Re: **LOW** Power applications

In article <1157472478.439417.94430@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
linnix <me@xxxxxxxxxxxxxxxxxx> wrote:

Ken Smith wrote:
In article <1157390698.941191.230410@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
linnix <me@xxxxxxxxxxxxxxxxxx> wrote:
[...]
The trick is power management. At 10mA, the Max232 is drawing several
time more power than the micro at idle. I am redesigning my circuit to
turn it on only when a cable is connected (A2D the DTR signal). The
Max232 (started out at Maxim, second sources from many) is about $1,
but it requires 4 1uF caps (a penny each). Siplex actually has a part
with build in caps, but they wants $3 each.

You can power the output circuit from the connected RS-232. This would
require that you not use the MAX232. I'm thinking along these lines:


I have tried charge pump circuits from cable signals (DTR and RX).
They work for low speeds and duty cycles, but fails under certain
conditions. The Max232 is more reliable, although for the price of
more PCB spaces.

Two points:

The Max232 you can't get is by its nature not reliable at all.

These were circuits you designed. Did you consider the posibility that
you did something wrong.

If Vcc is at least 3V, this will work. With the RS-232 cable not plugged
in, the circuit floats and only the leakage in C1 matters. The P-MOSFET's
leakage is taken out of the picture.

My Vcc goes down to 1.8V, when the BOD kick-in and shut it down.

I assume the micro isn't expected to do RS-232 when shut down. The 1.8V
level is a bit low to use directly but there are still ways to deal with
it.


--
--
kensmith@xxxxxxxxx forging knowledge

.

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