[time-nuts] Re: Death of a Capacitor

[Gerhard Hoffmann]

Am 27.09.21 um 10:16 schrieb nuts at lazygranch.com:
> I've only designed one LDO as a discrete chip (as opposed to a portion 
> of a chip where performance just has to be good enough), so I have no 
> guru status. That said, what spikes pass through a LDO if you do it 
> right is simply a capacitor divider comprised of the capacitance 
> across the pass device and the filter capacitor. This is a bit more 
> predictable with a PFET pass than a PNP. 
FET and predictable does not go together well. FET data sheets are 
seldom more than a page and normally don't promise hard limits. And 
then, like for the IF3602 there comes V2 with reduced claims after 20 
years, much more like what we used to measure in real life, still 
slightly optimistic.

> https://www.analog.com/en/products/lt3045.html You can see the PSRR 
> after a point (200kHz) rolls off and appears to flatten. I assume the 
> error amp is out of loop gain. It goes flat for a while. The idea here 
> is the drive on the pass device is constant and just maintains the DC 
> voltage. The AC rejection is mostly due to capacitance ratios. This 
> being a bipolar pass device there is some secondary effect here where 
> after 2MHz the rejection improves then goes flat again.
I would not call nearly 80 dB PSSRÂ  to 2 MHz bad. And @ 2MHz it is no 
longer really needed. A simple, cheap RC/LC pole does wonders there 
given it has some decades to develop its attenuation.

Where it really counts is in the low Hz region, when even costly 10000uF 
show barely any effect.

Gerhard