SNOAA35E December   2023  – June 2024 LM2901 , LM2901B , LM2901B-Q1 , LM2903 , LM2903-Q1 , LM2903B , LM2903B-Q1 , LM339 , LM339-N , LM393 , LM393-N , LM393B , LM397 , TL331 , TL331-Q1 , TL331B

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. Devices Covered in Application Note
    1. 1.1 Base Part Numbers
    2. 1.2 Input Voltage Offset Grades
    3. 1.3 Maximum Supply Voltage
    4. 1.4 High Reliability Options
  5. The New TL331B, TL391B, LM339B, LM393B, LM2901B and LM2903B B Versions
  6. PCN's to Change Classic Die to a New Die Design
    1. 3.1 PCN #1 for Single and Dual (TL331 and LMx93/LM2903)
    2. 3.2 PCN #2 for Single and Dual (TL331 and LMx93/LM2903)
    3. 3.3 PCN For Quad (LMx39/LM2901)
    4. 3.4 Device PCN Summary
    5. 3.5 Determining Die Version Used
      1. 3.5.1 Determine Die Used for Single TL331 and Dual LM293, LM393, and LM2903 - PCN #1 (Ji3)
      2. 3.5.2 Determine Die Used for Single TL331 and Dual LM293, LM393, and LM2903 - PCN #2 (TiB)
      3. 3.5.3 Determine Die Used for Quad LM139, LM239, LM339, and LM2901
  7. Changes to Package Top Markings
  8. Input Considerations
    1. 5.1  Input Stage Schematic – The Classic LM339 Family
    2. 5.2  Input Stage Schematic - New B and TiB Devices
    3. 5.3  Differences Between the Classic, B and Tib Die Devices
    4. 5.4  Input Voltage Range
    5. 5.5  Input Voltage Range vs. Common Mode Voltage Range
    6. 5.6  Reason for Input Range Headroom Limitation
    7. 5.7  Input Voltage Range Feature
    8. 5.8  Both Inputs Above Input Range Behavior
    9. 5.9  Negative Input Voltages
      1. 5.9.1 Maximum Input Current
      2. 5.9.2 Phase Reversal or Inversion
      3. 5.9.3 Protecting Inputs from Negative Voltages
        1. 5.9.3.1 Simple Resistor and Diode Clamp
        2. 5.9.3.2 Voltage Divider with Clamp
          1. 5.9.3.2.1 Split Voltage Divider with Clamp
    10. 5.10 Power-Up Behavior
    11. 5.11 Capacitors and Hysteresis
    12. 5.12 Output to Input Cross-Talk
  9. Output Stage Considerations
    1. 6.1 Output VOL and IOL
    2. 6.2 Pull-Up Resistor Selection
    3. 6.3 Short Circuit Sinking Current
    4. 6.4 Pulling Output Up Above Vcc
    5. 6.5 Negative Voltages Applied to Output
    6. 6.6 Adding Large Filter Capacitors To Output
  10. Power Supply Considerations
    1. 7.1 Supply Bypassing
      1. 7.1.1 Low VCC Guidance
      2. 7.1.2 Split Supply use
  11. General Comparator Usage
    1. 8.1 Unused Comparator Connections
      1. 8.1.1 Do Not Connect Inputs Directly to Ground
      2. 8.1.2 Unused Comparator Input Connections
      3. 8.1.3 Leave Outputs Floating
      4. 8.1.4 Prototyping
  12. PSpice and TINA TI Models
  13. 10Conclusion
  14. 11Related Documentation
    1. 11.1 Related Links
  15. 12Revision History

Adding Large Filter Capacitors To Output

Commonly designers add large capacitors (100pF to >10uF) directly from the output to ground in an attempt to filter the output to reduce noise on the output or reset timing circuits. TI discourages this practice as this can cause several problems.

A charged capacitor can source peak currents of several amps. When the output goes low, the output must then short the charged capacitor, which causes the output to go into current limit. Long-term, this can stress the output. If discharging a capacitor with the output is desired, as in a timing application, a series current limiting resistor must be used to keep the peak current below 10mA or less. Adding the series resistor, with the appropriate adjustment to the capacitor value, provides a more controlled discharge as the resistor swamps out the slight device variations in the short circuit current. If a series resistor is unacceptable, then an external discrete pass transistor or MOSFET must be used.

Of course, adding the output capacitor increases the propagation delay by directly affecting the risetime and falltime.

Adding a large output capacitor can affect any added hysteresis feedback by adding a delay and slowing the output edges, resulting in bursting or outright cancellation of the hysteresis.

Instead, TI recommends filtering the input signal and using hysteresis instead of brute-force filtering of the output with a capacitor. These techniques can maintain the proper propagation delay while minimizing chatter or false triggers on noisy signals. Please see AN-74 LM139/LM239/LM339 A Quad of Independently Functioning Comparators, application note for more information about adding hysteresis to filter noise.