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.

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.

01285306

FIGURE 3.

01285307

FIGURE 4. R

Ω

01285308

FIGURE 5. R

Ω

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