ZHCSD62F December   2012  – May 2021 LM25017

PRODUCTION DATA  

  1. 1 特性
  2. 2 应用
  3. 3 说明
  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 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 Overview
      2. 7.3.2  VCC Regulator
      3. 7.3.3  Regulation Comparator
      4. 7.3.4  Overvoltage Comparator
      5. 7.3.5  On-Time Generator
      6. 7.3.6  Current Limit
      7. 7.3.7  N-Channel Buck Switch and Driver
      8. 7.3.8  Synchronous Rectifier
      9. 7.3.9  Undervoltage Detector
      10. 7.3.10 Thermal Protection
      11. 7.3.11 Ripple Configuration
      12. 7.3.12 Soft-Start
    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 Application Circuit: 12.5-V to 48-V Input and 10-V, 650-mA Output Buck 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 RFB1, RFB2
          3. 8.2.1.2.3 Frequency Selection
          4. 8.2.1.2.4 Inductor Selection
          5. 8.2.1.2.5 Output Capacitor
          6. 8.2.1.2.6 Type III Ripple Circuit
          7. 8.2.1.2.7 VCC and Bootstrap Capacitor
          8. 8.2.1.2.8 Input Capacitor
          9. 8.2.1.2.9 UVLO Resistors
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Typical Isolated DC-DC Converter Using LM25017
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
          1. 8.2.2.2.1  Transformer Turns Ratio
          2. 8.2.2.2.2  Total IOUT
          3. 8.2.2.2.3  RFB1, RFB2
          4. 8.2.2.2.4  Frequency Selection
          5. 8.2.2.2.5  Transformer Selection
          6. 8.2.2.2.6  Primary Output Capacitor
          7. 8.2.2.2.7  Secondary Output Capacitor
          8. 8.2.2.2.8  Type III Feedback Ripple Circuit
          9. 8.2.2.2.9  Secondary Diode
          10. 8.2.2.2.10 VCC and Bootstrap Capacitor
          11. 8.2.2.2.11 Input Capacitor
          12. 8.2.2.2.12 UVLO Resistors
          13. 8.2.2.2.13 VCC Diode
        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. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. 11.1.1.1 Custom Design With WEBENCH® Tools
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 接收文档更新通知
    4. 11.4 支持资源
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 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

7.3.12 Soft-Start

A soft-start feature can be implemented with the LM25017 device using an external circuit. As shown in Figure 7-3, the soft-start circuit consists of one capacitor, C1, two resistors, R1 and R2, and a diode, D. During the initial start-up, the VCC voltage is established prior to the VOUT voltage. Capacitor C1 is discharged and diode D is thereby forward biased to pull up the FB pin voltage. The FB voltage exceeds the reference voltage (1.225 V) and switching is therefore disabled. As capacitor C1 charges, the voltage at node B gradually decreases and switching commences. VOUT will gradually rise to maintain the FB voltage at the reference voltage. Once the voltage at node B is less than a diode drop above the FB voltage, the soft-start sequence is finished and D is reverse biased.

During the initial part of the start-up, the FB voltage can be approximated as shown in Equation 5.

Equation 5. GUID-2231CDBF-569C-4CB1-B4A6-6693596FAAAF-low.gif

C1 is charged after the first start up. Diode D1 is optional and can be added to discharge C1 when the input voltage experiences a momentary drop to initialize the soft-start sequence.

To achieve the desired soft start, the following design guidance is recommended:

  • R2 is selected so that VFB is higher than 1.225 V for a VCC of 4.5 V, but is lower than 5 V when VCC is 8.55 V. If an external VCC is used, VFB should not exceed 5 V at maximum VCC.
  • C1 is selected to achieve the desired start-up time which can be determined from Equation 6.
    Equation 6. GUID-94F93EF7-18EB-4E95-9516-DF530B8A3CA8-low.gif
  • R1 is used to maintain the node B voltage at zero after the soft start is finished. A value larger than the feedback resistor divider is preferred. Note that the effect of resistor R1 is ignored in Equation 5.

Based on the schematic shown in Figure 7-3, selecting C1 = 1 uF, R2 = 1 kΩ, R1 = 30 kΩ results in a soft-start time of about 2 ms.

GUID-211B07F1-86EB-4FE6-97CF-5B407494CD0C-low.gifFigure 7-3 Soft-Start Circuit