LMC7215, LMC7225

SNOS882E – SEPTEMBER 1999 – REVISED MARCH 2013

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INPUT VOLTAGE RANGE

The LMC7215/25 have input voltage ranges that are larger than the supply voltage ensures that signals from

other parts of the system cannot overdrive the inputs. This allows sensing supply current by connecting one input

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 noticeable effect.

If the input current is limited to less than 5 mA by a series resistor, (see Figure 14), a threshold or zero crossing

detector, 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

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 important 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 circuits where

more than one output will be paralleled.

POWER DISSIPATION

The large output current ability makes it possible to exceed the maximum operating junction temperature of 85°C

and possibly even the absolute maximum junction temperature of 150°C.

The thermal resistance of the 8-pin SOIC package is 165°C/W. Shorting the output to ground with a 2.7V supply

will only result in about 5°C rise above ambient.

The thermal resistance of the much smaller 5-Pin SOT-23 package is 325°C/W. With a 2.7V supply, the raise is

only 10.5°C but if the supply is 5V and the short circuit current is 50 mA, this will cause a raise of 41°C in the 8-

Pin SOIC and 81°C in the 5-Pin SOT-23. This should be kept in mind if driving very low resistance loads.

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