Rao, your design looks ok. 4.4 v at output with mains supply and 3.2V with
backup. I've guess you have a rechargeable battery.
Schottky have almost the same dropout ( about 0.6V ) so if you need more
than 4.4V use a germanium diode and you may have 4.6V ( which is almost
the same )  12C508 is working at 4.4V
The better thing ( 78l05 or 2v7 zenner and a 0.6 Vbe transistor = 3.3V
output ) is what you have in your hands !

Cheers, Vasile



On Sun, 14 Oct 2001, Jeethu Rao wrote:

> Hi,
>         I've got no experience in battery bakup circuits for PICs.
>         Now, I've got an application with a 12c508 which needs battery
>         bakup. The application is a dusk timer so, its supposed to run
>         for years without intervention.
>
>         I've designed a trivial circuit for the purpose using a 3.2 volts
>         lithium battery. I guess there are quite a few shortcomings in it.
>         could you guys please look up the circuit and suggest any improvements.
>         One drawback I sense is that using 4148s is gonna cause quite a
>         bit of voltage drop. Do I use Schottky diodes ? The other one is the
>         78L05. I could'nt find a small equavelent for 7803. Since the size is
>         constrained, I could use a transistor - zener regulator. Whats better a
>         3.2 v simple transistor based reg or a 78L05 ?
>
> Thanks,
>
> Jeethu Rao

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