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 de-

sired, 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 surface mount 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 SOT23-5 pack-

age 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 SO-8 and 81˚C in the

SOT23-5. This should be kept in mind if driving very low re-

sistance loads.

SHOOT-THROUGH

Shoot-through is a common occurrence on digital circuits

and comparators where there is a push-pull output stage.

This occurs when a signal is applied at the same time to both

the N-channel and P-channel output transistors to turn one

off and turn the other on. (See

Figure 3.) If one of the output

devices responds slightly faster than the other, the fast one

can be turned on before the other has turned off. For a very

short time, this allows supply current to flow directly through

both output transistors. The result is a short spike of current

drawn from the supply.

The LMC7215 produces a small current spike of 300 µA

peak for about 400 ns with 2.7V supply and 1.8 mA peak for

400 ns with a 5V supply. This spike only occurs when the

output is going from high to low. It does not occur when going

from low to high.

Figure 4 and Figure 5 show what this cur-

rent pulse looks like on 2.7V and 5V supplies. The upper

trace is the output voltage and the lower trace is the supply

current as measured with the circuit in

Figure 6 .

If the power supply has a very high impedance, a bypass ca-

pacitor of 0.01 µF should be more than enough to minimize

the effects of this small current pulse.

DS012853-6

FIGURE 3.

DS012853-7

FIGURE 4. R

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