SLUSAO7C September   2011  – July 2024 UCC28063

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Revision History
  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  Principles of Operation
      2. 7.3.2  Natural Interleaving
      3. 7.3.3  On-Time Control, Maximum Frequency Limiting, and Restart Timer
      4. 7.3.4  Distortion Reduction
      5. 7.3.5  Zero-Current Detection and Valley Switching
      6. 7.3.6  Phase Management and Light-Load Operation
      7. 7.3.7  External Disable
      8. 7.3.8  Improved Error Amplifier
      9. 7.3.9  Soft Start
      10. 7.3.10 Brownout Protection
      11. 7.3.11 Dropout Detection
      12. 7.3.12 VREF
      13. 7.3.13 VCC
      14. 7.3.14 Control of Downstream Converter
      15. 7.3.15 System Level Protections
        1. 7.3.15.1 Failsafe OVP - Output Overvoltage Protection
        2. 7.3.15.2 Overcurrent Protection
        3. 7.3.15.3 Open-Loop Protection
        4. 7.3.15.4 VCC Undervoltage Lock-Out (UVLO) Protection
        5. 7.3.15.5 Phase-Fail Protection
        6. 7.3.15.6 CS-Open, TSET-Open and -Short Protection
        7. 7.3.15.7 Thermal Shutdown Protection
        8. 7.3.15.8 AC-Line Brownout and Dropout Protections
        9. 7.3.15.9 Fault Logic Diagram
    4. 7.4 Device Functional Modes
  9. 8 Applications 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  Inductor Selection
        2. 8.2.2.2  ZCD Resistor Selection (RZA, RZB)
        3. 8.2.2.3  HVSEN
        4. 8.2.2.4  Output Capacitor Selection
        5. 8.2.2.5  Selecting (RS) For Peak Current Limiting
        6. 8.2.2.6  Power Semiconductor Selection (Q1, Q2, D1, D2)
        7. 8.2.2.7  Brownout Protection
        8. 8.2.2.8  Converter Timing
        9. 8.2.2.9  Programming VOUT
        10. 8.2.2.10 Voltage Loop Compensation
      3. 8.2.3 Application Curves
        1. 8.2.3.1 Input Ripple Current Cancellation with Natural Interleaving
        2. 8.2.3.2 Brownout Protection
  10. 9 Power Supply Recommendations
  11. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  12. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. 11.1.1.1 Related Parts
      2. 11.1.2 Device Nomenclature
        1. 11.1.2.1 Detailed Pin Description
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  13. 12Mechanical, Packaging, and Orderable Information

7.3.9 Soft Start

Soft-start is a process for boosting the output voltage of the PFC converter from the peak of the ac-line input voltage to the desired regulation voltage under controlled conditions. Instead of a dedicated soft-start pin, the UCC28063 uses the voltage error amplifier as a controlled current source to increase the PWM duty-cycle by way of increasing the COMP voltage. To avoid excessive start-up time-delay when the ac-line voltage is low, a higher current is applied until VSENSE exceeds 3 V at which point the current is reduced to minimize the tendency for excess COMP voltage at no-load start-up.

The PWM gradually ramps from zero on-time to normal on-time as the compensation capacitor from COMP to AGND charges from zero to near its final value. This process implements a soft-start, with timing set by the output current of the error amplifier and the value of the compensation capacitors. In the event of a HVSEN FailSafe OVP, brownout, external-disable, UVLO fault, or other protection faults, COMP is actively discharged and the UCC28063 will soft-start after the triggering event is cleared. Even if a fault event happens very briefly, the fault is latched into the soft-start state and soft-start is delayed until COMP is fully discharged to 20 mV and the fault is cleared. See Figure 7-5 for details on the COMP current. See Figure 7-6 which illustrates an example of typical system behavior during soft-start.

UCC28063 Expanded Comp Output Current Curve
Expanded COMP output current curve including voltage-error amplifier transconductance and modifications applicable to soft-start and overvoltage conditions.
Figure 7-5 Expanded Comp Output Current Curve
UCC28063 Soft-Start Timing With Illustrative System BehaviorFigure 7-6 Soft-Start Timing With Illustrative System Behavior