SNVSBM6A April   2020  – January 2021 LM5181

PRODUCTION DATA  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Description (continued)
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Integrated Power MOSFET
      2. 8.3.2  PSR Flyback Modes of Operation
      3. 8.3.3  Setting the Output Voltage
        1. 8.3.3.1 Diode Thermal Compensation
      4. 8.3.4  Control Loop Error Amplifier
      5. 8.3.5  Precision Enable
      6. 8.3.6  Configurable Soft Start
      7. 8.3.7  External Bias Supply
      8. 8.3.8  Minimum On-Time and Off-Time
      9. 8.3.9  Overcurrent Protection
      10. 8.3.10 Thermal Shutdown
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Standby Mode
      3. 8.4.3 Active Mode
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Design 1: Wide VIN, Low IQ PSR Flyback Converter Rated at 5 V, 0.5 A
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
          1. 9.2.1.2.1  Custom Design With WEBENCH® Tools
          2. 9.2.1.2.2  Custom Design With Excel Quickstart Tool
          3. 9.2.1.2.3  Flyback Transformer – T1
          4. 9.2.1.2.4  Flyback Diode – DFLY
          5. 9.2.1.2.5  Zener Clamp Circuit – DF, DCLAMP
          6. 9.2.1.2.6  Output Capacitor – COUT
          7. 9.2.1.2.7  Input Capacitor – CIN
          8. 9.2.1.2.8  Feedback Resistor – RFB
          9. 9.2.1.2.9  Thermal Compensation Resistor – RTC
          10. 9.2.1.2.10 UVLO Resistors – RUV1, RUV2
          11. 9.2.1.2.11 Soft-Start Capacitor – CSS
      2. 9.2.2 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Examples
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
      2. 12.1.2 Development Support
        1. 12.1.2.1 Custom Design With WEBENCH® Tools
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Receiving Notification of Documentation Updates
    4. 12.4 Support Resources
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

PSR Flyback Modes of Operation

The LM5181 uses a variable-frequency, peak current-mode (VFPCM) control architecture with three possible modes of operation as illustrated in Figure 8-2.

GUID-DEBC16C9-97ED-4B46-B450-F5D22D799126-low.gifFigure 8-2 Three Modes of Operation Illustrated by Variation of Switching Frequency With Load

The LM5181 operates in boundary conduction mode (BCM) at heavy loads. The power MOSFET turns on when the current in the secondary winding reaches zero, and the MOSFET turns off when the peak primary current reaches the level dictated by the output of the internal error amplifier. As the load is decreased, the frequency increases to maintain BCM operation. The duty cycle of the flyback converter is given Equation 1.

Equation 1. GUID-73CADACE-ACB5-4F35-B6D0-98C81031B4B2-low.gif

where

  • VD is the forward voltage drop of the flyback diode as its current approaches zero

The output power in BCM is given by Equation 2, where the applicable switching frequency and peak primary current in BCM are specified by Equation 3 and Equation 4, respectively.

Equation 2. GUID-4F591685-015E-4F26-9EF1-42745663579A-low.gif
Equation 3. GUID-7502874D-A836-4A24-BC92-6A5573712FF4-low.gif
Equation 4. GUID-36E7FDD7-5649-4ADE-9647-610BD00128C1-low.gif

As the load decreases, the LM5181 clamps the maximum switching frequency to 350 kHz, and the converter enters discontinuous conduction mode (DCM). The power delivered to the output in DCM is proportional to the peak primary current squared as given by Equation 5 and Equation 6. Thus, as the load decreases, the peak current reduces to maintain regulation at 350-kHz switching frequency.

Equation 5. GUID-66AAE628-7EDC-47A8-BB1F-32475193E510-low.gif
Equation 6. GUID-2242E51F-ED32-431F-9999-98813715643D-low.gif
Equation 7. GUID-2EEFA598-04D2-4195-B1C9-8EBFCE46C422-low.gif

At even lighter loads, the primary-side peak current set by the internal error amplifier decreases to a minimum level of 0.15 A, or 20% of its 0.75-A peak value, and the MOSFET off-time extends to maintain the output load requirement. The system operates in frequency foldback mode (FFM), and the switching frequency decreases as the load current is reduced. Other than a fault condition, the lowest frequency of operation of the LM5181 is 12 kHz, which sets a minimum load requirement of approximately 0.5% full load.