Boost and buck-boost: raising voltage
Store energy in an inductor and release it stacked on top of the input, and the voltage rises. Boost, buck-boost, and the choice in battery devices.
Boost
When the switch is on, the inductor stores energy from the input current; when it turns off, the inductor voltage adds to the input voltage and goes to the output through the diode.
Vout = Vin / (1 - D)
3.7 V battery → 5 V: D ≈ 0.26
Points to watch: - The input current is larger than the output current. Making 5 V at 1 A draws about 1.5 A or more (efficiency included) from the battery. - An ordinary boost leaks input to output through the diode even when it is off. If you need disconnection, use an IC with output disconnect.
Buck-boost
Used when the input moves above and below the output. Making 3.3 V from a lithium battery (4.2 V → 3.0 V) is the typical case.
| Topology | Characteristics |
|---|---|
| Inverting buck-boost | Output polarity is reversed (negative supply) |
| 4-switch buck-boost | Polarity kept, efficient, the mainstream in modern ICs |
| SEPIC | Polarity kept, coupled inductor |
How to choose
| Input range against output | Choice |
|---|---|
| Always higher | Buck |
| Always lower | Boost |
| Spans it | Buck-boost, or boost followed by an LDO |