ZHCS576I January   2011  – August 2021 LMZ14201H

PRODUCTION DATA  

  1. 1 特性
  2. 2 应用
  3. 3 描述
  4. 4 Revision History
  5. 5 Pin Configuration and 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 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
      1. 7.1.1 COT Control Circuit Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Output Overvoltage Comparator
      2. 7.3.2 Current Limit
      3. 7.3.3 Thermal Protection
      4. 7.3.4 Zero Coil Current Detection
      5. 7.3.5 Prebiased Start-Up
    4. 7.4 Device Functional Modes
      1. 7.4.1 Discontinuous Conduction and Continuous Conduction Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Design Steps for the LMZ14201H Application
          1. 8.2.2.1.1 Enable Divider, RENT and RENB Selection
          2. 8.2.2.1.2 Output Voltage Selection
          3. 8.2.2.1.3 Soft-Start Capacitor, CSS, Selection
          4. 8.2.2.1.4 Output Capacitor, CO, Selection
            1. 8.2.2.1.4.1 Capacitance
            2. 8.2.2.1.4.2 ESR
          5. 8.2.2.1.5 Input Capacitor, CIN, Selection
          6. 8.2.2.1.6 ON-Time, RON, Resistor Selection
            1. 8.2.2.1.6.1 Discontinuous Conduction and Continuous Conduction Mode Selection
      3. 8.2.3 Application Curve
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Power Module SMT Guidelines
    2. 10.2 Layout Example
      1. 10.2.1 Power Dissipation and Board Thermal Requirements
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 接收文档更新通知
    3. 11.3 支持资源
    4. 11.4 Trademarks
    5. 11.5 静电放电警告
    6. 11.6 术语表
  12. 12Mechanical, Packaging, and Orderable Information

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机械数据 (封装 | 引脚)
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订购信息
8.2.2.1.6 ON-Time, RON, Resistor Selection

Many designs will begin with a desired switching frequency in mind. As seen in the Typical Characteristics section, the best efficiency is achieved in the 300 kHz to 400 kHz switching frequency range. Equation 12 can be used to calculate the RON value.

Equation 12. fSW(CCM) ≊ VO / (1.3 × 10-10 x RON)

This can be rearranged as

Equation 13. RON ≊ VO / (1.3 × 10-10 x fSW(CCM)

The selection of RON and fSW(CCM) must be confined by limitations in the ON-time and OFF-time for the COT Control Circuit Overview section.

The ON-time of the LMZ14201H timer is determined by the resistor RON and the input voltage VIN. It is calculated as follows:

Equation 14. tON = (1.3 × 10-10 × RON) / VIN

The inverse relationship of tON and VIN gives a nearly constant switching frequency as VIN is varied. RON should be selected such that the ON-time at maximum VIN is greater than 150 ns. The ON-timer has a limiter to ensure a minimum of 150 ns for tON. This limits the maximum operating frequency, which is governed by Equation 15:

Equation 15. fSW(MAX) = VO / (VIN(MAX) × 150 nsec)

This equation can be used to select RON if a certain operating frequency is desired so long as the minimum ON-time of 150 ns is observed. The limit for RON can be calculated as follows:

Equation 16. RON ≥ VIN(MAX) × 150 nsec / (1.3 × 10-10)

If RON calculated in Equation 13 is less than the minimum value determined in Equation 16 a lower frequency should be selected. Alternatively, VIN(MAX) can also be limited in order to keep the frequency unchanged.

Additionally, the minimum OFF-time of 260 ns (typical) limits the maximum duty ratio. Larger RON (lower FSW) should be selected in any application requiring large duty ratio.
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