SLUSDB2A August   2018  – December 2021 UCC28951

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and 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 Timing Requirements
    7. 6.7 Dissipation Ratings
    8. 6.8 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  Start-Up Protection Logic
      2. 7.3.2  Voltage Reference (VREF)
      3. 7.3.3  Error Amplifier (EA+, EA–, COMP)
      4. 7.3.4  Soft-Start and Enable (SS/EN)
      5. 7.3.5  Light-Load Power Saving Features
      6. 7.3.6  Adaptive Delay, (Delay Between OUTA and OUTB, OUTC and OUTD (DELAB, DELCD, ADEL))
      7. 7.3.7  Adaptive Delay (Delay Between OUTA and OUTF, OUTB and OUTE (DELEF, ADELEF)
      8. 7.3.8  Minimum Pulse (TMIN)
      9. 7.3.9  Burst Mode
      10. 7.3.10 Switching Frequency Setting
      11. 7.3.11 Slope Compensation (RSUM)
      12. 7.3.12 Dynamic SR ON/OFF Control (DCM Mode)
      13. 7.3.13 Current Sensing (CS)
      14. 7.3.14 Cycle-by-Cycle Current Limit Current Protection and Hiccup Mode
      15. 7.3.15 Synchronization (SYNC)
      16. 7.3.16 Outputs (OUTA, OUTB, OUTC, OUTD, OUTE, OUTF)
      17. 7.3.17 Supply Voltage (VDD)
      18. 7.3.18 Ground (GND)
    4. 7.4 Device Functional Modes
  8. 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  Power Loss Budget
        2. 8.2.2.2  Preliminary Transformer Calculations (T1)
        3. 8.2.2.3  QA, QB, QC, QD FET Selection
        4. 8.2.2.4  Selecting LS
        5. 8.2.2.5  Selecting Diodes DB and DC
        6. 8.2.2.6  Output Inductor Selection (LOUT)
        7. 8.2.2.7  Output Capacitance (COUT)
        8. 8.2.2.8  Select FETs QE and QF
        9. 8.2.2.9  Input Capacitance (CIN)
        10. 8.2.2.10 Current Sense Network (CT, RCS, R7, DA)
          1. 8.2.2.10.1 Voltage Loop Compensation Recommendation
      3. 8.2.3 Application Curves
  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
    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

Package Options

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

8.2.2.7 Output Capacitance (COUT)

The output capacitor is selected based on holdup and transient (VTRAN) load requirements.

The time it takes LOUT to change 90% of its full load current is calculated in Equation 67:

Equation 67. GUID-85FB66DE-1474-42AA-AA8C-B653471E4773-low.gif

During load transients most of the current will immediately go through the capacitors equivalent series resistance (ESRCOUT). Equation 68 and Equation 69 are used to select ESRCOUT and COUT based on a 90% load step in current. The ESR is selected for 90% of the allowable transient voltage (VTRAN), while the output capacitance (COUT) is selected for 10% of VTRAN.

Equation 68. GUID-A6FDA715-772F-459A-B128-E15669192715-low.gif
Equation 69. GUID-6B3DF569-FC6E-460F-8FDF-1186B029FF2B-low.gif

Before selecting the output capacitor, the output capacitor RMS current (ICOUT_RMS) must be calculated using Equation 70.

Equation 70. GUID-58895C11-B761-4526-B2F2-CDBBA84B4A45-low.gif

To meet the design requirements five 1500-µF, aluminum electrolytic capacitors are chosen for the design from United Chemi-Con™, part number EKY-160ELL152MJ30S. These capacitors have an ESR of 31 mΩ.

The number of output capacitors (n) is 5.

The total output capacitance is calculated usingEquation 71:

Equation 71. GUID-2D76C058-A2AE-4575-A0B5-6B8F1061DDAE-low.gif

The effective output capacitance ESR is calculated usingEquation 72:

Equation 72. GUID-77FA7A8B-6B94-4ED8-BD9A-22153DB22A77-low.gif

Calculate output capacitor loss (PCOUT) using Equation 73:

Equation 73. GUID-32D55CD8-FF3C-4591-83D1-B449DEF54745-low.gif

Recalculate the remaining Power Budget using Equation 74:

Equation 74. GUID-F93D18DA-A97A-4440-AFF1-FD07C48650A7-low.gif