ZHCSI45 April   2018 LM3478Q-Q1

PRODUCTION DATA.  

  1. 1 特性
  2. 2 应用
  3. 3 说明
    1.     Device Images
      1.      典型的高效升压转换器
  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 - LM3478Q-Q1
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 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 Overvoltage Protection
      2. 7.3.2 Slope Compensation Ramp
      3. 7.3.3 Frequency Adjust/Shutdown
      4. 7.3.4 Short-Circuit Protection
    4. 7.4 Device Functional Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Typical High Efficiency Step-Up (Boost) Converter
        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  Power Inductor Selection
          3. 8.2.1.2.3  Programming the Output Voltage
          4. 8.2.1.2.4  Setting the Current Limit
          5. 8.2.1.2.5  Current Limit with External Slope Compensation
          6. 8.2.1.2.6  Power Diode Selection
          7. 8.2.1.2.7  Power MOSFET Selection
          8. 8.2.1.2.8  Input Capacitor Selection
          9. 8.2.1.2.9  Output Capacitor Selection
          10. 8.2.1.2.10 Compensation
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Typical SEPIC Converter
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
          1. 8.2.2.2.1 Power MOSFET Selection
          2. 8.2.2.2.2 Power Diode Selection
          3. 8.2.2.2.3 Selection of Inductors L1 and L2
          4. 8.2.2.2.4 Sense Resistor Selection
          5. 8.2.2.2.5 Sepic Capacitor Selection
          6. 8.2.2.2.6 Input Capacitor Selection
          7. 8.2.2.2.7 Output Capacitor Selection
        3. 8.2.2.3 Application Curves
  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 使用 WEBENCH 工具创建定制设计
    2. 11.2 接收文档更新通知
    3. 11.3 文档支持
      1. 11.3.1 相关文档
    4. 11.4 商标
    5. 11.5 静电放电警告
    6. 11.6 术语表
  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

8.2.1.2.7 Power MOSFET Selection

The drive pin of the LM3478Q-Q1 must be connected to the gate of an external MOSFET. The drive pin (DR) voltage depends on the input voltage (see Typical Characteristics). In most applications, a logic level MOSFET can be used. For very low input voltages, a sub logic level MOSFET should be used. The selected MOSFET has a great influence on the system efficiency. The critical parameters for selecting a MOSFET are:

  1. Minimum threshold voltage, VTH(MIN)
  2. On-resistance, RDS(ON)
  3. Total gate charge, Qg
  4. Reverse transfer capacitance, CRSS
  5. Maximum drain to source voltage, VDS(MAX)

The off-state voltage of the MOSFET is approximately equal to the output voltage. Vds(max) must be greater than the output voltage. The power losses in the MOSFET can be categorized into conduction losses and switching losses. RDS(ON) is needed to estimate the conduction losses, Pcond:

Equation 28. Pcond = I2 x RDS(ON) x D

The temperature effect on the RDS(ON) usually is quite significant. Assume 30% increase at hot.

For the current I in Equation 28 the average inductor current may be used.

Especially at high switching frequencies the switching losses may be the largest portion of the total losses.

The switching losses are very difficult to calculate due to changing parasitics of a given MOSFET in operation. Often the individual MOSFET's data sheet does not give enough information to yield a useful result. Equation 29 and Equation 30 give a rough idea how the switching losses are calculated:

Equation 29. LM3478Q-Q1 10135508.gif
Equation 30. LM3478Q-Q1 10135510.gif