TIDUEY0A November   2020  – December 2025

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Li-ion Cell Formation Equipment
    2. 1.2 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 System Design Theory
      1. 2.2.1 Feedback Controller
      2. 2.2.2 DC/DC Start-Up
      3. 2.2.3 High-Resolution PWM Generation
      4. 2.2.4 Output Inductor and Capacitor Selection
      5. 2.2.5 Current and Voltage Feedback
    3. 2.3 Highlighted Products
      1. 2.3.1 TMS320F28P650DK
      2. 2.3.2 ADS9324
      3. 2.3.3 INA630
      4. 2.3.4 UCC27284
      5. 2.3.5 REF50E
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Software
      1. 3.2.1 Opening the Project Inside Code Composer Studio
      2. 3.2.2 Project Structure
      3. 3.2.3 Software Flow Diagram
    3. 3.3 Test Setup
      1. 3.3.1 Hardware Setup to Test Bidirectional Power Flow
      2. 3.3.2 Hardware Setup to Tune the Current and Voltage Loop
      3. 3.3.3 Hardware Setup for Current and Voltage Calibration
      4. 3.3.4 Lab Variables Definitions
      5. 3.3.5 Test Procedure
        1. 3.3.5.1 Lab 1. Open-Loop Current Control Single Phase
          1. 3.3.5.1.1 Setting Software Options for Lab 1
          2. 3.3.5.1.2 Building and Loading the Project and Setting up Debug Environment
          3. 3.3.5.1.3 Running the Code
        2. 3.3.5.2 Lab 2. Closed Loop Current Control Single Channel
          1. 3.3.5.2.1 Setting Software Options for Lab 2
          2. 3.3.5.2.2 Building and Loading the Project and Setting up Debug Environment
          3. 3.3.5.2.3 Run the Code
        3. 3.3.5.3 Lab 3. Open Loop Voltage Control Single Channel
          1. 3.3.5.3.1 Setting Software Options for Lab 3
          2. 3.3.5.3.2 Building and Loading the Project and Setting up Debug Environment
          3. 3.3.5.3.3 Running the Code
        4. 3.3.5.4 Lab 4. Closed Loop Current and Voltage Control Single Channel
          1. 3.3.5.4.1 Setting Software Options for Lab 4
          2. 3.3.5.4.2 Building and Loading the Project and Setting up Debug Environment
          3. 3.3.5.4.3 Running the Code
        5. 3.3.5.5 Lab 5. Closed Loop Current and Voltage Control Four Channels
          1. 3.3.5.5.1 Setting Software Options for Lab 5
          2. 3.3.5.5.2 Building and Loading the Project and Setting up Debug Environment
          3. 3.3.5.5.3 Running the Code
        6. 3.3.5.6 Calibration
    4. 3.4 Test Results
      1. 3.4.1 Current Load Regulation
      2. 3.4.2 Voltage Load Regulation
      3. 3.4.3 Current Linearity Test
      4. 3.4.4 Voltage Loop Linearity Test
      5. 3.4.5 Bidirectional Current Switching Time
      6. 3.4.6 Current Step Response
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Tools and Software
    3. 4.3 Documentation Support
    4. 4.4 Support Resources
    5. 4.5 Trademarks
  11. 5About the Author
  12. 6Revision History

2.2.5 Current and Voltage Feedback

The design targets for ± 0.02% FS current and voltage control and measurement accuracy over ±5°C temperature variation. Current sense is done with an instrumentation amplifier to measure the voltage across the shunt and scale to ADC input voltage.

The offset and gain errors are not a big concern since they can be calibrated in the equipment. However, gain and offset drift is the critical parameters for accuracy ±0.02% or ± 200 ppm. A smaller shunt resistor, helps reduce the thermal dissipation and total temperature drift when there is a large current going through the output stage. The reference design uses 2 mΩ, 25ppm/°C current shunt for 10A full-scale current range. INA630 is used for current sense, the indirect current feedback (ICFB) topology of this instrumentation amplifier makes it a cost-effective approach because this device eliminates the process of laser trimming the internal precision trimmed resistor, and the gain is set through a discrete external resistor.

The INA630 has 0.5μV/°C input offset drift and 3-ppm/°C typical gain drift, for a 2-mΩ sense resistor and 10-A full-scale current rating, the signal chain drift is 25.18ppm/℃. Since the gain is dictated by the matching of an external resistor divider, the gain drift can be reduced by even better temperature coefficients. For best performance, using matching resistor pairs can achieve lowest errors of gain drift.

The total ADC drift comes from the voltage reference, for with REF50E the temperature drift is 2.5 ppm/°C. The total unadjusted error of the current path is ±5℃×(252×(2.5)2 ppm/℃=±126.5ppm, which matches the design requirements.

The voltage sense path gives a better error margin over temperature change. The ADS9324 has 1MΩ input impedance and can support an input range of ±5 V through an integrated programmable gain amplifier. For both positive and negative analog input, it can connect to 5V battery directly, eliminating the external difference amplifier and reducing BOM cost. It also has 100dB minimum CMRR to support high-accuracy voltage sense. For a swing of ±1 V in a 5V cell, the error due to CMRR is ±5μV or ±10ppm of the full-scale voltage range. The offset drift is 0.5ppm/℃, so the total error can be much simpler to reach below 50ppm margin.