[time-nuts] Re: Death of a Capacitor

[nuts at lazygranch.com]

Generally this fet is internal and thus well known.

On Mon, 27 Sep 2021 11:57:19 +0200
Gerhard Hoffmann <ghf at hoffmann-hochfrequenz.de> wrote:

> 
> 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
> 
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