In post filtering of power supply noise from the switching regulator, when to use an LC filter and when to use an LDO?

Both LC filter and LDO has its functions in filtering power supply noise. It’s a matter of matching the power supply noise output to the maximum allowable noise of the RF device. If the low frequency noise exceeded the maximum limit, you should use an LDO. However, LDOs are power hungry. If the low frequency noise has good margin from the maximum limit but the switching noise exceeded, you can avoid using an LDO and opt to use an LC filter to optimize overall efficiency