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

6 General Considerations

In general the recommendations in the buck data sheet can be followed during the detailed design of the IBB, while taking account of the differences in operation outlined above. This should give a good starting point and a solution that can be performance tested. Typically the output capacitors may need to be adjusted from this point in order to meet load transient performance. The example PCB layout shown in the buck data sheet should also be used as a starting point for the IBB. As with any DC/DC design the IBB must be thoroughly tested before committing to production. This includes basic functionality and efficiency over the operating input voltage, output voltage, temperature and load. Bode plots and load transient testing should also be conducted, in order to ensure that the design has adequate stability margins.

Most regulators have Spice models available that can be used to simulate an ordinary buck application. However, these models cannot be used with an IBB, unless they are specifically designed for the inverting configuration. This is true since most Spice models have internal global ground connections. This prevents the use of the "GND" node as the negative output node. Always check the model before using it in an IBB application.