[time-nuts] Switching regulator replacement for 7805
attila at kinali.ch
Sun Dec 4 18:02:05 EST 2016
On Sun, 4 Dec 2016 16:41:13 -0500
John Ackermann N8UR <jra at febo.com> wrote:
> The little OCXO I used for testing goes from about 850ma cold to 200ma
> hot (at 5V) and I noticed a very different appearance of the spectrum as
> the current dropped -- at high current, the switching spurs were quite
> narrow, but as the current dropped (and particularly below some
> threshold I don't remember) the spurs widened out quite a bit.
This is exactly the PWM to PFM mode switch I described earlier.
The 850mA are probably high enough that the DC/DC converter works
in PWM mode, thus switching with a constant frequency, even if the
load changes. On the other hand 200mA of a 1A converter is low enough
that it's most likely in the PFM region, where every small change
(or noise) causes the switching frequency to change.
Any simple idea will be worded in the most complicated way.
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