SNOAA35D April   2023  – December 2023 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.   Application Design Guidelines for LM339, LM393, TL331 Family Comparators Including the New B-versions
  3.   Trademarks
  4. 1 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. 2 The New TL331B, TL391B, LM339B, LM393B, LM2901B and LM2903B B Versions
    1. 2.1 PCN to Change Classic Die to a New Die Design
      1. 2.1.1 Determine Die Used for Single TL331 and Dual LM293, LM393, and LM2903
      2. 2.1.2 Determine Die Used for Quad LM139, LM239, LM339, and LM2901
      3. 2.1.3 Device PCN Summary
    2. 2.2 Changes to Package Top Markings
  6. 3 Input Considerations
    1. 3.1  Input Stage Schematic – The Classic LM339 Family
    2. 3.2  Input Stage Schematic - New B Devices
    3. 3.3  Differences Between the Classic and B Die Devices
    4. 3.4  Input Voltage Range
    5. 3.5  Input Voltage Range vs. Common Mode Voltage Range
    6. 3.6  Reason for Input Range Headroom Limitation
    7. 3.7  Input Voltage Range Feature
      1. 3.7.1 Both Inputs Above Input Range Behavior
    8. 3.8  Negative Input Voltages
      1. 3.8.1 Maximum Input Current
      2. 3.8.2 Phase Reversal or Inversion
      3. 3.8.3 Protecting Inputs from Negative Voltages
        1. 3.8.3.1 Simple Resistor and Diode Clamp
        2. 3.8.3.2 Voltage Divider with Clamp
          1. 3.8.3.2.1 Split Voltage Divider with Clamp
    9. 3.9  Power-Up Behavior
    10. 3.10 Capacitors and Hysteresis
    11. 3.11 Output to Input Cross-Talk
  7. 4 Output Stage Considerations
    1. 4.1 Output VOL and IOL
    2. 4.2 Pull-Up Resistor Selection
    3. 4.3 Short Circuit Sinking Current
    4. 4.4 Pulling Output Up Above VCC
    5. 4.5 Negative Voltages Applied to Output
    6. 4.6 Adding Large Filter Capacitors To Output
  8. 5 Power Supply Considerations
    1. 5.1 Supply Bypassing
      1. 5.1.1 Low VCC Guidance
      2. 5.1.2 Split Supply use
  9. 6 General Comparator Usage
    1. 6.1 Unused Comparator Connections
      1. 6.1.1 Do Not Connect Inputs Directly to Ground
      2. 6.1.2 Unused Comparator Input Connections
      3. 6.1.3 Leave Outputs Floating
      4. 6.1.4 Prototyping
  10. 7 PSPICE and TINA TI Models
  11. 8 Conclusion
  12. 9 Related Documentation
    1. 9.1 Related Links
  13. 10Revision History

4.2 Pull-Up Resistor Selection

An open collector output requires a pull-up resistor for the output to go High. An often overlooked design item is the pull-up resistor value. If the pull-up resistor value is too low, the output low pull-up current is excessive, which results in the output low voltage (VOL) increasing, causing excessive output power dissipation and increased overall system supply currents. What is suitable for a 3.3 V pull-up voltage may not be suitable for a 24 V pull-up voltage!

If the pull-up resistor value is too high, this will result in a larger risetime. The risetime will vary with capacitive load as the risetime is dependent on the time constant of the pull-up resistor and the load capacitance. The result is an exponential risetime instead of a square edge and can effect the overall propagation delay. Falltime is not dependent on the pull-up resistor as the output transistor immediately shorts the output, quickly discharging the load capacitance through a low impedance.

The equation to determine risetime (10-90%), with 5k and 15pF example:

Equation 1. r i s e t i m e   =   2 . 2   *   ( R P U L L U P   *   C L O A D )   =   2 . 2   *   ( 5 k   *   15 p F )   =   165 n s

If risetime is not critical, a higher resistor value can be used to further save system power.

TI recommends a pull-up resistor sink current in the range of 100uA to 1mA for the best compromise of output swing and risetime. For example, With a 5 V pull-up voltage and 1mA current, the resistor value would be VPULL-UP / 1mA = 5kΩ. A 4.7k or 5.1k resistor would suffice, as the exact value is not critical. The proper pull-up resistor can be derived from the output saturation curve shown in Figure 4-1 above. With multiple channel devices, be sure to include the power dissipation of each channel in the total package power dissipation calculation.