signal passes
through the threshold. Using positive feedback to add hys-
teresis to the switching can reduce or eliminate this problem.
The positive feedback can be added by a high value resistor
(R
increasing signals and one for decreasing signals. A capaci-
tor can be added across R
and provide more short term hysteresis. This can result in
greater noise immunity for the circuit.
See Figure 4, Figure 5 and Figure 6.
Note that very heavy loading of the comparator output, such
as LED drive or bipolar logic gates, will change the output
voltage and shift the voltage thresholds.
R
R
F
F
F
@
@
). This will result in two switching thresholds, one for
R
R
FIGURE 4. Positive Feedback for Hysteresis
1
2
and
F
to increase the switching speed
(Continued)
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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
current may affect accuracy during normal operation. The
R-C time constant of R
also slow response time.
Without Positive Feedback
With Positive Feedback
(Hysteresis or Memory)
(No Hysteresis)
FIGURE 5.
FIGURE 6.
IN
and the diode capacitance may
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