SNVA856B May   2020  – October 2022 LM63615-Q1 , LM63625-Q1 , LM63635-Q1 , LMR33620 , LMR33620-Q1 , LMR33630 , LMR33630-Q1 , LMR33640 , LMR36006 , LMR36015 , TPS54360B , TPS54560B

 

  1.   Working With Inverting Buck-Boost Converters
  2.   Trademarks
  3. 1 Introduction
  4. 2 Inverting Buck-Boost Converter
  5. 3 Basic Operation
  6. 4 Operating Considerations of a Buck Based Inverting Buck-Boost
    1. 4.1 Voltage Stress
    2. 4.2 Current Stress
    3. 4.3 Power Loss and Efficiency
    4. 4.4 Small Signal Behavior
      1. 4.4.1 Measuring IBB Bode Plots
      2. 4.4.2 Testing Load Transients on an IBB
      3. 4.4.3 Simulation
  7. 5 Component Selection for the IBB
    1. 5.1 Inductor Selection
    2. 5.2 Capacitor Selection
    3. 5.3 External Feed-back Divider
  8. 6 General Considerations
  9. 7 Auxiliary Functions
    1. 7.1 Enable Input Level Shift
    2. 7.2 Synchronizing Input Level Shift
    3. 7.3 Power-Good Flag Level Shift
    4. 7.4 Output Clamp
    5. 7.5 Output Noise Filtering
  10. 8 Design Examples
    1. 8.1 Converting +12 V to –5 V at 3 A
    2. 8.2 Converting +5 V to –5 V at 1 A
  11. 9 Summary
  12. 10References
  13. 11Revision History

9 Summary

To convert a buck DC/DC converter to an IBB follow these steps:

  • Calculate the maximum voltage seen by the regulator from: VIN + |VOUT|
  • Based on an assumed efficiency and a ΔIL = 30% of IOUT, use Equation 4 and Equation 6 to calculate the maximum inductor currents.
  • Choose a suitable buck DC/DC regulator to use as the IBB.
  • Refer to the data sheet to size the device capacitors and feedback dividers, etc.
  • Decide if any auxiliary or level shifting circuits, etc. are required
  • Make the power connections shown in Figure 2-2 as follows:
    1. Reassign buck positive output as system ground.
    2. Reassign buck regulator ground nodes as the negative output voltage node.
    3. Positive input stays the same.
  • Build the design. Be sure to follow the PCB layout guidelines in the buck data sheet. Pay particular attention to the routing of the input and output capacitors and the FB connections.
  • Test the design. Check thoroughly over the entire input voltage and load current range at the expected limits of ambient temperature. Check load transients and stability margins using Bode plot tests. Finally, check the IC temperature rise to be sure that it is within the specified limits for the device.
  • Texas Instruments provides many resources for designing an IBB from a buck converter. In some cases the TI Webench Design Tool can be used www.ti.com/WEBENCH . In many cases application reports are available for specific Texas Instruments devices detailing the conversion to an IBB. Please visit TI.com for more information.