nulling offset in a comparator

[gearhead]

I need a blocking diode that will
conduct about ten amps. I have a
leftover comparator out of a quad,
so I decided to use parts the circuit
already has and run the comparator
off the charge pump that drives
the n-mos on the high side.

Now, if I try to string the
comparator across the mosfet,
the negative feedback coming
from the voltage at the mosfet's
source would cause oscillations.

But the circuit will also have
voltage and current sensing, so
I don't want the mosfet slamming
on and off. I thought I'd try
try something like this:


generator
|
|
| NTP90N02
|
| ,--|>|--,
| | |
+----+----+-+-, ,-+------,
| | S | v | D |
| 15K ------- |
| | --- | |
| | | | |
| '--------+ Ich|
| | | |
R2 | | |
| _|D V |
| ,--+--+--||_ pmos |
| | | | |S |
| | 27K Zdiode| |
| | | | | charge |
| | '--+--+--+---o pump |
+-|--10K---|----, ~24v |
| | | | |
| | |/-|-' |
| | / | 1/4 |
| '---+---< | LM339 |
| | \ | |
| - |\+|---10K----+
| ^ | |
|D | | |
'-- | | |
nmos ||-+----|-----2K2-----+
,-- | | |
|S _/ | |
+-----^ TL | _____
| |431 | 12v batt___
R1 | | |
| | | |
'-----+----+-------------+
|
gnd

Setting the values of R1 and R2 in
the constant current sink so that
the voltage across R2 is .7
volts. The voltage across the NTP90N02
will reach .7 volts when it has
between 1.5 and 2 amps flowing through
the body diode.
When that happens, the comparator
goes low, turning the mosfet on
and the current sink off. So now the
comparator's inverting input sees the
mosfet's source voltage. It won't turn
off until the voltage across the mosfet
drops to zero, +/- the offset of the
comparator.

Let's say the comparator
has a 9 mV max offset, and the mosfet
has 4.5 mOhms Rds. If the comparator
has the max offset in one direction,
then the battery can discharge
into the generator, and the comparator
won't turn it off until the discharge
current goes more than about 2 amps;
if the generator windings have more
than about 6 ohms resistance it won't
turn off.
With max offset in the other direction,
we get back into oscillation.
So
I went looking for offset nulling circuits.
This maxim appnote has some
http://www.maxim-ic.com/appnotes.cfm/an_pk/803
but the offset is a function of the voltage
applied to the pot. My battery voltage
will vary quite a bit, from about 11 to 15
volts. Does a nulling circuit exist
that doesn't depend on the accuracy
of a reference voltage?