For single battery applications, other devices need to use a standalone DC/DC converter. It is not possible to switch off an external boost converter, otherwise the MCU can never wake-up from sleep. Therefore, there is a big advantage in sleep mode current when the DC/DC converter is integrated on-chip. If the application needs more current, it is still very efficient to use C8051F900 with an external DC/DC converter because the C8051F900 will allow it to be switched completely off.