LMC7221_14 Datasheet, PDF (9/21 Pages) Texas Instruments – LMC7221 Tiny CMOS Comparator with Rail-To-Rail Input and Open Drain Output

www.ti.com
OPEN DRAIN OUTPUT
Output Stage
LMC7221
SNOS748E – SEPTEMBER 1999 – REVISED MARCH 2013
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 kΩ 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 R_IN and the diode capacitance may also slow response time.
Copyright © 1999–2013, Texas Instruments Incorporated
Product Folder Links: LMC7221
Submit Documentation Feedback
9