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參數資料
| 型號： | LMC7225IMX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 運動控制電子 |
| 英文描述： | Micro-Power, Rail-to-Rail CMOS Comparators with Open-Drain/Push-Pull Outputs and TinyPak⑩ Package |
| 中文描述： | COMPARATOR, 6000 uV OFFSET-MAX, 24000 ns RESPONSE TIME, PDSO8 |
| 封裝： | SOIC-8 |
| 文件頁數： | 7/11頁 |
| 文件大?。?/td> | 360K |
| 代理商： | LMC7225IMX |
Application Information
(Continued)
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
O
vs V
Supply
in the “Typical
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 85C and
possibly even the absolute maximum junction temperature of
150C.
The thermal resistance of the 8-pin surface mount package
is 165C/W. Shorting the output to ground with a 2.7V supply
will only result in about 5C rise above ambient.
The thermal resistance of the much smaller SOT23-5 pack-
age is 325C/W. With a 2.7V supply, the raise is only 10.5C
but if the supply is 5V and the short circuit current is 50 mA,
this will cause a raise of 41C in the SO-8 and 81C 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
S
= 100
www.national.com
7
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