BCC - KM - NB.

> I have an application where I'm considering just using the voltage drop
> produced by Rds(on) across a FQB22P10 fet in the place of a current
> sense resistor. =A0 ...

> Is there a reason I shouldn't do this, or why I wouldn't get at least
> somewhat accurate results?

This is definitely common enough practice in select circles.
Obviously the HV when FET is off must be dealt with and you will have
transients at the edges.
Monitoring how much Vd is below some low voltage local reference and
treating voltages above this as simply being at the upper limit
relieves you of the need to deal with much larger voltages.

Sensitivity of Rdson to junction temperature is a known issue - not of
as great an importance for you.

Fig3 here seems to do exactly what you want- but using high side PFET.
("exactly but" :-) )
LM3489
Hysteretic PFET driving buck converter controller. Sounds familiar :-).
http://www.ti.com/lit/ds/symlink/lm3489.pdf

Built into these synchronous buck drivers:
            http://www.intersil.com/data/fn/fn9135.pdf
            http://www.intersil.com/data/fn/fn9098.pdf

            http://images.ce-power.com/productattachment/40280311164e6e7301=
1655f8e38c0331.pdf


& high side switch (separate sense FET)
            http://www.irf.com/technical-info/appnotes/an-1118.pdf

IEEE discussion
http://ieeexplore.ieee.org/Xplore/login.jsp?url=3Dhttp%3A%2F%2Fieeexplore.i=
eee.org%2Fiel5%2F2220%2F5426952%2F05426992.pdf%3Farnumber%3D5426992&authDec=
ision=3D-203

Sense FET
Good & relevant paper albeit not what you want.
http://www.nxp.com/documents/application_note/AN10322.pdf


Good discussion (2007/2008)

    http://forum.allaboutcircuits.com/showthread.php?t=3D16686


 Russell

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