JAJSH61A April   2019  – September 2019 UC1843B-SP

PRODUCTION DATA.  

  1. 1 特長
  2. 2 アプリケーション
  3. 3 概要
    1.     Device Images
      1.      概略回路図
  4. 4 改訂履歴
  5. 5 Pin Configuration and Functions
    1.     Pin 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 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
        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. 11デバイスおよびドキュメントのサポート
    1. 11.1 ドキュメントの更新通知を受け取る方法
    2. 11.2 コミュニティ・リソース
    3. 11.3 商標
    4. 11.4 静電気放電に関する注意事項
    5. 11.5 Glossary
  12. 12メカニカル、パッケージ、および注文情報

8.2.2.5 Output Filter and Capacitor

The output capacitance value is picked such that there is enough capacitance for the required voltage ripple and output current load step. The UC1843B-SP design uses equations Equation 22 and Equation 24 to find a minimum capacitance.

Equation 22. C o u t > I o u t × D M A X V R i p p l e × f o s c
Equation 23. C o u t > 10   A ×0 .5 50   m V ×200   k H z =500   μ F
Equation 24. C o u t > Δ I s t e p 2 π × Δ V o u t × f c o
Equation 25. C o u t > 10   A 2 π ×0 .7   V ×2 .2   k H z =1   m F

A value of around 1145 µF was chosen to keep output voltage ripple low. Note that the output voltage ripple in the design was further decreased by adding an output filter and by adding an inductor after a small portion of the output capacitance. Six ceramic capacitors were picked to be placed before the output filter and then the large tantalum capacitors with some small ceramics were added to be part of the output filter. The initial ceramics will help with the initial current ripple, but have a very large output voltage ripple. This voltage ripple will be attenuated by the inductor and capacitor combination placed between the ceramic capacitors and the output. The equations below allow for finding the amount of attenuation that will come from a specific output filter inductance. An inductance of 500 nH was chosen to attenuate the output voltage ripple and the attenuation was sufficient for the design.

Equation 26. F r e s o n a n t = 1 2 π × L F i l t e r × C o B u l k
Equation 27. F r e s o n a n t = 1 2 π ×0 .5   n H ×1127   μ F =6 .7   k H z
Equation 28. F Z e r o = 1 2 π × C o B u l k × E S R o B u l k
Equation 29. F Z e r o = 1 2 π ×1127   μ F ×0 .009   Ω =15 .69   k H z
Equation 30. A t t e n u a t i o n f s w =40 × log10 f o s c f r e s o n a n t -20 × log10 f o s c f z e r o
Equation 31. A t t e n u a t i o n f s w =40 × log10 200   k H z 6 .7   k H z -20 × log10 200   k H z 15 .69   k H z   =36 .88   d B

Sometimes the output filter can cause peaking at high frequencies, this can be damped by adding a resistor in parallel with the inductor. For the UC1843B-SP design, 0.5 Ω was used as a very conservative value. The resistance needed to damp the peaking can be calculated using the following equations:

Equation 32. ω o = 2 ( C o C e r m + C o B u l k ) L F i l t e r × C o C e r m × C o B u l k
Equation 33. ω o = 2 (19   μ F +1127   μ F ) 500   n H ×19   μ F ×1127   μ F =463   k H z
Equation 34. R F i l t e r = R o × L F i l t e r × ( C o C e r m + C o B u l k ) - L F i l t e r ω o R o × ( C o C e r m + C o B u l k ) ω o - L F i l t e r × C o C e r m
Equation 35. R F i l t e r = 0 .5 ×500   n H × (19   μ F +1127   μ F ) - 500   n H 463   k H z 0 .5 × (19   μ F +1127   μ F ) 463   k H z -500   n H ×19   μ F =0 .232   Ω