Help with power issues

Mar 31, 2009 4 Replies
L

I am runing a sensor (1.5mA), op amp lm/lmv/lpv358 (1/0.2/0.1mA) and =B5C (0.5mA) on two button cells



The batteries range from 4V to 6V. The sensor needs at least 4V. The =B5C can have at most 3.3V. The op amp is a sigma-delta A2D from sensor to =B5C.



So, I am thinking about a 4V zener to the sensor, minus diode drop to the =B5C.



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.


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J

Please clarify that last sentence.

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

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

John

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L

Op amp analog input from sensor and digital output to =B5C. I am just worrying about peak output voltage hitting the op amp or the =B5C.

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

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

4V LDO: 0.30 Zenar: 0.05 Diode: 0.01

OK, will try that.

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J

You left out the series resistor. And its problem isn't the cost; it's the value.

How much will it cost to do it again?

John

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L

OK, OK. I will have both on the PCB and decide later.

A thousand jumpers by yours truly.

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