[time-nuts] Power Supply Noise Affects Thunderbolt 1 PPS

gary lists at lazygranch.com
Mon Feb 27 09:29:11 UTC 2012

On 2/27/2012 12:48 AM, Attila Kinali wrote:
> If you are really time-nutty, you can let the DC/DC converters produce
> a voltage about 1V above what you need and use low noise LDOs (ie not
> the 78xx or LM317&  Co) to produce the voltages for the thunderbolt.
> This should give you a 60-80dB damping of the noice produced by the
> DC/DC converters.
> 			Attila Kinali

Having designed LDO chips, people expect them to perform miracles well 
beyond reality. If you have a PNP pass and you are sitting near dropout, 
you get control loops that are an ugly combination of a path to keep the 
PNP from getting saturated plus one to control the voltage. With P-fet 
pass devices, the control is better (no sat killer needed with a fet), 
but still you are trying to regulate with what amount to be a variable 

You really don't get that much filtering at switcher frequencies with 
LDOs, plus some regulators can't handle too low of an ESR. If you care 
about noise, screw efficiency and go with a shunt regulator. There are 
hack circuits on the net to take 431s plus external components to roll a 
decent shunt. Note the shunt regulator makes the DC/DC happy by 
presenting a uniform load.

If you do a shunt right, the bypass capacitor will do all the work. This 
is somewhat true with a P-fet pass regulator, where Cds is forming a cap 
divider with your bypass. I never really warmed up to PNP pass devices, 
but they are best for high voltage applications.

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