[sdiy] Question about a FET I stumbled across in a datasheet

René Schmitz synth at schmitzbits.de
Fri May 8 21:35:04 CEST 2020

Hi Tom,

On 08.05.2020 20:47, Tom Corbitt wrote:
> So my daughters lamp failed the other day and I opened it up to 
> discover it was much more complicated than I imagined it to be. 
> Inside. along with a Holtek 8 bit micro was the awesomely named 
> VIPER12A; a google search shows it to be the brains of a switching 
> power supply circuit
> https://www.st.com/resource/en/datasheet/viper12a-e.pdf
> My question relates to the 4th connection shown in the block diagram 
> on page one (and later in the technical explanation) Just what the 
> heck is the bottom element shown on the FET? (the lowest element under 
> what I'd call the traditional source elements?) Is it the 
> body? Looking at wikipedia I see that the source can have a source 
> connected to a P type island (which I guess is connected to the 
> substrate?)
> image.png
> What makes it odder to me is in figure 5 they show a constant current 
> source "Is" which they say is proportional to the current flow across 
> the fet (Id)

It is a second source terminal. It is there to create a "tap" of the 
drain to source current. Most likely with a smaller geometry to the 
other "source", i.e. its so that only a fraction of the current flows 
through it.

This is easier to realize in a chip than a shunt resistor to measure the 

> Is this a special feature made/incorporated into this specific FET or 
> is this something that you can do with proper biasing of the body?

Yes, this is done on the die/chip level.

It is probably not connected to the body, as that would short it to the 
other source terminal.



synth at schmitzbits.de

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