JAJSHZ4A September   2019  – February 2020 LMR36520

PRODUCTION DATA.  

  1. 1 特長
  2. 2 アプリケーション
  3. 3 概要
    1.     Device Images
      1.      効率と出力電流との関係 VOUT = 5V、400kHz
      2.      概略回路図
  4. 4 改訂履歴
  5. 5 Description
  6. 6 Device Comparison Table
  7. 7 Pin Configuration and Functions
    1.     Pin Functions
  8. 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 Switching Characteristics
    8. 8.8 System Characteristics
    9. 8.9 Typical Characteristics
  9. 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, 2-A Buck Converter
        1. 10.2.1.1 Design Requirements
        2. 10.2.1.2 Detailed Design Procedure
          1. 10.2.1.2.1  Choosing the Switching Frequency
          2. 10.2.1.2.2  Setting the Output Voltage
          3. 10.2.1.2.3  Inductor Selection
          4. 10.2.1.2.4  Output Capacitor Selection
          5. 10.2.1.2.5  Input Capacitor Selection
          6. 10.2.1.2.6  CBOOT
          7. 10.2.1.2.7  VCC
          8. 10.2.1.2.8  CFF Selection
          9. 10.2.1.2.9  External UVLO
          10. 10.2.1.2.10 Maximum Ambient Temperature
      2. 10.2.2 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. 13デバイスおよびドキュメントのサポート
    1. 13.1 デバイス・サポート
      1. 13.1.1 開発サポート
    2. 13.2 ドキュメントのサポート
      1. 13.2.1 関連資料
    3. 13.3 ドキュメントの更新通知を受け取る方法
    4. 13.4 サポート・リソース
    5. 13.5 商標
    6. 13.6 静電気放電に関する注意事項
    7. 13.7 Glossary
  14. 14メカニカル、パッケージ、および注文情報

パッケージ・オプション

メカニカル・データ（パッケージ｜ピン）
サーマルパッド・メカニカル・データ
発注情報

9.3.3 Current Limit and Short Circuit

The LMR36520 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 at light loads (see the Glossary).

During overloads, the low-side current limit, ILIMIT, determines the maximum load current that the LMR36520 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. LMR36520 Ilim3_eq3.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 into 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 long as the short-circuit condition persists. This mode of operation reduces 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.

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.

LMR36520 Short_Applied_Removed.gifFigure 7. Short-Circuit Transient and Recovery
LMR36520 Short_Inductor_Current.gifFigure 8. Inductor Current Burst in Short Circuit Mode