TPS62140 Rise Time of Vout with no Soft Start capacitor

The rise time would be limited by the current limit and current capability of the input bus.  It also depends on the output capacitance and load current.

Your customer can measure it on the EVM, under their conditions.

Why do they not want soft start?

Hi Chris,

The device must have some minimum soft-start specification.

Equation 10 in the datasheet (Css = tss x 2.5uA / 1.25V) does not include inductor or output capacitor values.
Also figures 33 and 34 show the same start up time of 1600usec for 2 different load currents (100mA and 2A) with 3300pF Css.

Could you please inquire on the start up time is with SS tied to AVIN (as stated in the datasheet for "fastest start-up behavior")?

Thank you, Keith

Hi Keith,

Yes, when you set the SS time via the cap, we assume that you set it long enough that the external components, load, etc. do not extend the SS time beyond what is set by the cap.  That's the point of setting the SS time--you can set it long enough to always be the same under all operating conditions.  But if you hit current limit, for example, the SS time will extend beyond what is set by the cap.  The cap just sets the reference voltage that the IC attempts to track to.  If something else prevents it from tracking it, then the SS time is no longer as set by the cap.

If you have the shortest possible SS time, then other factors will extend the SS time beyond 0 nsec.  Examples of some of these factors are what I mentioned in my previous post.

After EN goes high, there is about a 50 usec delay before switching starts.  From some very old data, it seems that the minimum ramp time after that is about 50 usec as well.  But this depends on external factors to the IC.

Why does your customer want to do this?

Yep, that's a starting point for them.

We're always interested in what the market wants and why, so if you do find out what they're trying to do, let us know please.  Personally, I've seen too many customers with system start-up issues due to excessive inrush and so I highly recommend using the SS cap to reduce such issues.

I don't have any specific knowledge of TPS62140, but I can tell you that some devices have an internal slow start function that can be overridden by the addition of an external cap.  These devices will have a minimu slow start time that is appropriate for consistent start up.  Others have no provision for internal slow start.  If a slow start cap is not used, the internla SS current source can pull SS up extermely fast.  The effective slow start time will be only a few micro seconds.  This invaribly leads to problems such as overshoot, low head room drop out conditions, current limiting undervoltage protection latch off.. just to name a few possible problems.