SLUSD71A April   2018  – May 2018 UCC28742

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
      2.      Typical Efficiency of a 10-W, 5-V AC-to-DC Converter
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 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
  7. 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 VDD (Device Bias Voltage Supply)
        2. 7.3.1.2 GND (Ground)
        3. 7.3.1.3 VS (Voltage-Sense)
        4. 7.3.1.4 DRV (Gate Drive)
        5. 7.3.1.5 CS (Current Sense)
        6. 7.3.1.6 FB (Feedback)
      2. 7.3.2 Secondary-Side Optically Coupled Constant-Voltage (CV) Regulation
      3. 7.3.3 Control Law
      4. 7.3.4 Constant Current Limit and Delayed Shutdown
      5. 7.3.5 Valley-Switching and Valley-Skipping
      6. 7.3.6 Start-Up Operation
      7. 7.3.7 Fault Protection
    4. 7.4 Device Functional Modes
  8. 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  Custom Design With WEBENCH® Tools
        2. 8.2.2.2  VDD Capacitance, CDD
        3. 8.2.2.3  VDD Start-Up Resistance, RSTR
        4. 8.2.2.4  Input Bulk Capacitance and Minimum Bulk Voltage
        5. 8.2.2.5  Transformer Turns Ratio, Inductance, Primary-Peak Current
        6. 8.2.2.6  Transformer Parameter Verification
        7. 8.2.2.7  VS Resistor Divider and Line Compensation
        8. 8.2.2.8  Standby Power Estimate
        9. 8.2.2.9  Output Capacitance
        10. 8.2.2.10 Feedback Loop Design Consideration
      3. 8.2.3 Application Curves
    3. 8.3 Do's and Don'ts
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. 11.1.1.1 Custom Design With WEBENCH® Tools
      2. 11.1.2 Device Nomenclature
        1. 11.1.2.1  Capacitance Terms in Farads
        2. 11.1.2.2  Duty Cycle Terms
        3. 11.1.2.3  Frequency Terms in Hertz
        4. 11.1.2.4  Current Terms in Amperes
        5. 11.1.2.5  Current and Voltage Scaling Terms
        6. 11.1.2.6  Transformer Terms
        7. 11.1.2.7  Power Terms in Watts
        8. 11.1.2.8  Resistance Terms in Ω
        9. 11.1.2.9  Timing Terms in Seconds
        10. 11.1.2.10 Voltage Terms in Volts
        11. 11.1.2.11 AC Voltage Terms in VRMS
        12. 11.1.2.12 Efficiency Terms
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 Community Resources
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

7.3.1.5 CS (Current Sense)

The current sense pin is connected through a series resistor (RLC) to the current-sense resistor (RCS). The controller varies the internal current sense threshold between 190 mV and 770 mV, setting a corresponding control range for the peak-primary winding current to a 4-to-1 range. The series resistor RLC provides an input voltage feed-forward function. The voltage drop across this resistor reduces primary-side peak current as the line voltage increases, compensating for the increased di/dt and delays in the MOSFET turn-off. There is an internal leading-edge blanking time of 270 ns to eliminate sensitivity to the MOSFET turn-on leading edge current spike. If additional blanking time is needed, a small bypass capacitor, up to 30 pF, can be placed on between CS pin and GND pin. The value of RCS is determined by the target output current in constant current (CC) regulation. The values of RCS and RLC can be determined by the equations below. The term ηXFMR is intended to account for the energy stored in the transformer but not delivered to the secondary. This includes transformer core and copper losses, bias power, and primary leakage inductance losses.

Example: With a transformer core and copper losses of 3%, leakage inductance caused power losses 2%, and bias power to output power ratio of 0.5%. The transformer power transfer efficiency is estimated as ηXFMR = 100% - 3% - 2% - 0.5% = 94.5%

Equation 3. UCC28742 qu03_lusca8.gif

where

  • VCCR is a current regulation constant (see the Electrical Characteristics table),
  • NPS is the transformer primary-to-secondary turns ratio (a typical turns-ratio of 12 to 15 is recommended for 5-V output as an example),
  • IOCC is the target output current in constant-current limit (refer to Constant Current Limit and Delayed Shutdown for more detail),
  • ηXFMR is the transformer efficiency.
Equation 4. UCC28742 qu04_lusca8.gif

where

  • RS1 is the VS pin high-side resistor value,
  • RCS is the current-sense resistor value,
  • tD is the current-sense delay (typical 50 ns) plus MOSFET turn-off delay,
  • tGATE_OFF is the primary-side main MOSFET turn-off time,
  • NPA is the transformer primary-to-auxiliary turns-ratio,
  • LP is the transformer primary inductance,
  • KLC is a current-scaling constant (see the Electrical Characteristics table).