ZHCSYA6D May   1998  – May 2025 LM138 , LM338

PRODUCTION DATA  

  1.   1
  2. 1 特性
  3. 2 应用
  4. 3 说明
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 Recommended Operating Conditions
    3. 5.3 Thermal Information
    4. 5.4 Electrical Characteristics: LM138
    5. 5.5 Electrical Characteristics: LM338
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 NPN Darlington Output Drive
      2. 6.3.2 Overload Block
      3. 6.3.3 Programmable Feedback
    4. 6.4 Device Functional Modes
      1. 6.4.1 Normal Operation
      2. 6.4.2 Operation With Low Input Voltage
      3. 6.4.3 Operation at Light Loads
      4. 6.4.4 Operation in Self Protection
  8. 7 Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Applications
      1. 7.2.1 Constant 5-V Regulator
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
          1. 7.2.1.2.1 External Capacitors
          2. 7.2.1.2.2 Load Regulation
          3. 7.2.1.2.3 Protection Diodes
        3. 7.2.1.3 Application Curves
    3. 7.3 System Examples
    4. 7.4 Power Supply Recommendations
    5. 7.5 Layout
      1. 7.5.1 Layout Guidelines
      2. 7.5.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 接收文档更新通知
    2. 8.2 支持资源
    3. 8.3 Trademarks
    4. 8.4 静电放电警告
    5. 8.5 术语表
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

7.2.1.1 Design Requirements

R1: Because the LM138 produces a typical 1.24 V potential between the OUTPUT and ADJUST pins, placing a 270-Ω resistor between them causes 4.6 mA to flow through R1 and R2.

R2: To achieve a 5-V output, the sum of the voltages across R1 and R2 must equal 5 V. Therefore, Vr2 must equal 3.76 V when 4.6 mA is flowing through it. R2 = Vr2 / I = 3.76 V / 4.6 mA = ~820 Ω.

CIN: 0.1 µF of input capacitance helps filter out unwanted noise, especially if the regulator is located far from the power supply filter capacitors.

COUT: The regulator is stable without any output capacitance, but adding a 1-µF capacitor improves the transient response.

CADJ: A 10-µF capacitor bypassing the ADJUST pin to ground improves the regulators ripple rejection.

D1: Protection diode D1 is recommended if COUT is used. The diode provides a low-impedance discharge path to prevent the capacitor from discharging into the output of the regulator (see the Protection Diodes section).

D2: Protection diode D2 is recommended if CADJ is used. The diode provides a low-impedance discharge path to prevent the capacitor from discharging into the output of the regulator (see the Protection Diodes section).

Table 7-1 lists the design parameters for this typical application.

Table 7-1 Design Parameters
PARAMETER VALUE
Feedback resistor 1 (R1) 270 Ω
Feedback resistor 2 (R2) 820 Ω
Input capacitor (CIN) 0.1 µF
Output capacitor (COUT) 1 µF
Adjust capacitor(CADJ) 10 µF