Short version:

I have a project which requires 3.0-3.6v range, and is required to run 
off batteries for a Long Time (TM) at 4mA.

Can anyone suggest a suitable diode to drop 0.9v, or an alternative 
method for cheap, low to no power consumption voltage drop from 3 AA or 
AAA cells to 3.6V?

I purchased some diodes with a Vf of 0.9, but at my low currents they 
only drop .37v (50V 3A rectifier diodes, incidently).  How can I tell 
what  diode is going to really drop when I actually use it?

Long version:

It consumes less than 4mA in active mode.  I don't want to add any extra 
active components due to power consumption and cost.

I also want to protect "battery polarity challenged" individuals from 
themselves.

So I purchased some (large) diodes with a Vf of 0.9.  They only dropped 
.3 or so each, which made me puzzle.  I scrounged around until I found 2 
diodes with about .45v drop each - but the drop is different with 
different loads.  I'm using it with 3 AA batteries, and am shooting for 
about 400mAH before it hits 3.0v.  I already have the data sheets 
(duracell, energizer, panasonic) so it looks like I should have no 
problem as long as I can drop the 0.9.  And no, I don't care that I'm 
not using all the battery capacity - if it lasts for a year with an hour 
use per day then the end user is happy.

Since this is a prototype for production I can get by as is, but when it 
goes to production I want to have a 'better' solution, or at least 
understand the basics of diode voltage drops so I can justify this solution.

What affects voltage drop on a diode?  Is there a cheap diode which will 
drop .9V at 4mA?

Is there a better solution than this?  Maxim has a nifty buck-boost for 
just this application, but it's over $3 a chip without the inductor, 
diode, and resistors.  I haven't seen a linear regulator that meets my 
needs, nevermind one that costs as little as a diode.  It runs on 2 
batteries, but it may die/explode/bring armegeddon if the batteries are 
reversed, and can't get as much power (nominal 2 AA = 3v - it only runs 
for a hundred hours or so before going below 3).

Thanks!

-Adam
_______________________________________________
http://www.piclist.com
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist