SLUS458I July   2000  – June 2024 UCC28C40 , UCC28C41 , UCC28C42 , UCC28C43 , UCC28C44 , UCC28C45 , UCC38C40 , UCC38C41 , UCC38C42 , UCC38C43 , UCC38C44 , UCC38C45

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison Table
  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  Detailed Pin Description
        1. 7.3.1.1 COMP
        2. 7.3.1.2 FB
        3. 7.3.1.3 CS
        4. 7.3.1.4 RT/CT
        5. 7.3.1.5 GND
        6. 7.3.1.6 OUT
        7. 7.3.1.7 VDD
        8. 7.3.1.8 VREF
      2. 7.3.2  Undervoltage Lockout
      3. 7.3.3  ±1% Internal Reference Voltage
      4. 7.3.4  Current Sense and Overcurrent Limit
      5. 7.3.5  Reduced-Discharge Current Variation
      6. 7.3.6  Oscillator Synchronization
      7. 7.3.7  Soft-Start Timing
      8. 7.3.8  Enable and Disable
      9. 7.3.9  Slope Compensation
      10. 7.3.10 Voltage Mode
    4. 7.4 Device Functional Modes
      1. 7.4.1 Normal Operation
      2. 7.4.2 UVLO Mode
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1  Input Bulk Capacitor and Minimum Bulk Voltage
        2. 8.2.2.2  Transformer Turns Ratio and Maximum Duty Cycle
        3. 8.2.2.3  Transformer Inductance and Peak Currents
        4. 8.2.2.4  Output Capacitor
        5. 8.2.2.5  Current Sensing Network
        6. 8.2.2.6  Gate Drive Resistor
        7. 8.2.2.7  VREF Capacitor
        8. 8.2.2.8  RT/CT
        9. 8.2.2.9  Start-Up Circuit
        10. 8.2.2.10 Voltage Feedback Compensation
          1. 8.2.2.10.1 Power Stage Poles and Zeroes
          2. 8.2.2.10.2 Slope Compensation
          3. 8.2.2.10.3 Open-Loop Gain
          4. 8.2.2.10.4 Compensation Loop
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
        1. 8.4.1.1 Precautions
        2. 8.4.1.2 Feedback Traces
        3. 8.4.1.3 Bypass Capacitors
        4. 8.4.1.4 Compensation Components
        5. 8.4.1.5 Traces and Ground Planes
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Third-Party Products Disclaimer
    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

8.2.2.9 Start-Up Circuit

At start-up, the IC gets its power directly from the high-voltage bulk, through a high-voltage resistor (RSTART). The selection of the start-up resistor is the tradeoff between power loss and start-up time. The current flowing through RSTART at the minimum input voltage must be higher than the VDD current under UVLO conditions (100µA at its maximum value). A resistance of 420kΩ was chosen for RSTART, providing 250µA of start-up current at low-line conditions. The start-up resistor is physically comprised of two 210kΩ resistors in series to meet the high voltage requirements and power rating at high-line.

After VDD is charged up above the UVLO-ON threshold, the UCC28C42 starts to consume full operating current. The VDD capacitor is required to provide enough energy to prevent its voltage from dropping below the UVLO-OFF threshold during start-up, before the output is able to reach its regulated level. A large bulk capacitance would hold more energy but would result in slower start-up time. In this design, a 120µF capacitor is chosen to provide enough energy and maintain a start-up time of approximately 7 seconds. For faster start-up, the bulk capacitor value may be decreased or the RSTART resistor modified to a lower value.