The LM5168 is a synchronous buck converter with a wide input voltage range from 6 V to 115 V and maximum output current of 300 mA. The LM5168 can be configured as an inverting buck-boost (IBB) converter with a negative output voltage. This application note demonstrates how the LM5168 can be used as an inverting buck-boost converter, along with optional design considerations for inverting buck-boost converters such as a PGOOD or EN level-shifter. If higher output current is required, the LM5168 is pin-to-pin compatible with the 600-mA rated LM5169. Automotive grade versions, the LM5168-Q1 and LM5169-Q1, are also available.
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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.