Generally this fet is internal and thus well known. On Mon, 27 Sep 2021 11:57:19 +0200 Gerhard Hoffmann <[email protected]> wrote:
> > Am 27.09.21 um 10:16 schrieb [email protected]: > > 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 PSSR 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 > > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe > send an email to [email protected] To unsubscribe, go to > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
