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

7.1 Enable Input Level Shift

If it is required that the system control the enable function of the IBB, then a simple level shifter is required. Two possible circuits are shown in Figure 7-1.

GUID-8770D391-84CD-4E36-B439-B71740802A55-low.gifFigure 7-1 Enable Level Shifters

The circuit on the left has a large hysteresis in the threshold and requires a small amount of current from the controlling logic. However, it requires only one transistor. The circuit on the right has no hysteresis and requires no current from the logic, but requires two transistors. In any case, be sure that the EN input of the buck can withstand the full input voltage rating of the regulator (from datasheet Absolute Maximum Ratings section). If not, then a Zener diode, rated below the maximum enable voltage limit, must be used as shown. Even if the system is not required to control the enable of the regulator, this input must still be terminated correctly to keep the regulator turned on. In this case, consult the buck datasheet for the correct connections to the enable input.

In some systems the user may wish to use the enable control as an input undervoltage lockout (UVLO) feature. The best way to do this would be to use one of the enable level shifters shown, with an external op-amp/reference to provide the UVLO function. The TLV6713 , or similar product, could be used with one of the circuits shown in Figure 7-1 to provide an input UVLO feature for an IBB.