Application Information

RESPONSE TIME

Depending upon the amount of overdrive, the delay will

typically be between 10 µs to 200 µs. The curve showing

delay vs. overdrive in the “Typical Characteristics” section

shows the delay time when the input is preset with 100 mV

across the inputs and then is driven the other way by 1 mV

to 500 mV.

The transition from high to low or low to high is fast. Typically

1 µs rise and 400 ns fall.

With a small signal input, the comparators will provide a

square wave output from sine wave inputs at frequencies as

high as 25 kHz. Figure 1 shows a worst case example where

output is delayed by almost 180˚.

NOISE

Most comparators have rather low gain. This allows the

output to spend time between high and low when the input

signal changes slowly. The result is the output may oscillate

between high and low when the differential input is near

zero.

The exceptionally high gain of these comparators, 10,000

V/mV, eliminates this problem. Less then 1 µV of change on

the input will drive the output from one rail to the other rail.

If the input signal is noisy, the output cannot ignore the noise

unless some hysteresis is provided by positive feedback.

INPUT VOLTAGE RANGE

The LMC7215/25 have input voltage ranges that are larger

than the supply voltage guarantees that signals from other

parts of the system cannot overdrive the inputs. This allows

sensing supply current by connecting one input directly to

the V

resistor. The same is true if the sense resistor is in the

ground return line.

Sensing supply voltage is also easy by connecting one input

directly to the supply.

The inputs of these comparators are protected by diodes to

both supplies. This protects the inputs from both ESD as well

as signals that greatly exceed the supply voltages. As a

result, current will flow through these forward biased diodes

whenever the input voltage is more than a few hundred

millivolts larger than the supplies. Until this occurs, there is

essentially no input current. As a result, placing a large

resistor in series with any input that may be exposed to large

voltages, will limit the input current but have no other notice-

able effect.

If the input current is limited to less than 5 mA by a series

resistor, (see Figure 2), a threshold or zero crossing detec-

tor, that works with inputs from as low as a few millivolts to as

high as 5,000V, is made with only one resistor and the

comparator.

INPUTS

As mentioned above, these comparators have near zero

input current. This allows very high resistance circuits to be

used without any concern for matching input resistances.

This also allows the use of very small capacitors in R-C type

timing circuits. This reduces the cost of the capacitors and

amount of board space used.

CAPACITIVE LOADS

The high output current drive allows large capacitive loads

with little effect. Capacitive loads as large as 10,000 pF have

no effect upon delay and only slow the transition by about 3

µs.

OUTPUT CURRENT

Even though these comparators use less than 1 µA supply

current, the outputs are able to drive very large currents.

The LMC7215 can source up to 50 mA when operated on a

5V supply. Both the LMC7215 and LMC7225 can sink over

20 mA. (See the graph of Max I

Characteristics” section.)

This large current handling ability allows driving heavy loads

directly. LEDs, beepers and other loads can be driven easily.

The push-pull output stage of the LMC7215 is a very impor-

tant feature. This keeps the total system power consumption

to the absolute minimum. The only current consumed is the

less than 1 µA supply current and the current going directly

into the load. No power is wasted in a pull-up resistor when

the output is low. The LMC7225 is only recommended where

a level shifting function from one logic level to another is

desired, where the LMC7225 is being used as a drop-in

lower power replacement for an older comparator or in cir-

cuits where more than one output will be paralleled.

01285304

FIGURE 1.

01285305

FIGURE 2.

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