Re: Help with power issues

On Mar 31, 8:34 am, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Tue, 31 Mar 2009 07:36:22 -0700 (PDT), linnix

<m...@xxxxxxxxxxxxxxxxxx> wrote:
I am runing a sensor (1.5mA),
op amp lm/lmv/lpv358 (1/0.2/0.1mA)
and µC (0.5mA) on two button cells

The batteries range from 4V to 6V.
The sensor needs at least 4V.
The µC can have at most 3.3V.
The op amp is a sigma-delta A2D from sensor to µC.

Please clarify that last sentence.

Op amp analog input from sensor and digital output to µC. I am just
worrying about peak output voltage hitting the op amp or the µC.




So, I am thinking about a 4V zener to the sensor,
minus diode drop to the µC.

Should I power the op amp before or after the diode?
Also, 4V zener is hard to get.
Should I use 5V plus two more diodes?

I have to commit to PCB in a few days,
no time for more prototype/testing.

Zener regulators are terrible. When the battery is 4 volts and the
sensor needs 4 volts, the required series resistor is zero.

At 4V, the batteries are dead anyway. I should have said operating
range 4V somthing to 6V.

So when
the battery is 6 volts, everything fries. The math is terrible.

Use a low-dropout linear regulator to make the 4 volts; a diode from
there to the uP is OK, or use a second LDO.

But a 4V LDO is expensive, compared to the others.

4V LDO: 0.30
Zenar: 0.05
Diode: 0.01


If you really need to get this right, in a couple of days, post a
complete schematic that people can comment on.

OK, will try that.


John

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