ZHCSA29F October   2011  – June 2019 LMR24220

PRODUCTION DATA.  

  1. 1 特性
  2. 2 应用
  3. 3 说明
    1.     Device Images
      1.      典型应用
  4. 4 修订历史记录
  5. 5 Pin Configuration and Functions
    1.     Pin Descriptions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 Recommended Operating Ratings
    3. 6.3 Electrical Characteristics
    4. 6.4 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  CoT Control Circuit Overview
      2. 7.3.2  Start-up Regulator (VCC)
      3. 7.3.3  Regulation Comparator
      4. 7.3.4  Zero Coil Current Detect
      5. 7.3.5  Overvoltage Comparator
      6. 7.3.6  On-Time Timer, Shutdown
      7. 7.3.7  Current Limit
      8. 7.3.8  N-Channel MOSFET and Driver
      9. 7.3.9  Soft Start
      10. 7.3.10 Thermal Protection
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Detailed Design Procedure
        1. 8.2.1.1 Custom Design With WEBENCH® Tools
        2. 8.2.1.2 External Components
      2. 8.2.2 Application Curve
  9. 9 Layout
    1. 9.1 Layout Guidelines
    2. 9.2 Package Considerations
    3. 9.3 Thermal Derating
  10. 10器件和文档支持
    1. 10.1 器件支持
      1. 10.1.1 开发支持
        1. 10.1.1.1 使用 WEBENCH® 工具创建定制设计
    2. 10.2 接收文档更新通知
    3. 10.3 社区资源
    4. 10.4 商标
    5. 10.5 静电放电警告
    6. 10.6 Glossary
  11. 11机械、封装和可订购信息

封装选项

机械数据 (封装 | 引脚)
散热焊盘机械数据 (封装 | 引脚)
订购信息

7.3.9 Soft Start

The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing start-up stresses and current surges. Upon turnon, after VCC reaches the undervoltage threshold, an 8 µA internal current source charges up an external capacitor CSS connecting to the SS pin. The ramping voltage at the SS pin (and the non-inverting input of the regulation comparator as well) ramps up the output voltage VOUT in a controlled manner.

The soft start time duration to reach steady-state operation is given by Equation 7:

Equation 7. tSS = VREF × CSS / 8µA = 0.8 V × CSS / 8 µA

This equation can be rearranged as follows:

Equation 8. CSS= tSS × 8 µA / 0.8 V

Use of a 4.7-nF capacitor results in a 0.5-ms soft-start duration. This is a recommended value. Note that high values of CSS capacitance cause more output voltage drop when a load transient goes across the DCM-CCM boundary. If a fast load transient response is desired for steps between DCM and CCM mode the softstart capacitor value should be less than 18 nF (which corresponds to a soft-start time of 1.8 ms).

An internal switch grounds the SS pin if any of the following three cases happens: (i) VCC is below the UVLO threshold; (ii) a thermal shutdown occurs; or (iii) the EN pin is grounded. Alternatively, the output voltage can be shut off by connecting the SS pin to ground using an external switch. Releasing the switch allows the SS pin to ramp up and the output voltage to return to normal. The shutdown configuration is shown in Figure 19.

LMR24220 30167627.pngFigure 19. Alternate Shutdown Implementation