SLUSDD4B April   2019  – December 2020 UC1843B-SP

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 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 UVLO
      2. 7.3.2 Reference
      3. 7.3.3 Totem-Pole Output
    4. 7.4 Device Functional Modes
  8. 8 Application Information Disclaimer
    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 Switching Frequency
        2. 8.2.2.2 Transformer
        3. 8.2.2.3 RCD Diode Clamp
        4. 8.2.2.4 Output Diode
        5. 8.2.2.5 Output Filter and Capacitor
        6. 8.2.2.6 Compensation
        7. 8.2.2.7 Sense Resistor and Slope Compensation
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Feedback Traces
      2. 10.1.2 Input/Output Capacitors
      3. 10.1.3 Compensation Components
      4. 10.1.4 Traces and Ground Planes
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Receiving Notification of Documentation Updates
    2. 11.2 Support Resources
    3. 11.3 Trademarks
    4. 11.4 Electrostatic Discharge Caution
    5. 11.5 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

8.2.2.7 Sense Resistor and Slope Compensation

The sense resistor is used to sense the ripple current from the transformer as well as shutdown the switching cycle if the peak current of the converter is allowed to get too high. The voltage threshold of the CS pin is around 1 V, thus the equation to find the sense resistor from the peak current is shown in Equation 44.

Equation 44. Rcs=VCS Threshold-VSlope Comp OffsetIlimit
Equation 45. Rcs=1 V-0.1 V12 A=0.075 Ω

Note that Ilimit should be greater than IPriPeak, and that the voltage offset from the slope compensation will be dependant on the amount of slope compensation in the design. The value of 0.075 Ω for the sense resistance was found to be the optimum value adding some headroom for slope compensation offset of 0.1 V. Slope compensation was implemented with a BJT being turned off and on by the RC pin of the device. The BJT was placed between the REF pin and a resistor divider to the CS pin. The optimum slope compensation value can be found from the following equations after picking a value for the top of the divider:

Equation 46. Sc=Vout×Rcs×GcsLPRI×Nps
Equation 47. Sc=5 V×0.075 Ω×321 μH×3.33=16088
Equation 48. Sosc=fosc×VoscppDMIN
Equation 49. Sosc=200 kHz×1.7 V0.25=1360000
Equation 50. Rcsf=RtopSoscSc-1
Equation 51. Rcsf=11.8 kΩ136000016088-1=141 Ω

The UC1843B-SP design uses a much higher resistor of 1.47 kΩ, but this is an attempt to be very conservative. Note that the bottom resistor can be used as part of a filter to the CS pin as well, which is implemented in the design using a capacitor near the CS pin. Care was taken such that the RC filter would not filter the switching frequency by having the RC time constant be a decade less than the switching frequency.