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.
I would guess at 200mA the switcher is still operating PWM. With that
tiny little inductor, the inductor has probably fallen into
discontinuous current mode. Usually this is bad for noise generation. It
would be interesting to see what a scope trace of the switched leg of
the inductor looks like in both cases.
Dan
On 12/5/2016 12:00 PM, time-nuts-request@febo.com wrote:
> 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.
>
I would guess at 200mA the switcher is still operating PWM. With that
tiny little inductor, the inductor has probably fallen into
discontinuous current mode. Usually this is bad for noise generation. It
would be interesting to see what a scope trace of the switched leg of
the inductor looks like in both cases.
Dan
