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 Input Protection

If input signals are likely to exceed the common mode range

of the LMC7221, or it is likely that signals may be present

when power is off, damage to the LMC7221 may occur.

Large value (100 k

Ω to MΩ) input resistors may reduce the

likelihood of damage by limiting the input currents. Since the

LMC7221 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 3. Note that diode leakage

current may affect accuracy during normal operation.

The R-C time constant of R

also slow response time.

6.0 Layout Considerations

The LMC7221 is not an especially fast comparator, so high

speed design practices are not required. The LMC7221 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 LMC7221 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.

7.0 Push-Pull Outputs, Dual Versions

The LMC7211 is a comparator similar to the LMC7221, but

with push-pull outputs which can source current.

The performance of the LMC7221 is available in a dual de-

vice. Please see the LMC6772 datasheet. For a dual device

with push-pull outputs, please see the LMC6762 datasheet.

Rail-to-Rail Input Low Power Comparators —

Push-Pull Output

LMC7221

SOT23-5, SO-8

Single

LMC6762

SO-8, DIP

Dual

Open Drain Output

LMC7221

SOT23-5, SO-8

Single

LMC6772

SO-8, DIP

Dual

8.0 Additional SOT23-5 Tiny Devices

National Semiconductor has additional parts available in the

space saving SOT23 Tiny package, including amplifiers,

voltage references, and voltage regulators. These devices

include —

LMC7101 1 MHz gain-bandwidth rail-to-rail input and out-

put amplifier — high input impedance and high

gain 700 µA typical current 2.7V, 3V, 5V and 15V

specifications.

LMC7111 Low power 50 kHz gain-bandwidth rail-to-rail in-

put and output amplifier with 25 µA typical cur-

rent specified at 2.7V, 3.0V, 3.3V, 5V and 10V.

LM7131

Tiny Video amp with 70 MHz gain bandwidth 3V,

LP2980

Micropower SOT 50 mA Ultra Low-Dropout

Regulator.

LM4040

Precision micropower shunt voltage reference.

Fixed voltages of 2.500V, 4.096V, 5.000V,

8.192V and 10.000V.

LM4041

Precision micropower shut voltage reference

1.225V and adjustable.

LM385

Low current voltage reference. Fixed Voltages of

1.2V and 2.5V.

Contact your National Semiconductor representative for the

latest information.

9.0 Spice Macromodel

A Spice Macromodel is available for the LMC7221 compara-

tor on the National Semiconductor Amplifier Macromodel

disk. Contact your National Semiconductor representative to

obtain the latest version.

DS012346-18

FIGURE 3.

www.national.com

8