LMC7221 datasheet(7/12 Pages) NSC

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Application Information (Continued)

Zero Crossing Detector. Since the LMC7221’s common

mode input range extends below ground even when pow-

ered by a single positive supply, it can be used with large in-

put resistors as a zero crossing detector.

Low Input Currents and High Input Impedance. These

characteristics allow the LMC7221 to be used to sense high

impedance signals from sensors. They also make it possible

to use the LMC7221 in timing circuits built with large value

resistors. This can reduce the power dissipation of timing cir-

cuits. For very long timing circuits, using high value resistors

can reduce the size and cost of large value capacitors for the

same R-C time constant.

Direct Sensor Interfacing. The wide input voltage range

and high impedance of the LMC7221 may make it possible

to directly interface to a sensor without the use of amplifiers

or bias circuits. In circuits with sensors which can produce

outputs in the tens to hundreds of millivolts, the LMC7221

can compare the sensor signal with an appropriately small

reference voltage. This may be done close to ground or the

positive supply rail. Direct sensor interfacing may eliminate

the need for an amplifier for the sensor signal. Eliminating

the amplifier can save cost, space, and design time.

2.0 Low Voltage Operation

Comparators are the common devices by which analog sig-

nals interface with digital circuits. The LMC7221 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.

At V

+ = 2.7V propagation delays are t

PLH = 4 µs and tPHL =

4 µs with overdrives of 100 mV.

Please refer to the performance curves for more extensive

characterization.

3.0 Open Drain Output

Figure 2 shows the difference between push-pull output and

open drain output.

Push pull outputs will have a conventional high or low digital

output, the same as a logic gate. Low will be the negative

supply rail (usually ground) and high will be the positive sup-

ply rail.

This is useful if the chips you are interfacing to run on the

same supply voltage as the comparator. An example would

be an all +5V system.

Open drain outputs will only pull low — for the high output

they depend on an external pull-up resistor. This can pull up

to a voltage higher or lower than the comparator supply volt-

age. This voltage can be as high as 15V. This makes the

open drain parts useful in mixed voltage systems. An ex-

ample would be where the comparator runs at 5V and the

logic circuits are at 3.3V. The pull-up resistor would go to the

3.3V supply.

Open drain outputs are the CMOS equivalent of open collec-

tor outputs.

4.0 Output Short Circuit Current

The LMC7221 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

DS012346-16

FIGURE 1. Even at Low-Supply Voltage of 2.7V, an

Input Signal which Exceeds the Supply Voltages

Produces No Phase Inversion at the Output

Output Stage

DS012346-17

DS012346-21

FIGURE 2.

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