SLUSFG5 November   2024 UCC33421-Q1

ADVANCE INFORMATION  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison
  6. 5 Pin Configuration and Functions
  7. 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 Insulation Specifications
    6. 6.6 Safety-Related Certifications
    7. 6.7 Electrical Characteristics
    8. 6.8 External BOM Components
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Enable and Disable
      2. 7.3.2 Output Voltage Soft-Start
      3. 7.3.3 Output Voltage Steady-State Regulation
      4. 7.3.4 Protection Features
        1. 7.3.4.1 Input Under-voltage and Over-Voltage Lockout
        2. 7.3.4.2 Output Under-Voltage Protection
        3. 7.3.4.3 Output Over-Voltage Protection
        4. 7.3.4.4 Over-Temperature Protection
        5. 7.3.4.5 Fault Reporting and Auto-Restart
      5. 7.3.5 VCC Load Recommended Operating Area
      6. 7.3.6 Electromagnetic Compatibility (EMC) Considerations
    4. 7.4 Device Functional Modes
  9. 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
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Support Resources
    5. 9.5 Trademarks
    6. 9.6 Electrostatic Discharge Caution
    7. 9.7 Glossary
  11. 10Revision History
  12. 11Mechanical and Packaging Information

Package Options

Refer to the PDF data sheet for device specific package drawings

Mechanical Data (Package|Pins)
  • DHA|16
Thermal pad, mechanical data (Package|Pins)
Orderable Information

7.3.2 Output Voltage Soft-Start

The UCC33421-Q1 has soft-start mechanism that ensures a smooth and fast soft-start operation with minimum input inrush current. The output voltage Soft-Start diagram is shown in Figure 7-1. After VINP > VVINP_UVLO_R and EN/FLT is pulled high, the soft-start sequence starts with a primary duty cycle open loop control. The power stage operates with a fixed burst frequency with an incremental increasing duty cycle starting at 6.5% . The rate of change of the duty cycle is pre-programmed in the part to reduce the input inrush current while building the output voltage VCC. The primary side limits the maximum duty cycle to 62.5% during this phase till the secondary side VCC voltage passes VVCC_UVLO = 2.7V threshold before releasing this duty cycle limit. This limit will ensure minimum input current in case the device starts on a short circuit and the VCC is not building up.

The soft-start time will vary depending on the output capacitors, input voltage and loading conditions. The UCC33421-Q1 has a soft-start timeout feature by which the VCC output voltage state is monitored during soft-start. In certain conditions the VCC might not reach steady-state regulation threshold due to short circuit on the output voltage as shown in Figure 7-2 , heavy loading conditions above recommended operating conditions or higher output capacitor values as shown in Figure 7-3. In these conditions, if the soft-start timout duration of 16ms expires without the VCC reaching steady-state regulation, the part will shutdown and EN/FLT pin will be pulled low for 200us to report the fault condition. An auto-restart timer will start afterwards, the part will attempt to restart after that timer expires. More details regarding fault reporting and auto-restart can be found in Fault Reporting and Auto-Restart. If the same conditions continue to exist the same cycle will repeat again as shown in Figure 7-2and Figure 7-3 below.

UCC33421-Q1 Output Voltage Soft-Start Diagram Figure 7-1 Output Voltage Soft-Start Diagram
UCC33421-Q1 Soft-start under short-circuit output Figure 7-2 Soft-start under short-circuit output
UCC33421-Q1 Soft-start underhigh load or high output capacitor conditions Figure 7-3 Soft-start underhigh load or high output capacitor conditions