SNVAA61 February   2023 LM5168 , LM5168-Q1 , LM5169 , LM5169-Q1

 

  1.   Inverting Buck-Boost Application for LM5168
  2.   Trademarks
  3. 1Inverting Buck-Boost Topology
    1. 1.1 Concept
    2. 1.2 Output Current Calculations
    3. 1.3 Voltage Range of Inverting Buck-Boost Configuration
  4. 2Design Considerations
    1. 2.1 Bypass Capacitor and Optional Schottky Diode
  5. 3External Components
    1. 3.1 Capacitor Selection
    2. 3.2 Inductor Selection
  6. 4Digital Pin Configurations
    1. 4.1 Optional Enable (EN) Level Shifter
    2. 4.2 Power-Good (PG) Pin
  7. 5Typical Performance
    1. 5.1 VOUT = -12 V Typical Performance
    2. 5.2 VOUT = -24 V Typical Performance
    3. 5.3 VOUT = -48 V Typical Performance
  8. 6Conclusion
  9. 7References

1.1 Concept

For the standard buck converter, the inductor is connected to VOUT and the switch pin (SW) of the LM5168. In order to change to an inverting buck-boost topology, the VOUT and ground nodes of the circuit must be reversed. With the nodes reversed, the LM5168 can now invert the output voltage from the input voltage.

To change an LM5168 buck converter to an inverting buck-boost, reassign the buck converter VOUT to system ground, and the old buck system ground to -VOUT. The input capacitor will need to be reconnected to the new system ground, and a new bypass capacitor, CIO, is needed between VIN and -VOUT. The positive input and the feedback resistors will remain the same as in the buck converter. To adjust the output of the inverting buck-boost, calculate the feedback resistor values as if it was a buck converter. For further reading on the inverting buck-boost topology, refer to the Working with Inverting Buck-Boost Converters application note. The schematics in #ID-B6C9A22A-CA23-4644-FA3C-C4FA7281CD60 show the changes that have to be made when configuring the LM5168 buck converter as an inverting buck-boost converter.

Figure 1-1 Converting From Buck to Inverting Buck-Boost Topology