On 9/27/21 2:57 AM, Gerhard Hoffmann 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.
Actually, I like that it still has significant rejection even higher.
Sure a single RC or RLC has good effect, but you can use smaller L and C
to get the same "whole circuit" rejection. What we've done with these
is set the voltage of the regulator a few tenths higher to account for
the IR drop in the LC filters (which have parasitic R in the L) so that
the "at load" voltage is right. For most of the MMIC RF ampliifers, the
bias current is essentially constant (changing with temperature) since
we're in a (very) small signal Class A regime. The change in bias with
temperature (and any gain change too) gets calibrated out later -
because what's important to me is "quiet" from 0.1 to 30 MHz, and in
particular, suppressing 700-800 kHz (and harmonics) from the upstream
DC/DC converters.
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