Datasheet LMC7221 (Texas Instruments) - 9

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LMC7221 www.ti.com
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 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 RIN and the diode capacitance may also slow response time. Copyright © 1999–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LMC7221 Document Outline FEATURES Applications DESCRIPTION Connection Diagram Absolute Maximum Ratings Operating Ratings 2.7V Electrical Characteristics 5.0V and 15.0V Electrical Characteristics Leakage Characteristics AC Electrical Characteristics Typical Performance Characteristics Application Information BENEFITS OF THE LMC7221 TINY COMPARATOR Size Height Simplified Board Layout Low Supply Current Wide Voltage Range Digital Outputs Representing Signal Level Open Drain Output Driving LEDs (Light Emitting Diodes) Input range to Beyond Rail to Rail Zero Crossing Detector Low Input Currents and High Input Impedance Direct Sensor Interfacing LOW VOLTAGE OPERATION OPEN DRAIN OUTPUT Output Stage OUTPUT SHORT CIRCUIT CURRENT INPUT PROTECTION LAYOUT CONSIDERATIONS PUSH-PULL OUTPUTS, DUAL VERSIONS ADDITIONAL 5-Pin SOT-23 TINY DEVICES SPICE MACROMODEL Revision History