LMC7221

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SNOS748E – SEPTEMBER 1999 – REVISED MARCH 2013

OPEN DRAIN OUTPUT

Output Stage

Figure 17. Output Stage

Figure 17 shows the difference between push-pull output and open drain output.

Push pull outputs will have a conventional high or low digital output, the same as a logic gate. Low will be the

negative supply rail (usually ground) and high will be the positive supply rail.

This is useful if the chips you are interfacing to run on the same supply voltage as the comparator. An example

would be an all +5V system.

Open drain outputs will only pull low—for the high output they depend on an external pull-up resistor. This can

pull up to a voltage higher or lower than the comparator supply voltage. This voltage can be as high as 15V. This

makes the open drain parts useful in mixed voltage systems. An example would be where the comparator runs at

5V and the logic circuits are at 3.3V. The pull-up resistor would go to the 3.3V supply.

Open drain outputs are the CMOS equivalent of open collector outputs.

OUTPUT SHORT CIRCUIT CURRENT

The LMC7221 has short circuit protection of 40 mA. However, it is not designed to withstand continuous short

circuits, transient voltage or current spikes, or shorts to any voltage beyond the supplies. A resistor in series with

the output should reduce the effect of shorts. For outputs which send signals off PC boards additional protection

devices, such as diodes to the supply rails, and varistors may be used.

INPUT PROTECTION

If input signals are likely to exceed the common mode range of the LMC7221, or it is likely that signals may be

present when power is off, damage to the LMC7221 may occur. Large value (100 k

Ω to MΩ) input resistors may

reduce the likelihood of damage by limiting the input currents. Since the LMC7221 has very low input leakage

currents, the effect on accuracy will be small. Additional protection may require the use of diodes, as shown in

Figure 18. Note that diode leakage current may affect accuracy during normal operation.

The R-C time constant of RIN and the diode capacitance may also slow response time.

Copyright © 1999–2013, Texas Instruments Incorporated

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