ABS MAX and 6V battery, is it safe?

[dsicon]

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

[Dave]

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

Dave Purola,
N8NTA

[dsicon]

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

[Dave]

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