Hi all,
I am currently re-visiting my current APS power switch design to make sure
everything works (in theory at least) according to spec.  As I do so, I'm
finding myself slightly confused about the soft start/soft off features.
The TPS2490 controller IC offers this feature, but the documentation is
rather vague as to how it is implemented.  I think my understanding of
soft-start is reasonably sound, but I suppose I should first double check
this before finalizing any design.  As I understand it, the soft-start
function is turning on the FET slowly, rather than full on, to allow any
transients to die out before the switch is on.  If I'm wrong here, please
correct my thinking ASAP!! =)  Also as I understand it, a gate capacitor can
be added from the gate pin to ground, with a series resistor, to controll
the dV/dt of the FET's gate (this control over dV/dt being the soft-start
function).  If this thinking is correct, then we should be able to find the
value of this capacitor based on specs, right? e.g. "Must allow transients
of 100% for 100ms w/o fault to load."  This seems easy enough (if my
reasoning is correct, that is), however I found an excel tool (attached if
anyone's interested) which seems to point to the fact that the soft-start
function of the TPS2490 is implemented by connecting a capacitor from the
prog pin to ground.  This is implied in the line directly under the sample
schematic: "The calculations are for no external caps on prog (no
soft-start)..."  If this is indeed how soft-start is implemented,
my previous reasoning is totally wrong, which makes me a little nervous
about the rest of this circuit... Also, the datasheet offers no ideas on how
to calculate this particular capacitor value.  Hopefully, someone out there
can offer some insight on this matter and re-assure me that I'm not totally
backwards here.


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