30.6 A 98.6%-Peak-Efficiency 1.47A/mm2-Current-Density Buck-Boost Converter with Always Reduced Conduction Loss

Buck-Boost converters are commonly used in the Li-ion battery-powered mobile devices to convert a varying battery voltage (e.g., 2.7V to 4.2V) to a mid-3V output voltage
$(\mathrm{V}_{\text{OUT}})$
for supplying various function blocks such as RF power amplifiers. To extend the battery life and to meet the increasing loading demands, the converters should maintain a high efficiency throughout the whole range of the battery voltage
$(\mathrm{V}_{\text{IN}})$
with a high driving capability. In the conventional buck-boost topology [1], there are always two power switches connected to the inductor in the current paths, resulting in a large conduction loss and thus a low efficiency. To reduce the conduction loss, flying-capacitor based topologies that only have one power switch in the current paths are proposed in [2], [3]. However, one of the power switches has to withstand a voltage stress of
$\mathrm{V}_{\text{IN}}+\mathrm{V}_{\text{OUT}}$
in [2] or
$2\mathrm{V}_{\text{IN}}$
in [3], and thus a high-voltage process or stacked transistors is needed, which increases the fabrication cost and degrades the current density. Topologies using more power switches and flying capacitors are proposed in [4], [5] to overcome the voltage stress issue. However, the chip current density is significantly compromised and the switching loss is also increased. Moreover, compared with [1], the inductor current is not reduced in [2] or only reduced in the boost mode in [3–5], which causes high conduction loss.