Let's examines the Lithium-ion battery life of both a multi-mode buck-boost converter and a buck converter for a 3.3V micro hard disk drive application.
An arbitrarily large converter would provide better efficiency and a longer battery life than a smaller converter, regardless of topology. Therefore, for a fair comparison with meaningful results, converters of equal size were tested. Both converters had similar power MOSFET RDS(ON) and inductor DCR resistance, resulting in roughly equal losses and battery discharge characteristics above 3.6V. Both converters also used a 3x3mm IC package with similar sized external inductors and capacitors, so that the total solution footprint area was the same.
The buck battery life was defined as the time required for the output to decay to 5% below the nominal 3.3V output. The buck-boost converter battery life was defined as the time at which the battery and converter shut down.
A fully charged battery was applied to the input of the converter and a constant current load was applied to the output. The battery and converter voltage were recorded at specific intervals until the battery was fully discharged.
|| Buck Integrated Converter
||4 Switch Buck Boost Integrated Converter
||125mΩ 3x3mm TDFN
||2 x 100mΩ 3x3mm TDFN
| Inductor DCR
||3.3μH 130mΩ 3x3x1.4mm
||2.2μH, 110mΩ, 3x3x1.4mm
Table 1: Converter Characteristics