Is there a DC-DC converter topology that can output positive and negative?
In theory a buck converter can produce any output voltage between the input and ground, while an inverting buck-boost converter can produce any output voltage below ground.

The relay approach
As the two topologies are very similar, you could switch between them using a DPDT relay. While not very elegant, this incurs only a modest increase in part count and board area. Unfortunately, you do need a custom controller for controlling this, but any simple microcontroller with an ADC would likely do just fine.



simulate this circuit &ndash; Schematic created using CircuitLab

Solid state
I later came up with a solid state (relayless) four quadrant variant, which can source and sink current at both positive and negative voltages, while maintaining the same ground reference.



simulate this circuit
Just like the relay circuit, this topology combines a buck converter with a buck-boost converter.


The benefits are instantaneous switching between step-down and inverting mode, the lack of moving parts, and the more compact size.

The main drawback I see is that the gate drivers of M3 and M4 must be able to swing their outputs below ground, while the digital control inputs are probably going to be between zero and a logic high (e.g. 3.3 V). This complicates the selection of gate driver chips.