SLUSD37E October   2017  – November 2019 UCC28056


  1. Features
  2. Applications
  3. Description
    1.     No Load Power
      1.      Device Images
        1.       Simplified Application
  4. Revision History
  5. Device Comparison Tables
  6. Pin Configuration and Functions
    1.     Pin Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 CrM/DCM Control Principle
      2. 8.3.2 Line Voltage Feed-Forward
        1. Peak Line Voltage Detection
      3. 8.3.3 Valley Switching and CrM/DCM Hysteresis
        1. Valley Delay Adjustment
      4. 8.3.4 Transconductance Amplifier with Transient Speed-up Function
      5. 8.3.5 Faults and Protections
        1. Supply Undervoltage Lockout
        2. Two Level Over-Current Protection
          1. Cycle-by-Cycle Current Limit Ocp1
          2. Ocp2 Gross Over-Current or CCM Protection
        3. Output Over-Voltage Protection
          1. First Level Output Over-Voltage Protection (Ovp1)
          2. Second Level Over-Voltage Protection (Ovp2)
        4. Thermal Shutdown Protection
        5. Line Under-Voltage or Brown-In
      6. 8.3.6 High-Current Driver
    4. 8.4 Controller Functional Modes
      1. 8.4.1 Burst Mode Operation
      2. 8.4.2 Soft Start
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. Custom Design With WEBENCH® Tools
        2. Power Stage Design
          1. Boost Inductor Design
          2. Boost Switch Selection
          3. Boost Diode Selection
          4. Output Capacitor Selection
        3. ZCD/CS Pin
          1. Voltage Spikes on the ZCD/CS pin Waveform
        4. VOSNS Pin
        5. Voltage Loop Compensation
          1. Plant Model
          2. Compensator Design
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 VOSNS Pin
      2. 11.1.2 ZCD/CS Pin
      3. 11.1.3 VCC Pin
      4. 11.1.4 GND Pin
      5. 11.1.5 DRV Pin
      6. 11.1.6 COMP Pin
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Custom Design With WEBENCH® Tools
    2. 12.2 Receiving Notification of Documentation Updates
    3. 12.3 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

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

Valley Switching and CrM/DCM Hysteresis

The UCC28056 controller achieves maximum efficiency enabling power switching operation when the drain voltage of the MOSFET is at a minimum (sometimes referred to as valley), of the resonance that occurs during the TDCM period . Any energy stored in the Drain node capacitance (CDE ) dissipates in the power switch during its turnon transition time. Valley switching ensures minimum energy is stored in CDE prior to the turnon period and hence minimum switching loss. After the TDCM period, the controller waits for the next available valley on the drain voltage before initiating a new switching cycle. The actual TDCM duration is therefore always an integer multiple of the drain resonance period. If the calculated TDCM period extends over a valley boundary the actual TDCM duration steps up in value by one resonant period. This step change in TDCM duration causes a step change in Line current that rapidly decays as the TON(θ) computation iterates to a new solution to reflect the step change in TDCM duration. Line current distortion, resulting from valley transitions, is kept to a minimum by computing the TDCM duration from the COMP voltage. The COMP voltage varies little over the period of a Line cycle and hence the calculated TDCM duration changes very little over the period of a Line cycle.

Line current distortion is particularly severe during the transition from the first valley (CrM) to the second valley (DCM) operation while the input voltage is low. In this region, the first valley duration is extended by the clamping action of the power switch body diode. In this region Line current is reduced when switching on the first valley, (CrM) , because the inductor current is negative at the start of the on period. The reduction in Line current is not observed for second or subsequent valley (DCM) operation because the inductor current starts the on period from zero. UCC28056 implements hysteresis in the TDCM computation to virtually eliminate the possibility of repeated CrM/DCM transitions across a Line cycle. Such transitions can only occur if the twice Line frequency ripple on the COMP voltage is greater than 12% at the CrM/DCM boundary.

UCC28056 CrmDcmWfm.gifFigure 21. Drain Voltage and Inductor Current Transitioning from DCM to CrM