Yes you will need a USB DAC if you connect your Mobile via its USB port for FM, which you have mentioned in the post quoted below. If you connect a 3.5mm stereo cable to your mobile's headphone jack, then No, you do not need a DAC.
Thanks ... FM I meant while playing FM from mobile and mobile is connected via USB DAC
Yes, when connecting to the Mobile USB, you will need a DAC.
MaSh