[time-nuts] Linear voltage regulator hints...

[ed breya]

If the plan is to use a three-terminal regulator after all, I'd 
suggest not using a low-dropout (LDO) type if the raw input supply is 
noisy - the LDOs usually have PNP output transistors (for positive 
regulators), so may tend to have poorer HF input ripple rejection 
than equivalent ones with NPN passers. At low frequencies this is no 
problem since the regulator loop takes care of it, but as the loop 
rolls off, the PNP becomes a common-base amplifier, allowing more HF 
from the input to pass on through. I alluded to this in my previous 
post - from an input HF rejection perspective, it's usually best to 
use an NPN passer for positive supplies, and conversely a PNP for 
negative, working as an emitter-follower.

If the raw input comes from a switching supply, there will tend to be 
a lot of HF ripple, so this could be a concern. If this is the case, 
another option is to have a two-stage regulation scheme with as much 
pre-regulation and filtering as possible. This of course eats into 
the overhead budget, so may not be practical in many situations.

Ed