Battery Life Calculation

[arunkish]

Hi
I am using PIC16F684, having watchdog timer. Controller will be in sleep mode and wakes up every 10 sec (approx). During Sleep mode current consumption is 2.5uA and during wakeup it consumes about 3mA. Wakeup time is approximately 20 ms. Controller works on 3.3 V and the circuit is powered by 9V Alkaline Battery. Please advise the formula on calculating the battery life.

Thanks

[temtronic]

Microchip Application Note AN606

[languer]

That app note has a good explanation on the subject. I have my little spreadsheet which assigns each state a duty factor and then calculate total usage from there. The app note explanation just makes it easy to read.

[temtronic]

Potentially the biggest loss of power will be converting from 9V to 3.3V.

Better to just use 3V power supply and eliminate the 9V battery.
The ap note also explains how to decrease 90% of the power loss which may/may not be applicable with your hardware setup.

hth
jay

[asmboy]

I'm 1000% with Jay

whats wrong with two AA or AAA 1.5v alkaline batts for appx 3v ??

DO U NEED 9v for any aspect of the design?

[languer]

Or even better; how about 1 cell and a step-up device like this one: http://www.pololu.com/catalog/product/798

[bkamen]

I've done several projects with boost switchers.

Maxim makes some, Linear makes some.. TI, NS..

So many companies make 'em.

-Ben
_________________
Dazed and confused? I don't think so. Just "plain lost" will do. :D

[PCM programmer]

For your specs, I would recommend the Seiko S-817 series. It has
very low quiescent current
http://www.mouser.com/ds/2/360/S817_E-16206.pdf

We used this one in a hand-held instrument with a 5v PIC microcontroller.
A 9v battery was used: S-817B50AMC-CXNT2G
Mouser sells that one for $ .51 (US).
Seiko also sells version with 3.3v output, and up to 10v input.
But you want to get the version that has 1.2 ua quiescent current.

Regarding use of a 9v battery, sometimes the best case for your
instrument (from PacTec, for example) will only accept a 9v battery.

[Ttelmah]

Absolutely.
Critical thing then though is to realise that in the low power state the method you use to generate the 5v from the 9v, becomes the biggest consumer of power. There are some little Motorola regulators, which will better the one PCM programmer has noted, but these low power units also have the nasty habit of being more prone to oscillation than the more typical parts, which I suspect is one reason why PCM likes the Seiko unit.
I've used the TPS780xx regulators which push quiescent consumption down below 0.5uA, and with 6v batteries rather than 9v, ST do some designs that go down to a few nA.
Probably most of the consumption in the chip here is the watchdog (1.8uA typical), looking for one of the newer nanowatt chips could probably extend the battery life by several times.

Best Wishes

[ckielstra]

The original topic poster didn't specify why he/she is using a 9V alkaline battery to power a 3.3V PIC. Just my two cents worth:

Using a linear regulator the 9V can only be reduced to 3.3V by converting the 5.7V to warmth. So, even when the regulator consumes no power itself, even then 5.7V / 9V * 100% = 63% of battery power is lost.
A switching regulator can reach up to about 90% efficiency, a large improvement.

Physically the 9V battery consists of 6 stacked 1.5V cells. Effectively 4 of these cells are unused in this setup. A large waste. Would you use two separate 1.5V cells then a much higher capacity can be reached in the same volume.
For example, standard 9V alkaline cells have a capacity of about 550mAh. A 1.5V alkaline AAA battery is about 1100mAh, depending on brand and model. A combination of 2 x AAA is slightly will be smaller than the 9V battery but gives double usable capacity.

[Ttelmah]

And of course, 2*AAA, or a Lithium cell can be used without any regulator, which is the most efficient combination of all.

Best Wishes