SLUSCK4C September   2016  – October 2024 UCC28950-Q1 , UCC28951-Q1

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Timing Requirements
    7. 5.7 Dissipation Ratings
    8. 5.8 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1  Start-Up Protection Logic
      2. 6.3.2  Voltage Reference (VREF)
      3. 6.3.3  Error Amplifier (EA+, EA–, COMP)
      4. 6.3.4  Soft-Start and Enable (SS/EN)
      5. 6.3.5  Light-Load Power Saving Features
      6. 6.3.6  Adaptive Delay, (Delay Between OUTA and OUTB, OUTC and OUTD (DELAB, DELCD, ADEL))
      7. 6.3.7  Adaptive Delay (Delay Between OUTA and OUTF, OUTB and OUTE (DELEF, ADELEF)
      8. 6.3.8  Minimum Pulse (TMIN)
      9. 6.3.9  Burst Mode
      10. 6.3.10 Switching Frequency Setting
      11. 6.3.11 Slope Compensation (RSUM)
      12. 6.3.12 Dynamic SR ON/OFF Control (DCM Mode)
      13. 6.3.13 Current Sensing (CS)
      14. 6.3.14 Cycle-by-Cycle Current Limit Current Protection and Hiccup Mode
      15. 6.3.15 Synchronization (SYNC)
      16. 6.3.16 Outputs (OUTA, OUTB, OUTC, OUTD, OUTE, OUTF)
      17. 6.3.17 Supply Voltage (VDD)
      18. 6.3.18 Ground (GND)
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1  Power Loss Budget
        2. 7.2.2.2  Preliminary Transformer Calculations (T1)
        3. 7.2.2.3  QA, QB, QC, QD FET Selection
        4. 7.2.2.4  Selecting LS
        5. 7.2.2.5  Selecting Diodes DB and DC
        6. 7.2.2.6  Output Inductor Selection (LOUT)
        7. 7.2.2.7  Output Capacitance (COUT)
        8. 7.2.2.8  Select FETs QE and QF
        9. 7.2.2.9  Input Capacitance (CIN)
        10. 7.2.2.10 Current Sense Network (CT, RCS, R7, DA)
          1. 7.2.2.10.1 Voltage Loop Compensation Recommendation
      3. 7.2.3 Application Curves
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Development Support
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Community Resources
    5. 8.5 Trademarks
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

6.3.8 Minimum Pulse (TMIN)

The resistor RTMIN from the TMIN pin to GND sets a fixed minimum pulse width. This pulse is applied to the transformer and enables ZVS at light load. If the output PWM pulse demanded by the feedback loop is shorter than TMIN, then the controller proceeds to burst mode operation where an even number of TMIN pulses are followed by the off time dictated by the feedback loop. The proper selection of the TMIN duration is dictated by the time it takes to raise sufficient magnetizing current in the power transformer to maintain ZVS. The TMIN pulse is measured from the rising edge of OUTA to the falling edge of OUTD – or from the rising edge of OUTB to the falling edge of OUTC. The minimum pulse TMIN is then defined by Equation 8.

Equation 8. UCC28950-Q1 UCC28951-Q1

where

  • TMIN is in ns
  • RTMIN is in kΩ

Various propagation and response time delays in the power circuit modify (usually increase) the pulse width that is measured at the transformer. Select the correct TMIN setting using an iterative process due to the propagation and response time delays in the power circuit.

Note:

The minimum allowed resistance on the TMIN pin, RTMIN is 10kΩ.

The related plot is shown in Figure 6-7.

UCC28950-Q1 UCC28951-Q1 Minimum Time TMIN Over Setting Resistor RTMINFigure 6-7 Minimum Time TMIN Over Setting Resistor RTMIN

The value of minimum duty cycle DMIN is determined by Equation 9.

Equation 9. UCC28950-Q1 UCC28951-Q1

where

  • FSW(osc) is oscillator frequency in kHz
  • TMIN is the minimum pulse in ns
  • and DMIN is in percent