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 k

Ω 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.

7.0 Layout Considerations

The LMC7211 is not an especially fast comparator, so high

speed design practices are not required. The LMC7211 is

capable of operating with very high impedance inputs, so

precautions should be taken to reduce noise pickup with

high impedance (

∼ 100 kΩ and greater) designs and in elec-

trically noisy environments.

Keeping high value resistors close to the LMC7211 and mini-

mizing 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 re-

duce noise.

8.0 Open Drain Output, Dual Versions

The LMC7221 is a comparator similar to the LMC7211, but

with an open drain output which allows the output voltage to

be different (higher or lower) than the supply voltage. The

open drain output is like the open collector output of a logic

gate. This makes the LMC7221 very useful for mixed voltage

systems. Many systems will have different voltages for the

analog and microprocessor sections. Please see the

LMC7221 datasheet for details.

The performance of the LMC7211 is available in dual de-

vices. Please see the LMC6762 datasheet for details on a

dual push-pull output device. For a dual device with open

drain outputs, please see the LMC6772 datasheet.

Rail-to-Rail Input Low Power Comparators —

Push-Pull Output

LMC7211

SOT23-5, SO-8

Single

LMC6762

SO-8, DIP

Dual

Open Drain Output

LMC7221

SOT23-5, SO-8

Single

LMC6772

SO-8, DIP

Dual

DS012337-9

FIGURE 4. Positive Feedback for Hysteresis

Without Positive Feedback

(No Hysteresis)

DS012337-10

FIGURE 5.

With Positive Feedback

(Hysteresis or Memory)

DS012337-11

FIGURE 6.

DS012337-12

FIGURE 7.

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