SLUSC60B December   2017  – October 2019 UCC28064A


  1. Features
  2. Applications
  3. Description
    1.     Device Images
      1.      Simplified Application
  4. Revision History
  5. Description (Continued)
  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  Principles of Operation
      2. 8.3.2  Natural Interleaving
      3. 8.3.3  On-Time Control, Maximum Frequency Limiting, Restart Timer and Input Voltage Feed-Forward compensation
      4. 8.3.4  Distortion Reduction
      5. 8.3.5  Zero-Current Detection and Valley Switching
      6. 8.3.6  Phase Management and Light-Load Operation
      7. 8.3.7  Burst Mode Operation
      8. 8.3.8  External Disable
      9. 8.3.9  Improved Error Amplifier
      10. 8.3.10 Soft Start
      11. 8.3.11 Brownout Protection
      12. 8.3.12 Line Dropout Detection
      13. 8.3.13 VREF
      14. 8.3.14 VCC
      15. 8.3.15 System Level Protections
        1. Failsafe OVP - Output Over-voltage Protection
        2. Overcurrent Protection
        3. Open-Loop Protection
        4. VCC Undervoltage Lock-Out (UVLO) Protection
        5. Phase-Fail Protection
        6. CS - Open, TSET - Open and Short Protection
        7. Thermal Shutdown Protection
        8. Fault Logic Diagram
    4. 8.4 Device Functional Modes
  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.  Inductor Selection
        3.  ZCD Resistor Selection RZA, RZB
        4.  HVSEN
        5.  Output Capacitor Selection
        6.  Selecting RS For Peak Current Limiting
        7.  Power Semiconductor Selection (Q1, Q2, D1, D2)
        8.  Brownout Protection
        9.  Converter Timing
        10. Programming VOUT
        11. Voltage Loop Compensation
      3. 9.2.3 Application Curves
        1. Input Ripple Current Cancellation with Natural Interleaving
        2. Brownout Protection
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Package Option Addendum
    1. 12.1 Packaging Information
  13. 13Device and Documentation Support
    1. 13.1 Device Support
      1. 13.1.1 Development Support
        1. Custom Design With WEBENCH® Tools
    2. 13.2 Documentation Support
      1. 13.2.1 Related Documentation
    3. 13.3 Receiving Notification of Documentation Updates
    4. 13.4 Community Resources
    5. 13.5 Trademarks
    6. 13.6 Electrostatic Discharge Caution
    7. 13.7 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

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

Brownout Protection

As the power line RMS voltage decreases, RMS input current must increase to maintain a constant output voltage for a specific load. Brownout protection helps prevent excess system thermal stress (due to the higher RMS input current) from exceeding a safe operating level. Power-line voltage is sensed at VINAC pin. When the VINAC fails to exceed the brownout threshold for the brownout filter time (tBODLY), a brownout condition is detected and both gate drive outputs are turned off. During brownout, COMP is actively pulled low and soft-start condition is initiated. When VINAC rises above the brownout threshold, the power stage soft-starts as COMP rises with controlled current.

The brownout threshold and its hysteresis are set by the voltage-divider ratio and resistor values. Brownout protection is based on VINAC peak voltage; the threshold and hysteresis are also based on the line peak voltage. Hysteresis is provided by a 2-μA current-sink (IBOHYS) enabled whenever Brownout protection is activated. As soon as the Brownout protection is activated an additional timer is started that counts the tBORST time. During this time the device is forced to stay in a Brownout condition. So, during tBORST time, the device is not allowed to switch, COMP is pulled low and the 2-uA current sink (IBOHYS) is active regardless of the voltage on VINAC pin. After tBORST is elapsed the device can exit from Brownout condition only if VINAC pin exceeds VBOTHR threshold. When the device operates in burst mode, several blocks inside the IC are turned off to reduce IC current consumption. The Brownout management block is also turned off. Each time the system stops switching, because of burst mode, the Brownout filter timer is reset. So if the system is operating in burst mode, the Brownout protection, generally is not triggered. The main purpose of Brownout is to avoid excess system thermal stress. When the system is operating in burst-mode the load is low enough to avoid thermal stress. The peak VINAC voltage can be easily translated into an RMS value. Example resistor values for the voltage divider are 8.61 MΩ ±1% from the rectified input voltage to VINAC and 133 kΩ ±1% from VINAC to ground. These resistors set the typical thresholds for RMS line voltages, as shown in Table 1.

Table 1. Brownout Thresholds (For Conditions Stated in the Text)

Falling 67 V
Rising 81 V

Equation 13 and Equation 14 can be used to calculate the VINAC divider-resistors values based on desired brownout and brown-in voltage levels. VAC_OK is the desired RMS turnon voltage, VAC_BO is the desired RMS turnoff brownout voltage, and VLOSS is total series voltage drop due to wiring, EMI-filter, and bridge-rectifier impedances at VAC_BO. VBOTHR, and IBOHYS are found in the data-tables of this datasheet.

Equation 13. UCC28064A eq_rax_slusc60.gif
Equation 14. UCC28064A eq_rbx_slusc60.gif

When standard values for the VINAC divider-resistors RA and RB are selected, the actual turn-on and brownout threshold RMS voltages for the ac-line can be back-calculated with Equation 15 and Equation 16:

Equation 15. UCC28064A eq_vacbox_slusc60.gif
Equation 16. UCC28064A eq_vacokx_slusc60.gif

An example of the timing for the brownout function is illustrated in Figure 28.