Application Information

1.0 Benefits of the LMC7211 Tiny Comparator

Size. The small footprint of the SOT 23-5 packaged Tiny

Comparator, (0.120 x 0.118 inches, 3.05 x 3.00 mm) saves

space on printed circuit boards, and enable the design of

smaller electronic products. Because they are easier to

carry, many customers prefer smaller and lighter products.

Height. The height (0.056 inches, 1.43 mm) of the Tiny

Comparator makes it possible to use it in PCMCIA type III

cards.

Simplified Board Layout. The Tiny Comparator can sim-

plify board layout in several ways. First, by placing a com-

parator where comparators are needed, instead of routing

signals to a dual or quad device, long pc traces may be

avoided.

By using multiple Tiny Comparators instead of duals or

quads, complex signal routing and possibly crosstalk can be

reduced.

Low Supply Current. The typical 7 µA supply current of the

LMC7211 extends battery life in portable applications, and

may allow the reduction of the size of batteries in some ap-

plications.

Wide Voltage Range. The LMC7211 is characterized at

15V, 5V and 2.7V. Performance data is provided at these

popular voltages. This wide voltage range makes the

LMC7211 a good choice for devices where the voltage may

vary over the life of the batteries.

Digital Outputs Representing Signal Level. Comparators

provide a high or low digital output depending on the voltage

levels of the (+) and (−) inputs. This makes comparators use-

ful for interfacing analog signals to microprocessors and

other digital circuits. The LMC7211 can be thought of as a

one-bit a/d converter.

Push-Pull Output. The push-pull output of the LMC7211 is

capable of both sourcing and sinking milliamp level currents

even at a 2.7 volt supply. This can allow the LMC7211 to

drive multiple logic gates.

Driving LEDs (Light Emitting Diodes). With a 5 volt power

supply, the LMC7211’s output sinking current can drive

small, high efficiency LEDs for indicator and test point cir-

cuits. The small size of the Tiny package makes it easy to

find space to add this feature to even compact designs.

Input range to Beyond Rail to Rail. The input common

mode range of the LMC7211 is slightly larger than the actual

power supply range. This wide input range means that the

comparator can be used to sense signals close to the power

supply rails. This wide input range can make design easier

by eliminating voltage dividers, amplifiers, and other front

end circuits previously used to match signals to the limited

input range of earlier comparators. This is useful to power

supply monitoring circuits which need to sense their own

power supply, and compare it to a reference voltage which is

close to the power supply voltage. The wide input range can

also be useful for sensing the voltage drop across a current

sense resistor for battery chargers.

Zero Crossing Detector. Since the LMC7211’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 LMC7211 to be used to sense high

impedance signals from sensors. They also make it possible

to use the LMC7211 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 LMC7211 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 LMC7211

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 LMC7211 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.

4 µs with overdrives of 100 mV.

Please refer to the performance curves for more extensive

characterization.

3.0 Shoot-Through Current

The shoot-through current is defined as the current surge,

above the quiescent supply current, between the positive

and negative supplies of a device. The current surge occurs

when the output of the device switches states. The

shoot-through current results in glitches in the supply volt-

ages. Usually, glitches in the supply lines are prevented by

bypass capacitors. When the glitches are minimal, the value

of the bypass capacitors can be reduced.

DS012337-5

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

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