TIDUEB2A July   2022  – July 2022

 

  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  Power Multiplexing Circuit Design Parameters
      2. 2.2.2  Input Connections and Filter
      3. 2.2.3  Reverse Polarity Protection
      4. 2.2.4  Battery Charger Input
      5. 2.2.5  Battery Ideal Diode-OR
      6. 2.2.6  Input and Battery Switchover Mechanics
      7. 2.2.7  LM74800 (U1) HGATE
      8. 2.2.8  Battery LM74800 HGATE
      9. 2.2.9  BQ25731 Design Considerations
      10. 2.2.10 BQ25731 Component Selection
      11. 2.2.11 ILIM Circuit
      12. 2.2.12 MCU and I2C Bus Design Considerations
      13. 2.2.13 MSP430FR2475
      14. 2.2.14 I2C Bus Overview
      15. 2.2.15 MSP430 Connectors
      16. 2.2.16 MSP430 Power Supply
      17. 2.2.17 Sensing Circuits
      18. 2.2.18 Current Sensing
      19. 2.2.19 Voltage Sensing
      20. 2.2.20 Input Comparators
      21. 2.2.21 Software Flow Chart
    3. 2.3 Highlighted Products
      1. 2.3.1 BQ25731
      2. 2.3.2 LM7480-Q1
      3. 2.3.3 LM74700-Q1
      4. 2.3.4 MSP430FR2475
      5. 2.3.5 PCA9546A
  8. 3Hardware, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Test Setup
    3. 3.3 Test Results
      1. 3.3.1 Adaptive Charge Current Limiting
      2. 3.3.2 Battery ORing System
      3. 3.3.3 Circuit Switchover From Adapter to Battery
  9. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Documentation Support
    3. 4.3 Support Resources
    4. 4.4 Trademarks
  10. 5Revision History

2.2.11 ILIM Circuit

For this design a circuit was added to limit the system current to 8 A. This is most impactful when the device is being used with a 12-V car adapter and can pull the battery voltage below the input threshold of the system. This circuit includes the INA213B to amplify the current across a 2-mΩ sense resistor. This INA213B has a gain of 50 V/V so an input of 8 A results in an 800-mV output.

GUID-20220516-SS0I-WVFN-JD5J-JW99SRJWV5ZJ-low.gif Figure 2-4 Current-Limit Circuit

To convert this signal into a usable voltage for the ILIM pin of the BQ25731 devices, an op amp configured as a difference amplifier was used. The voltage on the BQ25731 ILIM pin is converted to charge current based on Equation 5.

Equation 5. V(ILIM_HIZ) = 1 V+40×IDPM×Rac

where

  • IDPM is the target input current limit
  • Rac is the 5- or 10-mΩ resistor chosen for the BQ25731
Figure 2-5 Difference Amplifier

For the difference amplifier when:

Equation 6. Vout=V2-V1

where

  • R 1 = R 2 = R f = R g ,   V o u t = V 2 - V 1

To correctly set the BQ25731 charge current, set the ILIM pin at 1.0 V with 8 A of system current and 1.8 V with 0-A system current. Limit the charge current for each of the 2 charger devices to between 0 A and 4 A.

Set V2 at 1.8 V in this case. With V2 at 1.8 V, the voltage at the amplifiers positive input is equal to V2/2, which is 0.9 V. This voltage then needs to be created with the system 3.3-V rail and a resistor divider to match the calculated set point. This was implemented with a R7 and R9 as shown in the schematic above.