I've been pulling my hair for several hours the past few days out trying to
figure out a very low power low voltage shut off circuit.

Goal: when the battery voltage (battery voltage is constantly dropping due
to usage), reaches a minimum (say, 4.4 V), output a logic low signal/open
collector/whatever, which is used to hold a switching supply's SHUTDOWN pin
low (so it turns off).

I can understand how to do it with a zener diode - just take the signal
between the anode and a resistor to ground - when the zener stops "breaking
down" (that is, below the zener voltage), line will be pulled low. But I
can't find an appropriate zener.

Then I came across this:
http://archive.electronicdesign.com/files/29/6403/figure_01.gif and
discovered the adjustable shunt voltage reference.

I searched around and then found this chip and example circuit on the
right: http://www.national.com/mpf/LM/LM431.html#Overview

I see how they're "adjusting" the device with a resistor divider. Problem i=
s
- the supply voltage itself (battery voltage) constantly changes. Wouldn't
this also cause the resistor divider output to constantly drop, therefore
constantly adjusting the "breakdown" voltage as well?

How would I go about doing this?
--=20
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