5V 5mA, best option for power? Power regulator info?

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#1
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Hi All

I need 5V 5mA for a sensor. I am using Li-ion battery so max voltage is 4.2V. My AVR is running of 3.3V.

I think it will be overkill to to use a Step-up IC like the max668.

I do not have any availble counters that i can do PWM on my AVR.

So what would be my best option? Could i generate a pulse some other way to get voltage doubler then regulting it using a zener?

I think this would be the best option, as i require very low current. And it can be done cheaply, on bread board.

The other option is to use a 8.4V cell, at the moment i am using 4.2V 1000mA. So i would use 8.4V 500mA. This way the size of cell stays the same.

But the only issue would be this, say my circuit takes 100mA,

Then with 1000mA, i could have 10 hours of life

but with 500mA its only 5 hours.

But could the power loss from 8.4V to 4.2V, also give me a power to run for the extra 5 hours like the 1000mA cell does.

To be honest i prefer to use 4.2V cells.

Regards

DJ

Thanks

Regards

DJ

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Use a charge pump circuit driven by an output from the AVR.

Leon

Leon Heller G1HSM

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See the thing is i am using the Zibee Bitcloud on the AVR. So i am limited, but i will read up on how i can over come this.

Thanks

DJ

Thanks

Regards

DJ

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Are you using a SMPS? If not, doubling the cell voltage wouldn't halve the current, it would just waste power. if you do go with a higher input voltage and want to do s SMPS to step it down to 5v, this may be of use to you:

http://www.romanblack.com/smps/s...

Especially if you can run your AVR at 5v

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The LTC1044 might work.
http://cds.linear.com/docs/Datas...
Or some other switched cap doubler such as those in RS232 chips.

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Is your battery rechargeable? Because I don't see you worry about the current consumption.

Step up/down circuits, zeners or linear regulators usually are not switable for battery systems because of their efficiency-current consumption.

Why don't you use 4xAA batteries in series? By connecting a diode in series you will have aprox. 5.3V for the sensor and the AVR.

Michael.

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If you use a higher voltage battery then Texas has some very low consumption LDOs.

Michael.

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The perfect solution for you is the Maxim Semi MAX5008 which is a charge pump with regulated output. It takes 2.95 to 5.5 V in and outputs regulated +5 at 125 ma load.
This should work.

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Take a look at this fantastic LDO.

http://focus.ti.com/lit/ds/symli...

Michael.

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icarus1 wrote:
Take a look at this fantastic LDO.

http://focus.ti.com/lit/ds/symli...

Its a nice part but it can only output up to 95% of input or .95 x 4.2V the poster mentioned. That would be 3.99 VDC, not the 5VDC he wants. So we still have to BOOST his 4.2 up to 5.

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No, I was thinking a higher voltage battery.

Michael.

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To boost you have 2 choices: charge pump or inductive boost converter. The charge pump is much simpler (you don't have to deal with magnetics). That's probably what I'd go with, although there are inductive converter solutions that are very small, low ripple, nice guys. I'm not sure if you need the 100mA all at 5V or is some of it running off your battery? These only supply closer to 50mA, but there are higher current versions that are still low in supply current:

LTC3221-5 charge pump, Iq = 8uA

TPS61222 inductive boost would work and has an Iq = 5.5uA

The MAX5008 is really for a different application. It hogs current but has programmable current limit to support USB power supplies.

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See the thing is i am using 4.2V battery, i need 5V only once every hour for few seconds. The 5V is for a sensor that uses max 5mA.

DJ

Thanks

Regards

DJ

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As I said previously, you don't need a separate chip. Just use a charge pump driven off an output. You don't need a regulator if the load is constant.

Leon

Leon Heller G1HSM

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True, you can just run an output as an oscillator, use 2 diodes and 2 caps and you have a boosted output. The only problem I see in this application is that your Li+ batt voltage will vary from say 4.2V to 3.0V over its discharge cycle. If you use the right duty cycle for 4.2V to 5.0V then you'll get around 3.6V when the battery is low. So you'll see something like +/- 0.7V around where you center the design. Depending on the sensor this could be a problem. You may not be able to effectively even use the sensor at such a low voltage and may not like being over-voltaged to avoid it.

I've seen people do sensor voltage correction in software (and temperature, memory effects, etc). It adds complexity though, you have to characterize sensors across lots of manufacture to understand the transfer curves and how they vary (maybe your sensor doesn't care or you don't need the accuracy?).

If you only need to supply your 5mA sensor then a regulated charge pump basically makes that problem go away.

If you care about current consumption then a charge pump IC can be very efficient compared to an oscillator with diodes and caps, you'll lose a lot to diode voltage drops at this low of a voltage.

The LT3221 is only 2mm x 2mm and only needs 2-3 caps to run it. It will be very small, maybe a pain in the butt to solder at home.

I definitely would *not* change your battery cell structure to avoid this!