LMC7211AIM National Semiconductor, LMC7211AIM Datasheet - Page 10

• Manufacturer Part Number: LMC7211AIM
• Description: Voltage Comparator IC
• Manufacturer: National Semiconductor

Specifications of LMC7211AIM

• No. Of Comparators: 1
• Response Time: 4µs
• No. Of Pins: 8
• Output Type: Push Pull
• Single Supply Voltage Min (+v): 2.7V
• Mounting Type: Surface Mount
• Rail To Rail I/o Type: Rail Rail Inputs
• Number Of Elements: 1
• Technology: CMOS
• Input Offset Voltage: 5mV
• Input Bias Current (typ): 40pA
• Single Supply Voltage (typ): 3/5/9/12V
• Dual Supply Voltage (typ): Not RequiredV
• Supply Current (max): 0.014@15VmA
• Power Supply Requirement: Single
• Common Mode Rejection Ratio: 75dB
• Voltage Gain In Db: 100dB
• Power Supply Rejection Ratio: 80dB
• Single Supply Voltage (min): 2.7V
• Single Supply Voltage (max): 15V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 8
• Package Type: SOIC N
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

www.national.com
For non-precision circuits, such as using a comparator to de-
termine 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 expen-
sive bypass capacitors in non-critical circuits.
4.0 Output Short Circuit Current
The LMC7211 has short circuit protection of 40 mA. However,
it is not designed to withstand continuous short circuits, tran-
sient voltage or current spikes, or shorts to any voltage be-
yond 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
creasing signals and one for decreasing signals. A capacitor
can be added across R
provide more short term hysteresis. This can result in greater
noise immunity for the circuit.
See
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 in-
R
R
Figure
1
2
and
FIGURE 4. Positive Feedback for Hysteresis
4,
Figure 5
Without Positive Feedback
(No Hysteresis)
F
and
to increase the switching speed and
FIGURE 5.
Figure
6.
1233709
1233710
10
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 likelihood 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
may affect accuracy during normal operation. The R-C time
constant of R
sponse time.
7.0 Layout Considerations
The LMC7211 is not an especially fast comparator, so high
speed design practices are not required. The LMC7211 is ca-
pable of operating with very high impedance inputs, so pre-
cautions should be taken to reduce noise pickup with high
impedance (
noisy environments.
Keeping high value resistors close to the LMC7211 and min-
imizing 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
reduce noise.
∼
IN
100 kΩ and greater) designs and in electrically
and the diode capacitance may also slow re-
With Positive Feedback
(Hysteresis or Memory)
Figure
FIGURE 6.
FIGURE 7.
7. Note that diode leakage current
1233712
1233711