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

8.2.3 Ripple Configuration

LM5161 uses a Constant-On-Time (COT) control scheme, in which the ON-time is terminated by a one-shot, and the OFF-time is terminated by the feedback voltage (VFB) falling below the reference voltage. Therefore, for stable operation, the feedback voltage must decrease monotonically and in phase with the inductor current during the OFF-time. Furthermore, this change in feedback voltage (VFB) during OFF-time must be large enough to dominate any noise present at the feedback node.

Table 5 presents three different methods for generating appropriate voltage ripple at the feedback node. Type 1 and Type 2 ripple circuits couple the ripple from the output of the converter to the feedback node (FB). The output voltage ripple has two components:

  1. Capacitive ripple caused by the inductor current ripple charging or discharging the output capacitor.
  2. Resistive ripple caused by the inductor current ripple flowing through the ESR of the output capacitor and R3.

The capacitive ripple is out-of-phase with the inductor current. As a result, the capacitive ripple does not decrease monotonically during the OFF-time. The resistive ripple is in phase with the inductor current and decreases monotonically during the OFF-time. The resistive ripple must exceed the capacitive ripple at output (VOUT) for stable operation. If this condition is not satisfied unstable switching behavior is observed in COT converters, with multiple ON-time bursts in close succession followed by a long OFF-time.

Type 3 ripple method uses a ripple injection circuit with RA, CA and the switch node (SW) voltage to generate a triangular ramp. This triangular ramp is then AC-coupled into the feedback node (FB) using the capacitor CB. Because this circuit does not use the output voltage ripple, it is suited for applications where low output voltage ripple is imperative. See application note Controlling Output Ripple and Achieving ESR Independence in Constant On-Time (COT) Regulator Designs for more details for each ripple generation method.

Table 5. Ripple Configuration

TYPE 1 TYPE 2 TYPE 3
Lowest Cost Reduced Ripple Minimum Ripple
LM5161 esr_ripple_snvsae3.gif
LM5161 reduced_rippleinj_snvsae3.gif
LM5161 ripple_inj_snvsae3.gif
Equation 22. LM5161 eq22_low_cost_snvsa03.gif
Equation 23. LM5161 eq23_reduce_ripple_snvsa03.gif
Equation 24. LM5161 eq24_snvsae3.gif