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Buck- Boost Loop Voltage Mode

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Buck-Boost (Non-Isolated)

Used for:

Stepping a voltage up or down  from a positive input to a negative output with high efficiency

Pros:

• Steps positive input up to any negative value or vice versa
• Simple circuit
• Current limiting/short circuit protection relatively easy because of series switch
• Requires high-side gate drive

Cons:

• High output ripple - can be > output DC current so need large, low ESR output caps
• Only negative output available (for +ve input) or vice versa

Note:

There is a version of a buck-boost called the four transistor buck boost which gives a positive output for a positive input, lower or higher than the output, using one inductor.

This is simply a circuit which can be physically switched between classic buck or boost. It is more expensive because it has redundant switches and is more lossy as it has an extra switch element in the output path as a buck converter and in the input path as a boost converter. There are some control ICs like LM5175 that can do the switch selection automatically.

The inductor can’t be optimum for both modes though. Why not just use a SEPIC?

Can be useful at low power when the transistors are integral to the control IC