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

7.4.1 Forced Pulse Width Modulation (FPWM) Mode

The Synchronous Rectifier section gives a brief introduction to the LM5161 diode emulation feature. The FPWM pin allows the power supply designer to select either CCM or DCM mode of operation at light loads. When the FPWM pin is connected to ground or left floating (FPWM = 0), a pulse-skipping mode and the zero-cross current detector circuit is enabled. The zero-cross detector turns off the low-side FET when the inductor current falls close to zero (IZX, seeElectrical Characteristics ). This feature allows the LM5161 regulator to operate in DCM mode at light loads. In the DCM state, the switching frequency decreases with lighter loads.

When the FPWM pin is left open or shorted to ground, the user can take the advantage of the internal ripple injection circuit, enabled in this mode, for a typical Buck application circuit. This feature is applicable over the entire load and input voltage ranges. It eliminates the need for an external feedback ripple injection circuit.

For wide VIN applications where VIN > 72 V, an external VCC supply is commonly used to minimize the power dissipation in the IC. In such applications at TJ>125°C, it is recommended to add a BST resistor (> 3Ω) in series with the BST capacitor, in order to protect the internal VCC-BST diode during a full load transient operation. The addition of the external resistor will reduce the fast (dv/dt) of the switch node that can impact the normal IC operation.

If the FPWM pin is pulled high, the LM5161 will operate in CCM mode regardless of the load conditions. The CCM operation reduces efficiency at light load but improves the output transient response to step load changes and provides nearly constant switching frequency. Moreover, the Fly-Buck topology always requires the continuous conduction mode during its operation.

The internal ripple injection circuit is disabled in the CCM mode. An external ripple injection circuit or an additional ESR resistor in series with the output capacitor is required to generate the optimal ripple at the FB node. Also, there is no need to add any BST resistor in series with the BST capacitor in either forced CCM Buck or Fly-Buck application.

Table 1. FPWM Pin Mode Summary

FPWM PIN CONNECTION LOGIC STAGE DESCRIPTION
GND or Floating (High Z) 0 The FPWM pin is grounded or left floating. DCM enabled at light loads. Internal Ripple circuit is enabled. No external ripple circuit/ addition required.
VCC 1 The FPWM pin is connected to VCC. The LM5161 then operates in CCM mode at light loads. Internal ripple injection disabled. External ripple injection needed.