Re: **LOW** Power applications


Ken Smith wrote:
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.

Why? I have no problem getting the Generic Max232. I got 5 TI and 10
Sipex on hand for testing. I don't see any problem getting enough
Sipex, TI or ST Max232 for productions.


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

Yes, it is possible. I just don't want to waste more time when there
is a easy alternative. The charge pumps have 3 transistors to store
energy from incoming signals. It is fine for prototypes (work Ok at
9600 baud at less than 50% duty cycles with good Vcc). I just don't
want to risk it for production.


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.

It will try to transmit data on the way down (i.e. dropping Vcc) as
much as possible.
And here could be a problem, the micro is transmitting most of the
time. There is not enough receving signal transitions to trigger the
charge pumps.



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