LMC7221AIM datasheet(8/13 Pages) NSC

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Application Information (Continued)

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 cir-

cuits 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 =4µsand 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

supply 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

voltage. 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 col-

lector outputs.

01234616

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

01234617

01234621

FIGURE 2.

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