The iDAC2 micro just like the original iDAC it replaces / draws it's ppower from the USB connection. The iPhone and many Android devices either cannot supply sufficient power or have small batteries which would be quickly depleted.
We have tested the iDAC2 micro as drawing between 200 and 250mA (depending on sample rate and loudness of headphone Amplifier. Given the usual design of the USB Host mode on Android and iOS devices this translates into a draw of around 1.5Wh or with a common Lithium based battery of 430mA/h. So if a given Phone delivers (say) 10 Hours playback time while fitted with a 4,300mAh battery, powering the iDAC2 micro will cut this playback time in half.
When using Laptops or desktop PC's the extra 1.5Wh power draw is usually inconsequential to operating time. The iDAC2 micro is designed for semi-stationary use on Desktop, for a portable/mobile solution the iDSD nano and iDSD micro are recommended, as they both offer up to around 8...10 Hours playback time in mobile operatioin without drawing power from Phone/Tablet etc.