SNVSCL1 November   2023 LM25185

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Description (continued)
  6. 5 Pin Configuration and Functions
  7. 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
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Power MOSFET Gate Driver
      2. 7.3.2  PSR Flyback Modes of Operation
      3. 7.3.3  High Voltage VCC Regulator
      4. 7.3.4  Setting the Output Voltage
        1. 7.3.4.1 Diode Thermal Compensation
      5. 7.3.5  Control Loop Error Amplifier
      6. 7.3.6  Precision Enable
      7. 7.3.7  Configurable Soft Start
      8. 7.3.8  Minimum On-Time and Off-Time
      9. 7.3.9  Current Sensing and Overcurrent Protection
      10. 7.3.10 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Standby Mode
      3. 7.4.3 Active Mode
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Design 1: Wide VIN, Low IQ PSR Flyback Converter Rated at 16.4 V, 1 A
        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  Custom Design With Excel Quickstart Tool
          3. 8.2.1.2.3  Flyback Transformer T1 and Current-Sense Resistor (RCS)
          4. 8.2.1.2.4  Flyback Diode – DFLY
          5. 8.2.1.2.5  Leakage Inductance Clamp Circuit – DF, DCLAMP
          6. 8.2.1.2.6  Feedback Resistor – RFB
          7. 8.2.1.2.7  Thermal Compensation Resistor – RTC
          8. 8.2.1.2.8  UVLO Resistors – RUV1, RUV2
          9. 8.2.1.2.9  Soft-Start Capacitor – CSS
          10. 8.2.1.2.10 Compensation Components
        3. 8.2.1.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
      2. 8.4.2 Layout Examples
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Development Support
        1. 9.1.1.1 Custom Design With WEBENCH® Tools
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Support Resources
    5. 9.5 Trademarks
    6. 9.6 Electrostatic Discharge Caution
    7. 9.7 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information
8.2.1.2.3 Flyback Transformer T1 and Current-Sense Resistor (RCS)

The turns ratio of the transformer is selected such that the maximum duty cycle is smaller than 70%. While the maximum duty cycle can approach 80% if needing a particularly wide input voltage application, the maximum duty cycle increases the peak current stress of the secondary-side components. The turns ratio for this design is 1:1.

Equation 14. NPS<DMAX1-DMAX×VIN(min)VOUT+VD=0.71-0.7×20 V16.4 V+0.3 V=2.8

The magnetizing inductance is selected based on the switching frequency being 250 kHz for the nominal VIN at 24 V at full load 1 A. The 250 kHz is usually a good tradeoff for flyback design in terms of transformer size and overall efficiency, and so forth. Use Equation 1, Equation 4, and Equation 3 to calculate the required inductance. A value of 12 µH is chosen for this design. Use Equation 4 to calculate the primary peak current and the peak current with the selected 12-µH 1:1 transformer is 3.7 A. By giving a 15% margin, the peak current limit is around 4.3 A. Use Equation 15 to calculate the RCS. In this design, RCS is set to 20 mΩ. A small RC filter (100 Ω, 100 pF) is added to overcome the leading edge noise of the current sense signal.

Equation 15. RCS≤VCS-MAXIpk=100mV4.3A=23.3mΩ

Note that a higher magnetizing inductance provides a larger operating range for BCM and FFM, but the leakage inductance can increase based on a higher number of primary turns, NP. Equation 16 and Equation 17 give the primary and secondary winding RMS currents, respectively.

Equation 16. IPRI-RMS=D3×IPRI-PK
Equation 17. ISEC-RMS=2×IOUT×IPRI-PK×NPS3