TIDUE82A November   2021  – April 2024

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
    3. 2.3 Highlighted Products
    4. 2.4 System Design Theory
      1. 2.4.1 Inverting Power Supply Design Using TPS62933
  9. 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 Load Transient
      3. 3.3.3 Output Ripple
      4. 3.3.4 Thermal Performance
  10. 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
  11. 5About the Author
  12. 6Revision History

2.2 Design Considerations

Details of the main differences for when the buck converter is configured as an inverting buck boost follow:

  • The input voltage that can be applied to an inverting buck-boost converter IC is less than the input voltage that can be applied to the same buck converter IC. This is because the ground pin of the IC is connected to the (negative) output voltage. Therefore, the input voltage across the device is VIN to VOUT, not VIN to ground. Thus, the input voltage range of the TPS62933 device is 3.8 V to 30 V – VOUT, where VOUT is a positive value.
  • The output voltage is the same as when configured as a buck converter, but negative. So, set the output voltage for the inverting buck-boost topology from –0.8 V to –22 V. The output voltage is set the same way as in the buck configuration, with two resistors connected to the FB pin.
  • In the buck configuration, the average inductor current equals the average output current because the inductor always supplies current to the load during both the on and off times of the control MOSFET. However, in the inverting buck-boost configuration, the load is supplied with current only from the output capacitor and is completely disconnected from the inductor during the on time of the control MOSFET. During the off time, the inductor connects to both the output capacitor and the load. Thus, the output current that the device can hold in the buck-boost application is lower than in the buck application. The output current varies with the value of inductor, input voltage, and output voltage.
  • The TPS62933 device is enabled when the voltage at the EN pin trips the threshold, and the input voltage is above the UVLO threshold. The device stops operation when the voltage on the EN pin falls below the threshold, or the input voltage falls below the UVLO threshold. However, when configured as a buck-boost application, the GND pin of the TPS62933 device is tied to the negative output voltage and not the zero voltage (system ground), which can cause difficulties enabling or disabling the device. So, level-shifting circuitry is needed to solve the problem.