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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