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LMC7215IM Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LMC7215IM Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 11 page 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 VSupply 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 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 S = 100Ω www.national.com 7 |
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