Query: How to protect uPower 5V rail at u leakage currents 
for the protector.


Starters for 10 points:


A FET may do very well.
Speed not up to tranzorb speed but may well be fast enough.

A FET has a Vth gate voltage below which it is formally off 
and you get channel leakage only.
While Vth is a bit iof a variable feast you should be able 
to produce something that triggers at less than a say 2:1 
voltage increase and draws about zilch when off.

You could add a gate divider with ultra low current gain at 
the expense of slowing it down.
A speedup cap may get you enough speed with a tolerably low 
divider.

If you run a low Iq LDO at just above dropout you can place 
a transistor across it such that the transistor is off 
usually but on when Vin rises above !~= 0.5V or a bit less. 
(LM2936 may work). Olin mentioned something similar recently 
and I have used it successfully for another uPower 
application.

A smps with low enough Iq can supply a half isolated supply 
(common ground) that cannot fry if Vin rises high. The smps 
per se has to fail to allow high Vout. I've used a CD40106 
as a SMPS with minuscle Iq when no load current required.

A 1N4148 is specd at 0.375V at 10 uA. Put 13 in series (!!! 
:-) ) and you get a 10 uA leakage at 5V diode that conducts 
100 uA at 6.5 V and 1  mA at 8V. (Temperature an issue). 
Feed appropriate clamp trigger.

An LM385 can be operated at as low as 10 uA - but again 
speed may be an issue.


        Russell


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