JAJSE93B March   2016  – November 2017 LM5161

PRODUCTION DATA.  

  1. 1 特長
  2. 2 アプリケーション
  3. 3 概要
    1.     Device Images
      1.      代表的な降圧アプリケーション回路
      2.      代表的なFly-Buckアプリケーション回路
  4. 4 改訂履歴
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Switching Characteristics
    7. 6.7 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  Control Circuit
      2. 7.3.2  VCC Regulator
      3. 7.3.3  Regulation Comparator
      4. 7.3.4  Soft-Start
      5. 7.3.5  Error Transconductance (GM) Amplifier
      6. 7.3.6  On-Time Generator
      7. 7.3.7  Current Limit
      8. 7.3.8  N-Channel Buck Switch and Driver
      9. 7.3.9  Synchronous Rectifier
      10. 7.3.10 Enable / Undervoltage Lockout (EN/UVLO)
      11. 7.3.11 Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Forced Pulse Width Modulation (FPWM) Mode
      2. 7.4.2 Undervoltage Detector
  8. 8 Applications and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 LM5161 Synchronous Buck (15-V to 95-V Input, 12-V Output, 1-A Load)
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1  Custom Design With WEBENCH® Tools
          2. 8.2.1.2.2  Output Resistor Divider Selection
          3. 8.2.1.2.3  Frequency Selection
          4. 8.2.1.2.4  Inductor Selection
          5. 8.2.1.2.5  Output Capacitor Selection
          6. 8.2.1.2.6  Series Ripple Resistor - RESR (FPWM = 1)
          7. 8.2.1.2.7  VCC and Bootstrap Capacitor
          8. 8.2.1.2.8  Input Capacitor Selection
          9. 8.2.1.2.9  Soft-Start Capacitor Selection
          10. 8.2.1.2.10 EN/UVLO Resistor Selection
        3. 8.2.1.3 Application Curves
      2. 8.2.2 LM5161 Isolated Fly-Buck (36-V to 72-V Input, 12-V, 12-W Isolated Output)
        1. 8.2.2.1 LM5161 Fly-Buck Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
          1. 8.2.2.2.1 Selection of VOUT and Turns Ratio
          2. 8.2.2.2.2 Secondary Rectifier Diode
          3. 8.2.2.2.3 External Ripple Circuit
          4. 8.2.2.2.4 Output Capacitor (CVISO)
        3. 8.2.2.3 Application Curves
      3. 8.2.3 Ripple Configuration
    3. 8.3 Do's and Don'ts
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11デバイスおよびドキュメントのサポート
    1. 11.1 WEBENCH®ツールによるカスタム設計
    2. 11.2 関連資料
    3. 11.3 ドキュメントの更新通知を受け取る方法
    4. 11.4 コミュニティ・リソース
    5. 11.5 商標
    6. 11.6 静電気放電に関する注意事項
    7. 11.7 Glossary
  12. 12メカニカル、パッケージ、および注文情報

8.2.1.2.4 Inductor Selection

The inductor is selected to limit the inductor ripple current to a value between 20 and 40 percent of the maximum load current. The minimum value of the inductor required in this application can be calculated from Equation 10:

Equation 10. LM5161 eq10_snvsa03.gif

Based on Equation 10 , the minimum value of the inductor is calculated to be 85 µH for VIN = 80-V (max) and inductor current ripple will be 40 percent of the maximum load current. Allowing some margin for inductance variation and inductor saturation, a higher standard value of L1 (L) = 100 µH is selected for this design.

The peak inductor current at maximum load must be smaller than the minimum current limit threshold of the high side FET as given inElectrical Characteristics table. The inductor current ripple at any input voltage is given by:

Equation 11. LM5161 eq11_snvsa03.gif

The peak-to-peak inductor current ripple is calculated to be 81 mA and 341 mA at the minimum and maximum input voltages respectively. The maximum peak inductor current in the buck FET is given by Equation 12:

Equation 12. LM5161 eq12_snvsa03.gif

In this design with maximum output current of 1-A, the maximum peak inductor current is calculated to be approximately 1.17 A at VIN,max = 80 V, which is less than the minimum high-side FET current limit threshold.

The saturation current of the inductor must also be carefully considered. The peak value of the inductor current will be bound by the high side FET current limit during overload or short circuit conditions. Based on the high side FET current limit specification in theElectrical Characteristics, an inductor with saturation current rating above 1.9 A (max) should be selected.