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

Doug Ronald doug at dougronald.com
Thu Dec 11 13:24:53 EST 2014

There exists a newer generation of linear regulators with much lower noise, designed for sensitive analog loads. Here are some representative parts. Check Analog Devices' website for other options...
6.5 V, Ultra Low Noise, High PSRR, Fast Transient Response CMOS LDO

Doug R.
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of ed breya
Sent: Thursday, December 11, 2014 9:48 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Linear voltage regulator hints...

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.


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