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

2.2.5 Battery Ideal Diode-OR

To discharge the two independent batteries in the design, an ideal diode-ORing circuit is used. In this circuit, Q3 and Q4 are driven by U5 and U7 to regulate the FET forward voltage drop to 20 mV. This allows for the efficient discharge of both batteries at the same time, while also preventing current from traveling from one battery to the other. This circuit also allows for passive battery pack voltage balancing to occur in the system by drawing more current from the battery with a higher voltage. Equation 2 shows an example calculation of battery discharge current based on voltage and equivalent series resistance (ESR). For this example, it is assumed that BAT_A is at 16.8 V and has an ESR of 50 mΩ, BAT_B is at 16.6 V and also has an ESR of 50 mΩ, and the system is drawing 8 A.

ILOAD=IBATA+IBATB
VLoad=VBATA-VAf-(IBATA×ESRBATA)
VLoad=VBATB-VBf-(IBATB×ESRBATB)
VBATA-VAf-IBATA×ESRBATA=VBATB-VBf-(IBATB×ESRBATB)
16.8 V-20 mV-IBATA×50 mΩ=16.7 V-20 mV-IBATB×50 mΩ
Equation 2. IBATA=IBATB+2.0, where IBATB≥0

In this example, BAT_A supplies 2 A more current that BAT_B until the higher current discharge causes the voltage differential between the batteries to be reduced.