The load conditions of an application can vary significantly. The system can be in idle or power down mode for long periods of time consuming only microAmps. Then, in active mode the current draw can be in the tens of milliAmps. A typical external DC-DC converter is not designed to adapt to changing load conditions, which affects the overall conversion efficiency adversely. The boost converter of ATtiny43U is tightly coupled with the MCU and it is capable of quickly adapting to the varying loads. When there is enough load, say more than 0.6mA, the boost converter will work in Regulated Mode and the efficiency of the converter is typically between 75 to 85%. At very light load current  the efficiency would dramatically decrease.To avoid this, the converter switches to operate in Low Current Mode. This greatly improves the conversion efficiency and increases battery life. Toggling between these two modes is automatic and requires no MCU control.