ABS MAX and 6V battery, is it safe?

Hi all
i am working on a new design, power source to be a small 6V alkaline battery.
Open circuit voltage on fresh battery samples shows a consistent 6.50V
(there is also a lithium in the same package, same open ckt voltage).

ABS MAX VDD for the PIC is 6.5V.
The part will mostly be sleeping and quiescent current is planned to be 1-5uA.

There will be a series diode both for polarity protection and to trim the top off the open ckt voltage.

So the question is this:
am I living too dangerously close to the ABS MAX ?
a 4148 diode data sheet says that Vf will be only about 275mV at 1uA so the part will be sitting for most of it's life just a few hundred millivolts under the ABS MAX.

Does anyone have any experience or advice with this ? Long term experience with similar designs, good or bad ?

Using 2 diodes in series that does not appear to be an option for certain reasons.

dsicon, What are the "certain reasons"? Most pic processors will operate down to about 3.3 volts quite nicely...

Dave Purola,
N8NTA

Hi Dave,
the system needs around 4.0V minimum for reasons other than the PIC itself, also the PIC will be doing a low battery detect with the on board PLVD module which is internally tied to VDD, so an extra diode drop will mess with the limited trip points available

i sort of suspect there must be a lot of PIC systems using a series diode and 4 alkalines which are in the same boat

BTW, a regulator is not an option either due to the requirement of several uA standby
oh and did i mention the board is about 0.8 inches sq with about 25 parts including a sw reg? :)
so frankly i am not sure i could even fit another diode

dsicon, You mention a switching regulator. What is it for? Does it have a shutdown input that you are using?

Dave Purola,
N8NTA

the sw reg is fed from the battery before the series diode to the pic
and yes it has a shutdown but interestingly since it is a boost reg the full battery dc voltage appears at the output (thru the inductor) during shutdown so the shutdown pin is worthless to me
so i had to put a load sw (P+N mosfet) in front of the sw reg to fully shut it down
this also has the benefit of reverse protection to the sw reg due to the body diode in the P fet

it is really a challenge to get this thing down to a few uA's of standby!

but i still welcome any comments from people who run their pics for years near 6.5V abs max

dsicon, I have used a MAX1724 in the past with 2 diodes on the output (anodes), 1 cathode going to the load and 1 cathode going to the PIC VCC. Connected to the PIC VCC I had a second diode cathode connected. The anode of this second diode went to the battery source. The battery source was 2 AA batterys in series to give about 3 volts. This provided low power to the PIC and when the PIC awakened it placed + 3 volts on the shutdown pin of the MAX1724 which turned it on. This then produced +5 volts to the high current circuitry (100 Ma.) @ 4.5 volts (thru a diode drop). This switching regulator will work down to 2.6 volts on the input. There are ways to shin a cat at 2.6 volts.....

Dave Purola,
N8NTA

I have a design thats up and running for years now. One version of it is based on 16F628A and the other around a 16f917. Both of them run off 4xA sized dry/alkaline cells. In series with the cells is a 1N4007 for polarity reversal protection. The device usually lies in sleep mode and gets woken by a keypress. So, I guess, your app would work Ok.

thanks for your comment
can you confirm that with fresh batteries your sleep voltage post diode is about 6.2V ?
ds

Jerson
did you use such a high voltage rating (1N4007) for any particular reason (higher Vf ?)
ds