Re: static drain-source on resistance



Fred Bloggs wrote:


Spehro Pefhany wrote:

On Mon, 30 Jul 2007 14:56:26 -0500, "Anthony Fremont"
<anyone@xxxxxxxxxx> wrote:


kell wrote:

On the data*** for the STP80PF55
http://www.st.com/stonline/books/pdf/docs/8177.pdf
I can't make head or tail of figure 6. What is Vgs? It can't be 10
volts, the numbers all wrong for that.


It's a pretty strange looking graph but, if I understand, the horizontal axis represents Vgs's from 2.5V thru ~7V. The vertical axis represents the amount of drain current (Id) required to create an arbitrarily chosen 5V drop (Vds). Since there is a well defined curved line, I suppose that proves that Rdson is fairly static for a given Vgs. I don't know why they felt the need to make a puzzle out of it. This looks allot like the work of "The Center of Confusion". ;-)



One hint that something is seriously wrong is that a graph called
"Static drain-source on resistance" does not have ohms as the units
for the dependent variable. A second hint is that if you interpret the
Y axis as ohms, it's about 10:1 too high to match the typical Rds(on)
figure of 16m ohms. A third hint is that Vgs is not stated. Garbage.
Best regards, Spehro Pefhany


Mmmmmmmmmmmmmmm..nah. The graph is okay. You might note that in both active and saturation region the Gfs is a product of mu.sub.p x Cox (per unit area) x (W/L) x (VGS-VTH). RDS,ON is 1/Gfs in active region. Computing the Gfs from Fig 6 at the various VGS in saturation region, allows for accurate extrapolation to RDS,ON in active region for a given VGS...


I should correct that to RDS,ON= 1/(dId/dVds) in active region, which is related to 1/(dId/dVGS)=1/Gfs in saturation region, the same product factors. I guess you could call RDS,ON= 1/Gds in active region. Gds=dId/dVds there.

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