SLUSEV2C June   2022  – March 2023 UCC28C50-Q1 , UCC28C51-Q1 , UCC28C52-Q1 , UCC28C53-Q1 , UCC28C54-Q1 , UCC28C55-Q1 , UCC28C56H-Q1 , UCC28C56L-Q1 , UCC28C57H-Q1 , UCC28C57L-Q1 , UCC28C58-Q1 , UCC28C59-Q1

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 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. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Detailed Pin Description
        1. 8.3.1.1 COMP
        2. 8.3.1.2 FB
        3. 8.3.1.3 CS
        4. 8.3.1.4 RT/CT
        5. 8.3.1.5 GND
        6. 8.3.1.6 OUT
        7. 8.3.1.7 VDD
        8. 8.3.1.8 VREF
      2. 8.3.2  Undervoltage Lockout
      3. 8.3.3  ±1% Internal Reference Voltage
      4. 8.3.4  Current Sense and Overcurrent Limit
      5. 8.3.5  Reduced-Discharge Current Variation
      6. 8.3.6  Oscillator Synchronization
      7. 8.3.7  Soft Start
      8. 8.3.8  Enable and Disable
      9. 8.3.9  Slope Compensation
      10. 8.3.10 Voltage Mode
    4. 8.4 Device Functional Modes
      1. 8.4.1 Normal Operation
      2. 8.4.2 UVLO Mode
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1  Primary-to-Secondary Turns Ratio of the Flyback Transformer (NPS)
        2. 9.2.2.2  Primary Magnetizing Inductance of the Flyback Transformer (LM)
        3. 9.2.2.3  Number of Turns of the Flyback Transformer Windings
        4. 9.2.2.4  Current Sense Resistors (R24, R25) and Current Limiting
        5. 9.2.2.5  Primary Clamp Circuit (D7, D1, D3, R2, R28) to Limit Voltage Stress
        6. 9.2.2.6  Primary-Side Current Stress and Input Capacitor Selection
        7. 9.2.2.7  Secondary-Side Current Stress and Output Capacitor Selection
        8. 9.2.2.8  VDD Capacitors (C12, C18)
        9. 9.2.2.9  Gate Drive Network (R14, R16, Q6)
        10. 9.2.2.10 VREF Capacitor (C18)
        11. 9.2.2.11 RT/CT Components (R12, C15)
        12. 9.2.2.12 HV Start-Up Circuitry for VDD (Q1, Q2, D2, D4, D6, D8, R5)
        13. 9.2.2.13 Desensitization to CS-pin Noise by RC Filtering, Leading-Edge Blanking, and Slope Compensation
        14. 9.2.2.14 Voltage Feedback Compensation
          1. 9.2.2.14.1 Power Stage Gain, Poles, and Zeroes
          2. 9.2.2.14.2 Compensation Components
          3. 9.2.2.14.3 Bode Plots and Stability Margins
          4. 9.2.2.14.4 Stability Measurements
      3. 9.2.3 Application Curves
    3. 9.3 PCB Layout Recommendations
      1. 9.3.1 PCB Layout Routing Examples
    4. 9.4 Power Supply Recommendations
  10. 10Device and Documentation Support
    1. 10.1 Device Support
      1. 10.1.1 Development Support
    2. 10.2 Documentation Support
      1. 10.2.1 Related Documentation
    3. 10.3 Related Links
    4. 10.4 Support Resources
    5. 10.5 Trademarks
    6. 10.6 Electrostatic Discharge Caution
    7. 10.7 Glossary
  11. 11Mechanical, Packaging, and Orderable Information

Package Options

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

9.2.2.7 Secondary-Side Current Stress and Output Capacitor Selection

Similar to the input capacitors, the output capacitors must limit the voltage ripple and support an RMS current. However, for DCM operation, the high peak secondary current results in a relatively substantial RMS current in the output capacitors, usually requiring multiple capacitors in parallel.

First, estimate the maximum ESR of the output capacitors (RESR_MAX) based on the output ripple requirement and highest secondary peak current at full load (ISEC_PEAK). When an electrolytic capacitor is used, the output ripple magnitude is mainly determined by the ESR ripple. Paralleling the two output capacitors, C9 and C10, reduces the total ESR less than RESR_MAX.

Equation 22. ISEC_PEAK=NPS2×40 WLM×fSW×η=20.5 A
Equation 23. RESR_MAX=VOUT_RIPPLEISEC_PEAK=0.5 V20.5 A=24 mΩ

Next, calculate the minimum required output capacitance to meet the output voltage ripple requirement assuming full-load and 90% of RESR_MAX

Equation 24. COUT≥IOUT×1-DVINVOUT_RIPPLE-ISEC_PEAK×90%×RESR_MAX×fSW=1196 µF

Finally, calculate the RMS current the output capacitors must withstand at full load (ICOUT_RMS), considering the demagnetizing time (i.e. duty cycle (DDEMAG)) during DCM operation. Note that paralleling two or more output capacitors, C9 and C10, shares the total RMS current and also reduces the power loss contributed by the ESR.

Equation 25.  DDEMAG=IM_FL×LMVOUT+VF×NPS×fSW=0.297
Equation 26. ICOUT_RMS=ISEC_PEAK2×DDEMAG3-IOUT2=6.45 A