I have a buck converter (LT8631) that is operating at 400kHz with 250mVpp and I'm looking for an LDO (output of 5V) that can smooth out that ripple to below 1mVpp (roughly 50dB attenuation). Since the output is a triangle wave with a fundamental at 400kHz, I need something that has good PSRR at 400kHz (and above due to the higher frequency components in the triangle wave). I've gone through the datasheets for about 30 chips so far, and they all have PSRR at that frequency of no more than about 36dB. Current capacity must be over 1A, and low dropout is preferred to keep the thermal dissipation low.
Alternatively, I may be going about this all wrong, and it would be better to use a PI filter (or ferrite bead PI filter) prior to the LDO. Feel free to comment if I'm taking the wrong approach. I've also heard of people having good success with a capacitance multiplier prior to the LDO, although that will burn more power which isn't ideal for a battery-powered situation.