SNVS771C May   1998  – December 2016 LM138 , LM338


  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  6. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 Recommended Operating Conditions
    3. 6.3 Thermal Information
    4. 6.4 Electrical Characteristics: LM138
    5. 6.5 Electrical Characteristics: LM338
    6. 6.6 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 NPN Darlington Output Drive
      2. 7.3.2 Overload Block
      3. 7.3.3 Programmable Feedback
    4. 7.4 Device Functional Modes
      1. 7.4.1 Normal Operation
      2. 7.4.2 Operation With Low Input Voltage
      3. 7.4.3 Operation at Light Loads
      4. 7.4.4 Operation in Self Protection
  8. Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Constant 5-V Regulator
        1. Design Requirements
        2. Detailed Design Procedure
          1. External Capacitors
          2. Load Regulation
          3. Protection Diodes
        3. Application Curves
    3. 8.3 System Examples
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Receiving Notification of Documentation Updates
    2. 11.2 Related Links
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information


  • Specified 7-A Peak Output Current
  • Specified 5-A Output Current
  • Adjustable Output Down to 1.2 V
  • Specified Thermal Regulation
  • Current Limit Constant With Temperature
  • P + Product Enhancement Tested
  • Output is Short-Circuit Protected


  • Adjustable Power Supplies
  • Constant Current Regulators
  • Battery Chargers

Available Packages

LM138 LM338 front_page_group_glam_shot_snvs771.gif

Typical Application Circuit

LM138 LM338 00906006.gif


The LM138 series of adjustable 3-terminal positive voltage regulators is capable of supplying in excess of 5 A over a 1.2-V to 32-V output range. They are exceptionally easy to use and require only 2 resistors to set the output voltage. Careful circuit design has resulted in outstanding load and line regulation, comparable to many commercial power supplies. The LM138 family is supplied in a standard 3-lead transistor package.

A unique feature of the LM138 family is time-dependent current limiting. The current limit circuitry allows peak currents of up to 12 A to be drawn from the regulator for short periods of time. This allows the LM138 to be used with heavy transient loads and speeds start-up under full-load conditions. Under sustained loading conditions, the current limit decreases to a safe value protecting the regulator. Also included on the chip are thermal overload protection and safe area protection for the power transistor. Overload protection remains functional even if the adjustment (ADJ) pin is accidentally disconnected.

Normally, no capacitors are needed unless the device is situated more than 6 inches from the input filter capacitors in which case an input bypass is needed. An output capacitor can be added to improve transient response, while bypassing the adjustment pin increases the ripple rejection of the regulator.

Besides replacing fixed regulators or discrete designs, the LM138 is useful in a wide variety of other applications. Because the regulator is floating and receives only the input-to-output differential voltage, supplies of several hundred volts can be regulated as long as the maximum input to output differential is not exceeded; do not short-circuit output to ground. The part numbers in the LM138 series which have a K suffix are packaged in a standard steel TO-CAN package, while those with a T suffix are packaged in a TO-220 plastic package. The LM138 is rated for TJ = –55°C to 150°C, and the LM338 is rated for TJ = 0°C to 125°C.

Device Information(1)

LM138 TO-CAN (2) 25.40 mm × 38.94 mm
LM338 TO-220 (3) 10.16 mm × 14.986 mm
TO-CAN (2) 25.40 mm × 38.94 mm
  1. For all available packages, see the orderable addendum at the end of the data sheet.

Revision History

Changes from B Revision (April 2013) to C Revision

  • Added Device Information table, Pin Configuration and Functions section, Specifications section, Recommended Operating Conditions table, Thermal Information table, Detailed Description section, Application and Implementation section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and Mechanical, Packaging, and Orderable Information sectionGo
  • Deleted RETS138K military specification reference from Absolute Maximum Ratings tableGo
  • Changed Junction to Ambient, RθJA, value in Thermal Information table From: 50°C/W To: 22.9°C/W (NDE)Go
  • Changed Junction to Case, RθJC(top), value in Thermal Information table From: 4°C/W To: 15.7°C/W (NDE)Go

Changes from A Revision (April 2013) to B Revision

  • Changed layout of National Semiconductor Data Sheet to TI formatGo