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TPS63051: Clarification on the soft start implementation

Genius 14569 points
Part Number: TPS63051

Hello Experts,

About the attachement from this post - TPS63051: About Soft Start issue - Power management forum

The second scope capture (Css = 101 nF) looks very similar to what I'm seeing for Vout. That is, two rising portions separated by a very long plateau. I was concerned because I didn't know whether Vout should look more "ramp-y", like the SS pin.  But it sounds like everything is working fine.  FWIW, here's a screenshot of what I'm seeing in the lab:

Regarding the Vout waveform: I've found that increasing Css mostly affects the length of the plateau & not so much the two rising edges. This is a bummer because the edges are what drive inrush current. So increasing Css gives a much longer plateau but only slightly longer rising edges - meaning the inrush current isn't as affected as one would hope. Does this sound like normal operation?

Lastly, I wanted to give you a heads-up that I think there's an error in the datasheet. Sec 8.3.5 says that, "Figure 3 shows the value of the soft-start capacitor in respect to the soft-start time," yet Figure 3 actually contains completely different information.

Thanks ahead for your assistance.

Archie A.

  • Hi Archie,

    Thanks for your comment about the dismatch between description and Figure, yes, you are correct.

    Also, observed relationship between Css and actual inrush current is 100% correct too. it is a fixed behavior for TPS6305x.