SNVSAC2A March   2015  – June 2020 LM317A

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
    1.     Typical Application
  3. 3 Description
    1.     Revision History
  4. 4 Device Comparison Table
  5. 5 Pin Configuration and Functions
    1.     Pin 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 Load Regulation
    4. 7.4 Device Functional Modes
      1. 7.4.1 External Capacitors
      2. 7.4.2 Protection Diodes
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1  1.25-V to 25-V Adjustable Regulator
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
        3. 8.2.1.3 Application Curve
      2. 8.2.2  5-V Logic Regulator With Electronic Shutdown
      3. 8.2.3  Slow Turnon 15-V Regulator
      4. 8.2.4  Adjustable Regulator With Improved Ripple Rejection
      5. 8.2.5  High-Stability 10-V Regulator
      6. 8.2.6  High-Current Adjustable Regulator
      7. 8.2.7  Emitter-Follower Current Amplifier
      8. 8.2.8  1-A Current Regulator
      9. 8.2.9  Common-Emitter Amplifier
      10. 8.2.10 Low-Cost 3-A Switching Regulator
      11. 8.2.11 Current-Limited Voltage Regulator
      12. 8.2.12 Adjusting Multiple On-Card Regulators With Single Control
      13. 8.2.13 AC Voltage Regulator
      14. 8.2.14 12-V Battery Charger
      15. 8.2.15 Adjustable 4-A Regulator
      16. 8.2.16 Current-Limited 6-V Charger
      17. 8.2.17 Digitally-Selected Outputs
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Thermal Considerations
        1. 10.1.1.1 Heatsink Requirements
        2. 10.1.1.2 Heatsinking Surface Mount Packages
          1. 10.1.1.2.1 Heatsinking the SOT-223 (DCY) Package
          2. 10.1.1.2.2 Heatsinking the TO-252 (NDP) Package
    2. 10.2 Layout Examples
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Support Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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10.1.1.1 Heatsink Requirements

The LM317A regulators have internal thermal shutdown to protect the device from over-heating. Under all operating conditions, the junction temperature of the LM317A should not exceed the rated maximum junction temperature (TJ) of 125°C. A heatsink may be required depending on the maximum device power dissipation and the maximum ambient temperature of the application. To determine if a heatsink is needed, the power dissipated by the regulator, PD, must be calculated by Equation 2:

Equation 2. PD = ((VIN − VOUT) × IL) + (VIN × IG)

Figure 36 shows the voltage and currents which are present in the circuit.

The next parameter which must be calculated is the maximum allowable temperature rise, TR(MAX) in Equation 3:

Equation 3. TR(MAX) = TJ(MAX) − TA(MAX)

where

  • TJ(MAX) is the maximum allowable junction temperature (125°C for the LM317A),
  • and TA(MAX) is the maximum ambient temperature that will be encountered in the application.

Using the calculated values for TR(MAX) and PD, the maximum allowable value for the junction-to-ambient thermal resistance (θJA) can be calculated by Equation 4:

Equation 4. θJA = (TR(MAX) / PD)
LM317A 906360.gifFigure 36. Power Dissipation Diagram

If the calculated maximum allowable thermal resistance is higher than the actual package rating, then no additional work is needed. If the calculated maximum allowable thermal resistance is lower than the actual package rating, either the power dissipation (PD) needs to be reduced, the maximum ambient temperature TA(MAX) needs to be reduced, the thermal resistance (θJA) must be lowered by adding a heatsink, or some combination of these measures should be implemented.

If a heatsink is needed, the value can be calculated from Equation 5:

Equation 5. θHA ≤ (θJA – (θCH + θJC))

where

  • θCH is the thermal resistance of the contact area between the device case and the heatsink surface
  • θJC is thermal resistance from the junction of the die to surface of the package case

When a value for θHA is found using the equation shown, a heatsink must be selected that has a value that is less than or equal to this number.

The θHA rating is specified numerically by the heatsink manufacturer in the catalog, or shown in a curve that plots temperature rise vs power dissipation for the heatsink.