SNOS977F May   2001  – May 2016 LM397

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Input Stage
      2. 7.3.2 Output Stage
    4. 7.4 Device Functional Modes
      1. 7.4.1 Hysteresis
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Input Voltage Range
        2. 8.2.2.2 Minimum Overdrive Voltage
        3. 8.2.2.3 Output and Drive Current
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Community Resources
    2. 11.2 Trademarks
    3. 11.3 Electrostatic Discharge Caution
    4. 11.4 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

8.1 Application Information

LM397 will typically be used to compare a single signal to a reference or two signals against each other.

8.2 Typical Application

LM397 20022109.gif Figure 9. Inverting Comparator With Hysteresis

8.2.1 Design Requirements

For this design example, use the parameters listed in Table 1 as the input parameters.

Table 1. Design Parameters

DESIGN PARAMETER EXAMPLE VALUE
Input voltage range 0 V to VS – 1.5 V
Supply voltage 5 V to 30 V
Logic supply voltage (RPULLUP voltage) 5 V to 30 V
Output current (VLOGIC/RPULLUP) 1 µA to 20 mA
Input overdrive voltage 100 mV
Reference voltage 5.5 V

8.2.2 Detailed Design Procedure

When using TL331 in a general comparator application, determine the following:

  • Input voltage range
  • Minimum overdrive voltage
  • Output and drive current

8.2.2.1 Input Voltage Range

When choosing the input voltage range, the input common mode voltage range (VCM) must be taken in to account. If temperature operation is above or below 25°C the VCM can range from 0 V to VS – 1.5 V. This limits the input voltage range to as high as VS – 1.5 V and as low as 0 V. Operation outside of this range can yield incorrect comparisons.

Below is a list of input voltage situation and their outcomes:

  1. When both IN– and IN+ are both within the common mode range:
    1. If IN– is higher than IN+ and the offset voltage, the output is low and the output transistor is sinking current
    2. If IN– is lower than IN+ and the offset voltage, the output is high impedance and the output transistor is not conducting
  2. When IN– is higher than common mode and IN+ is within common mode, the output is low and the output transistor is sinking current
  3. When IN+ is higher than common mode and IN– is within common mode, the output is high impedance and the output transistor is not conducting
  4. When IN– and IN+ are both higher than common mode, the output is low and the output transistor is sinking current

8.2.2.2 Minimum Overdrive Voltage

Overdrive Voltage is the differential voltage produced between the positive and negative inputs of the comparator over the offset voltage. To make an accurate comparison; the overdrive voltage should be higher than the input offset voltage. Overdrive voltage can also determine the response time of the comparator, with the response time decreasing with increasing overdrive.

8.2.2.3 Output and Drive Current

Output current is determined by the pullup resistance (RPULLUP) and VS voltage. The output current will produce a output low voltage (VOL) from the comparator. In which VOL is proportional to the output current. Use Figure 3 to determine VOL based on the output current. The output current can also effect the transient response.

8.2.3 Application Curves

LM397 20022105.gif Figure 10. Response Time for Various Input Overdrives – tPHL
LM397 20022106.gif Figure 11. Response Time for Various Input Overdrives – tPLH