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TPS54362-HT slow start, threshold, pullup, voltage rating

Other Parts Discussed in Thread: TPS40200, SM320F28335-HT

Hello.

Changing our 1.9V supply from TPS40200 to TPS54382-HT to get the reset circuit to reset the SM320F28335-HT 8µs before 1.9 V reaches 1.5 V.

Reading the datasheet, there are a few questions left regarding soft start and voltage ratings at pins.

Soft start: There is showed a current source to charge the soft start capacitor. What is the threshold or range for the soft start? for the TPS40200 it is well shown at which voltage the soft start is completed. In the block diagram the voltage goes through two amplifiers or buffers into the error amp. So the error amp input reaches 0.8 V, but how does that relate to the voltage at the soft start pin? only a buffer? Clamped to a voltage somewhere? Reccomended voltage range is 0-6.5 V and 8V max at the pin, but that is out of our control. :S

In ht circuits the applied voltage is important for the estimated life time, and then the voltage at the pin is at interest.

Equation 4 in the datasheet says that Css can not be very big (in our case 20 nF). Why and where did this equation come from? Why can we not connect a 100GF capacitor to spend a year during soft start? Our consern is to not draw too much current during charging the output capacitors.

Cdly, what voltage is that clamped to? Same 0-6.5 V /8 V max spec on this pin, but not easy to know what we can experience. If it is normally clamped to 3 V it is another world to us than 8 V.