lmc6772aimx National Semiconductor Corporation, lmc6772aimx Datasheet - Page 7

• Manufacturer Part Number: lmc6772aimx
• Description: Dual Micropower Rail-to-rail Input Cmos Comparator With Open Drain Output
• Manufacturer: National Semiconductor Corporation

1.0 Input Common-Mode Voltage
Range
A wide input voltage range means that the comparator can
be used to sense signals close to ground and also to the
power supplies. This is an extremely useful feature in power
supply monitoring circuits.
An input common-mode voltage range that exceeds the sup-
plies, 20 fA input currents (typical), and a high input imped-
ance makes the LMC6772 ideal for sensor applications. The
LMC6772 can directly interface to sensors without the use of
amplifiers or bias circuits. In circuits with sensors which pro-
duce outputs in the tens to hundreds of millivolts, the
LMC6772 can compare the sensor signal with an appropri-
ately small reference voltage. This reference voltage can be
close to ground or the positive supply rail.
2.0 Low Voltage Operation
Comparators are the common devices by which analog sig-
nals interface with digital circuits. The LMC6772 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.
FIGURE 1. An Input Signal Exceeds the LMC6772
Power Supply Voltages with No Output Phase
(Continued)
Inversion
DS012347-24
7
At V
4 µs with overdrives of 100 mV. Please refer to the perfor-
mance curves for more extensive characterization.
3.0 Output Short Circuit Current
The LMC6772 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 is 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.
4.0 Hysteresis
If the input signal is very noisy, the comparator output might
trip several times as the input signal repeatedly passes
through the threshold. This problem can be addressed by
making use of hysteresis as shown below.
The capacitor added across the feedback resistor increases
the switching speed and provides more short term hyster-
esis. This can result in greater noise immunity for the circuit.
5.0 Spice Macromodel
A Spice Macromodel is available for the LMC6772. The
model includes a simulation of:
and many more characteristics as listed on the macromodel
disk.
• Input common-mode voltage range
• Quiescent and dynamic supply current
• Input overdrive characteristics
FIGURE 3. Canceling the Effect of Input Capacitance
FIGURE 2. Even at Low-Supply Voltage of 2.7V, an
Input Signal which Exceeds the Supply Voltages
+
= 2.7V, propagation delays are t
Produces No Phase Inversion at the Output
PLH
= 4 µs and t
DS012347-25
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DS012347-26
PHL
=