ZHCSI00D April   2018  – September 2020 LMR36015

PRODUCTION DATA  

  1. 特性
  2. 应用
  3. 说明
  4. Revision History
  5. 说明(续)
  6. Device Comparison Table
  7. Pin Configuration and Functions
    1.     Pin Functions
  8. Specifications
    1. 8.1 Absolute Maximum Ratings
    2. 8.2 ESD Ratings
    3. 8.3 Recommended Operating Conditions
    4. 8.4 Thermal Information
    5. 8.5 Electrical Characteristics
    6. 8.6 Timing Requirements
    7. 8.7 System Characteristics
    8. 8.8 Typical Characteristics
  9. Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Power-Good Flag Output
      2. 9.3.2 Enable and Start-up
      3. 9.3.3 Current Limit and Short Circuit
      4. 9.3.4 Undervoltage Lockout and Thermal Shutdown
    4. 9.4 Device Functional Modes
      1. 9.4.1 Auto Mode
      2. 9.4.2 Forced PWM Operation
      3. 9.4.3 Dropout
      4. 9.4.4 Minimum Switch On-Time
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design 1: Low Power 24-V, 1.5-A PFM Converter
        1. 10.2.1.1 Design Requirements
        2. 10.2.1.2 Detailed Design Procedure
          1. 10.2.1.2.1  Custom Design With WEBENCH Tools
          2. 10.2.1.2.2  Choosing the Switching Frequency
          3. 10.2.1.2.3  Setting the Output Voltage
          4. 10.2.1.2.4  Inductor Selection
          5. 10.2.1.2.5  Output Capacitor Selection
          6. 10.2.1.2.6  Input Capacitor Selection
          7. 10.2.1.2.7  CBOOT
          8. 10.2.1.2.8  VCC
          9. 10.2.1.2.9  CFF Selection
            1. 10.2.1.2.9.1 External UVLO
          10. 10.2.1.2.10 Maximum Ambient Temperature
      2. 10.2.2 Application Curves
      3. 10.2.3 Design 2: High Density 24-V, 1.5-A FPWM Converter
        1. 10.2.3.1 Design Requirements
        2. 10.2.3.2 Detailed Design Procedure
        3. 10.2.3.3 Application Curves
    3. 10.3 What to Do and What Not to Do
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
      1. 12.1.1 Ground and Thermal Considerations
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Device Support
      1. 13.1.1 Development Support
        1. 13.1.1.1 Custom Design With WEBENCH® Tools
    2. 13.2 Documentation Support
      1. 13.2.1 Related Documentation
    3. 13.3 接收文档更新通知
    4. 13.4 支持资源
    5. 13.5 Trademarks
    6. 13.6 静电放电警告
    7. 13.7 术语表
  14. 14Mechanical, Packaging, and Orderable Information

Current Limit and Short Circuit

The LMR36015 incorporates valley current limit for normal overloads and for short-circuit protection. In addition, the high-side power MOSFET is protected from excessive current by a peak current limit circuit. Cycle-by-cycle current limit is used for overloads, while hiccup mode is used for short circuits. Finally, a zero current detector is used on the low-side power MOSFET to implement diode emulation mode (DEM) at light loads (see Section 13.7).

During overloads, the low-side current limit, ILIMIT, determines the maximum load current that the LMR36015 can supply. When the low-side switch turns on, the inductor current begins to ramp down. If the current does not fall below ILIMIT before the next turnon cycle, then that cycle is skipped, and the low-side MOSFET is left on until the current falls below ILIMIT. This is somewhat different than the more typical peak current limit and results in Equation 1 for the maximum load current.

Equation 1. GUID-3309F119-618F-42BD-BDB8-2AB7C7833B2A-low.gif

where

  • fSW = switching frequency
  • L = inductor value

If, during current limit, the voltage on the FB input falls below about 0.4 V due to a short circuit, the device enters hiccup mode. In this mode, the device stops switching for tHC or about 94 ms, and then goes through a normal re-start with soft start. If the short-circuit condition remains, the device runs in current limit for about 20 ms (typical) and then shuts down again. This cycle repeats, as shown in Figure 9-6 as long as the short-circuit condition persists. This mode of operation helps to reduce the temperature rise of the device during a hard short on the output. Of course, the output current is greatly reduced during hiccup mode. Once the output short is removed and the hiccup delay is passed, the output voltage recovers normally as shown in Figure 9-6.

The high-side-current limit trips when the peak inductor current reaches ISC. This is a cycle-by-cycle current limit and does not produce any frequency or load current foldback. It is meant to protect the high-side MOSFET from excessive current. Under some conditions, such as high input voltages, this current limit can trip before the low-side protection. Under this condition, ISC determines the maximum output current. Note that ISC varies with duty cycle.

GUID-02F9D67F-529A-4843-83ED-8EDA6D653A05-low.pngFigure 9-6 Short-Circuit Transient and Recovery