TIDUE18 November   2022 TPS51215A

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 DCR current sense
      2. 2.2.2 Subtract and Amplify
      3. 2.2.3 Active Voltage Scale
      4. 2.2.4 IMON Output
    3. 2.3 Highlighted Products
    4. 2.4 System Design Theory
      1. 2.4.1 DCR Current Sense
      2. 2.4.2 Subtract and Amplify
      3. 2.4.3 Active Voltage Scale
      4. 2.4.4 IMON Output
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Test Setup
    3. 3.3 Test Results
      1. 3.3.1 Start-up
      2. 3.3.2 VID Change
      3. 3.3.3 Load Transient
      4. 3.3.4 Load-Line Test
  9. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
      3. 4.1.3 PCB Layout Recommendations
      4. 4.1.4 Altium Project
      5. 4.1.5 Gerber Files
      6. 4.1.6 Assembly Drawings
    2. 4.2 Documentation Support
    3. 4.3 Support Resources
    4. 4.4 Trademarks
  10. 5About the Authors

2.4.1 DCR Current Sense

Figure 2-3 DCR Current Sense Block

A voltage proportional to inductor DC current (VISNS) can be got with DCR current sense block as shown in Figure 2-3. First stage filter with Rfilter1 and Cfilter1 is used to get inductor current information from SW pulse voltage. Second stage filter is used to get DC value from sensed inductor current and filter the ripple. RNTC is a negative thermal coefficient resistor, which is used to compensate the DCR variation with temperature. Radjust is used for matching the thermal coefficient of RNTC and DCR. It’s targeted to let the VISNS not changed with temperature variation.

Why do we need a second stage RC filter to extract DC current information?

  • Low frequency and middle frequency inductor current information are needed for feedback to reflect output current DC value and dynamic changing.
  • High frequency response needs to be limited, since the inductor current ripple needs to be filtered.
  • Two stage filter can bring -40 dB/dec gain slope at high frequency. Figure 2-4 shows how the two stage filter can have lower gain at high frequency while maintaining larger gain in middle frequency range.
Figure 2-4 Comparison of Frequency Response with One-Stage RC Filter and Two-Stage RC Filter