In general, the purpose of a soft start feature is to limit the turn on inrush current into the regulator as a whole. This is important because in some applications a high inrush current could "brown out" the input voltage rail which may be unacceptable. Although I have not looked at the datasheet for this particular regulator, a have worked with other regulators that use a capacitor to ground to provide soft-start functionality. Typically, a fixed current source inside the regulator IC charges the capacitor, and the pin voltage increases linearly with time. The IC then uses the pin voltage to adjust its current limit in some fashion. Obviously, the larger the capacitor value, the longer the soft-start time.

Regarding the idea of slowing down the turn on time of the FETs, this is usually a bad idea. If you slow down the FET turn on (or turn off) time, then the FET will spend a greater percentage of its time in its linear region of operation. As a result, the AC losses of the FET will increase and its temperature will increase. In the end, this means that you have to thermally size the FET larger (i.e. higher part cost) than you would otherwise have to.

On the other hand, it is true that fast FET transients often produce radiated emissions (EMI) problems. However, these emissions can usually be eliminated by careful attention to the layout and the addition of some high frequency filtering.


Scott Schuehle wrote:
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. Thanks!!! Scott ------------------------------------------------------------------------

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