LMC7215_06 Datasheet, PDF (7/11 Pages) National Semiconductor (TI) – Micro-Power, Rail-to-Rail CMOS Comparators with Push-Pull/Open-Drain Outputs and TinyPak™ Package

Application Information (Continued)
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 SOT23
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 SOT23. This should be kept in mind if
driving very low resistance 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.
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FIGURE 4. R_S = 100Ω
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
current 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
capacitor of 0.01 µF should be more than enough to mini-
mize the effects of this small current pulse.
FIGURE 3.
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FIGURE 5. R_S = 10Ω
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