ZHCSFE2A August   2016  – November 2017 LM5161-Q1

PRODUCTION DATA.  

  1. 1 特性
  2. 2 应用
  3. 3 说明
    1.     Device Images
      1.      典型降压应用电路
      2.      典型 Fly-Buck 应用电路
  4. 4 修订历史记录
  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
      12. 7.3.12 Ripple Configuration
    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-Q1 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-Q1 Isolated Fly-Buck (36-V to 72-V Input, 12-V, 12-W Isolated Output)
        1. 8.2.2.1 LM5161-Q1 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. 8.3 Do's and Don'ts
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11器件和文档支持
    1. 11.1 器件支持
      1. 11.1.1 开发支持
        1. 11.1.1.1 使用 WEBENCH® 工具创建定制设计
    2. 11.2 相关文档
    3. 11.3 商标
    4. 11.4 接收文档更新通知
    5. 11.5 社区资源
    6. 11.6 静电放电警告
    7. 11.7 Glossary
  12. 12机械、封装和可订购信息

封装选项

机械数据 (封装 | 引脚)
散热焊盘机械数据 (封装 | 引脚)
订购信息

8.2.1.2.3 Frequency Selection

The duty cycle required to maintain output regulation at the minimum input voltage restricts the maximum switching frequency of LM5161-Q1. The maximum value of the minimum forced OFF-time TOFF,min (max), limits the duty cycle and therefore the switching frequency. The maximum frequency that avoids output dropout at minimum input voltage can be calculated from Equation 10.

Equation 10. LM5161-Q1 eq07_snvsa03.gif

For this design example, the maximum frequency based on the minimum OFF-time limitation for TOFF,min(typical) = 170 ns is calculated to be FSW,max(@VIN,min) = 1.2 MHz. This value is above 1 MHz, the maximum possible operating frequency of the LM5161-Q1. Therefore, the minimum OFF-time parameter restricts the maximum achievable switching frequency calculation in this application.

At the maximum input voltage, the maximum switching frequency of LM5161-Q1 is restricted by the minimum ON-time, TON,min which limits the minimum duty cycle of the converter. The maximum frequency at maximum input voltage can be calculated using Equation 11.

Equation 11. LM5161-Q1 eq08_snvsa03.gif

Using Equation 11 and TON,min (typ) = 150 ns, the maximum achievable switching frequency is FSW,max(@VIN,min)= 1000 kHz. Taking this value as the maximum possible operational switching frequency over the input voltage range in this application, a nominal switching frequency of FSW = 300 kHz is chosen for this design.

The value of the resistor, RON sets the nominal switching frequency based on Equation 12.

Equation 12. LM5161-Q1 RON_VS_FSW_eq_R1_snvu504.gif

For this particular application with FSW = 300 kHz, RON calculates to be 396 kΩ . Selecting a standard value for R1 (RON) = 402 kΩ (±1%) results in a nominal frequency of 296 kHz. The resistor value may need to adjusted further in order to achieve the required switching frequency as the switching frequency in Constant ON-Time converters varies slightly(±10%) with input voltage and/or output current. Operation at a lower nominal switching frequency will result in higher efficiency but increase in the inductor and capacitor values leading to a larger total solution size.