SLUSC61A January   2015  – December 2020 UCC28700-Q1

PRODUCTION DATA  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
    1. 5.1 Pin Functions
  6. Specifications
    1. 6.1 Absolute Maximum Ratings (1)
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Switching Characteristics
    7. 6.7 Typical Characteristics
  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 CBC (Cable Compensation)
      2. 7.3.2 Fault Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Primary-Side Voltage Regulation
      2. 7.4.2 Primary-Side Current Regulation
      3. 7.4.3 Valley-Switching
      4. 7.4.4 Start-Up Operation
  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 Transformer Parameter Verification
        2. 8.2.2.2 Output Capacitance
        3. 8.2.2.3 VDD Capacitance, CDD
        4. 8.2.2.4 VDD Start-Up Resistance, RSTR
        5. 8.2.2.5 VS Resistor Divider, Line Compensation, and Cable Compensation
        6. 8.2.2.6 Input Bulk Capacitance and Minimum Bulk Voltage
        7. 8.2.2.7 Transformer Turns Ratio, Inductance, Primary-Peak Current
        8. 8.2.2.8 Standby Power Estimate
      3. 8.2.3 Application Curves
  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 Device Nomenclature
        1. 11.1.1.1  Capacitance Terms in Farads
        2. 11.1.1.2  Duty Cycle Terms
        3. 11.1.1.3  Frequency Terms in Hertz
        4. 11.1.1.4  Current Terms in Amperes
        5. 11.1.1.5  Current and Voltage Scaling Terms
        6. 11.1.1.6  Transformer Terms
        7. 11.1.1.7  Power Terms in Watts
        8. 11.1.1.8  Resistance Terms in Ω
        9. 11.1.1.9  Timing Terms in Seconds
        10. 11.1.1.10 Voltage Terms in Volts
        11. 11.1.1.11 AC Voltage Terms in VRMS
        12. 11.1.1.12 Efficiency Terms
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Trademarks

Package Options

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

Transformer Turns Ratio, Inductance, Primary-Peak Current

The maximum primary-to-secondary turns ratio can be determined by the target maximum switching frequency at full load, the minimum input capacitor bulk voltage, and the estimated DCM quasi-resonant time.

Initially determine the maximum available total duty cycle of the on time and secondary conduction time based on target switching frequency and DCM resonant time. For DCM resonant time, assume 500 kHz if you do not have an estimate from previous designs. For the transition mode operation limit, the period required from the end of secondary current conduction to the first valley of the VDS voltage is ½ of the DCM resonant period, or 1 µs assuming 500-kHz resonant frequency. DMAX can be determined using the equation below.

Equation 21. GUID-6821CF65-415D-40C0-98D2-7D3E0BAD573E-low.png

Once DMAX is known, the maximum turns ratio of the primary to secondary can be determined with the equation below. DMAGCC is defined as the secondary diode conduction duty cycle during constant-current, CC, operation. It is set internally by the UCC28700-Q1 at 0.425. The total voltage on the secondary winding needs to be determined; which is the sum of VOCV, the secondary rectifier VF, and the cable compensation voltage (VOCBC). For the 5-V USB charger applications, a turns ratio range of 13 to 15 is typically used.

Equation 22. GUID-F8498F9D-E018-467E-B07F-2A50B709FE82-low.gif

Once an optimum turns ratio is determined from a detailed transformer design, use this ratio for the following parameters.

The UCC28700-Q1 controller constant-current regulation is achieved by maintaining a maximum DMAG duty cycle of 0.425 at the maximum primary current setting. The transformer turns ratio and constant-current regulating voltage determine the current sense resistor for a target constant current.

Since not all of the energy stored in the transformer is transferred to the secondary, a transformer efficiency term is included. This efficiency number includes the core and winding losses, leakage inductance ratio, and bias power ratio to rated output power. For a 5-V, 1-A charger example, bias power of 1.5% is a good estimate. An overall transformer efficiency of 0.9 is a good estimate to include 3.5% leakage inductance, 5% core and winding loss, and 1.5% bias power.

Equation 23. GUID-2B83FB7F-E2B3-4D22-BABA-1695BF049C89-low.png

The primary transformer inductance can be calculated using the standard energy storage equation for flyback transformers. Primary current, maximum switching frequency and output and transformer power losses are included in the equation below. Initially determine transformer primary current.

Primary current is simply the maximum current sense threshold divided by the current sense resistance.

Equation 24. GUID-AEF53F14-6424-4344-BE61-072240CE430E-low.gif
Equation 25. GUID-23585701-6B52-44C3-AD2C-8EBBE511B5BC-low.gif

The secondary winding to auxiliary winding transformer turns ratio (NAS) is determined by the lowest target operating output voltage in constant-current regulation and the VDD UVLO of the UCC28700-Q1. There is additional energy supplied to VDD from the transformer leakage inductance energy which allows a lower turns ratio to be used in many designs.

Equation 26. GUID-4BC44EE4-0B77-478E-B008-5861D0E583AF-low.gif