ZHCSHR3B June   2017  – August 2020 LMR23615

PRODUCTION DATA  

  1. 1 特性
  2. 2 应用
  3. 3 说明
  4. 4 Revision History
  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 Timing Characteristics
    7. 6.7 Switching Characteristics
    8. 6.8 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  Fixed-Frequency, Peak-Current-Mode Control
      2. 7.3.2  Adjustable Frequency
      3. 7.3.3  Adjustable Output Voltage
      4. 7.3.4  Enable/Sync
      5. 7.3.5  VCC, UVLO
      6. 7.3.6  Minimum ON-Time, Minimum-OFF Time, and Frequency Foldback at Dropout Conditions
      7. 7.3.7  Internal Compensation and CFF
      8. 7.3.8  Bootstrap Voltage (BOOT)
      9. 7.3.9  Overcurrent and Short-Circuit Protection
      10. 7.3.10 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Active Mode
      3. 7.4.3 CCM Mode
      4. 7.4.4 Light Load Operation
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1  Custom Design With WEBENCH® Tools
        2. 8.2.2.2  Output Voltage Setpoint
        3. 8.2.2.3  Switching Frequency
        4. 8.2.2.4  Inductor Selection
        5. 8.2.2.5  Output Capacitor Selection
        6. 8.2.2.6  Feedforward Capacitor
        7. 8.2.2.7  Input Capacitor Selection
        8. 8.2.2.8  Bootstrap Capacitor Selection
        9. 8.2.2.9  VCC Capacitor Selection
        10. 8.2.2.10 Undervoltage Lockout Setpoint
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Compact Layout for EMI Reduction
      2. 10.1.2 Ground Plane and Thermal Considerations
      3. 10.1.3 Feedback Resistors
    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 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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7.3.6 Minimum ON-Time, Minimum-OFF Time, and Frequency Foldback at Dropout Conditions

Minimum ON-time, TON_MIN, is the smallest duration of time that the HS switch can be on. TON_MIN is typically 60 ns in the LMR23615. Minimum OFF-time, TOFF_MIN, is the smallest duration that the HS switch can be off. TOFF_MIN is typically 100 ns in the LMR23615. In CCM operation, TON_MIN and TOFF_MIN  limit the voltage conversion range given a selected switching frequency.

The minimum duty cycle allowed is:

Equation 3. DMIN = TON_MIN × fSW

And the maximum duty cycle allowed is:

Equation 4. DMAX = 1 – TOFF_MIN × fSW

Given fixed TON_MIN and TOFF_MIN, the higher the switching frequency the narrower the range of the allowed duty cycle. In the LMR23615 device, a frequency foldback scheme is employed to extend the maximum duty cycle when TOFF_MIN is reached. The switching frequency decreases once longer duty cycle is needed under low VIN conditions. Wide range of frequency foldback allows the LMR23615 output voltage stay in regulation with a much lower supply voltage VIN. This leads to a lower effective drop-out voltage.

Given an output voltage, the choice of the switching frequency affects the allowed input voltage range, solution size, and efficiency. The maximum operation supply voltage can be found by:

Equation 5. GUID-1A2ADA29-83BB-4C2D-9DA6-B85C5E42D4E5-low.gif

At lower supply voltage, the switching frequency decreases once TOFF_MIN is tripped. The minimum VIN without frequency foldback can be approximated by:

Equation 6. GUID-540428EC-B8DA-4677-86C8-031BD9D8221D-low.gif

Taking considerations of power losses in the system with heavy load operation, VIN_MAX is higher than the result calculated in Equation 5. With frequency foldback, VIN_MIN is lowered by decreased fSW.

GUID-0817DD3A-F0D7-415A-9590-DCD67D1F9DF8-low.gifFigure 7-8 Frequency Foldback at Dropout (VOUT = 5 V, fSW = 2100 kHz)