SLUS846D September   2008  – August 2024 UCC25600

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Revision History
  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 Electrical Characteristics
    6. 6.6 Typical Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Soft Start
      2. 7.3.2 Overcurrent Protection
      3. 7.3.3 Gate Driver
      4. 7.3.4 Overtemperature Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Burst-Mode Operation
      2. 7.4.2 VCC
  9. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Principal of Operation
      2. 8.1.2 Adjustable Dead Time
      3. 8.1.3 Oscillator
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
      3. 8.2.3 Application Curves
  10. 9 Power Supply Recommendations
  11. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  12. 11Device and Documentation Support
    1. 11.1 Support Resources
    2. 11.2 Trademarks
    3. 11.3 Electrostatic Discharge Caution
  13. 12Glossary
  14. 13Mechanical, Packaging, and Orderable Information

8.1.3 Oscillator

With variable switching frequency control, UCC25600 relies on the internal oscillator to vary the switching frequency. The oscillator is controlled by the current flowing out of the RT pin. Except during soft start, the relationship between the gate signal frequency and the current flowing out of the RT pin can be represented as:

Equation 18. UCC25600

Because the switching frequency is proportional to the current, by limiting the maximum and minimum current flowing out of the RT pin, the minimum and maximum switching frequency of the converter could be easily limited. As shown in Figure 8-7, putting a resistor from the RT pin to ground limits the minimum current and putting a resistor in series with the opto-coupler limits the maximum current.

UCC25600 Maximum and Minimum Frequency Setting for UCC25600Figure 8-7 Maximum and Minimum Frequency Setting for UCC25600

The frequency limiting resistor can be calculated based on following equations.

Equation 19. UCC25600
Equation 20. UCC25600
Equation 21. UCC25600
Equation 22. UCC25600