The approach I like to take is to use a very low quiescent
current (micropower) low-dropout regulator, like the T.I. TPS7250=

to keep the battery voltage regulated. It uses a P-channel mosfet=

as the pass device and therefore uses very little current for
regulation (unlike most Bipolar linear regulators). The PIC
supply is regulated to 5V, then when the voltage drops below 5V
the regulator drops out of regulation and just passes whatever
battery voltage is present. If safe shutdown is required below
some minimum voltage, a low power supervisior circuit like a
Maxim MAX810 can be used to hold the PIC in reset when the
batteries are deemed discharged. I prefer to use a supervisor
circuit (if at all possible) to guarantee reliable startup and
shutdown. You might also use a regulator that has a micropower
comparator onboard to provide a PIC reset function.

If you were to use the "LF" version of the PIC, you could
probably use a 3.3V (or lower) regulator to run the PIC until the=

batteries are completely discharged. I haven't looked at the
discharge curves of your battery, but you should be able to come=

up with a circuit that "drains them dry"!

Using dropping diodes is bad as you can never "fully drain" the
battery due to the diode voltage drops -- any battery power lost=

as drops across the diodes is wasted. You want every uA of
current possible going to the PIC and nowhere else.

Matt Pobursky
Maximum Performance Systems


On Tue, 19 Feb 2002 14:53:26 +1300, Paul Gaastra wrote:
>I'm using a PIC16F628 powered by 6V lithium batteries (KL2CR5).
>When new their open circuit voltage is 6.5V.  The max Vcc in=
 the
>specs for a 628 is 5.5V.  I use two 1N4007 equivalents to drop
>the battery voltage down to below this.  Does anyone ignore the
>spec and use just one diode or no diode at all?
>
>I didn't use a switching supply because I was pushed for time.
>The device has to last 5 years on a battery and I didn't want=
 to
>spend time calculating which would be the best solution.
>

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