MSP430F412 and 3 AA batteries

[ljbeng]

I need to power a MSP430F412 project with 3AA batteries. This adds up to 4.5V and the MSP has a max of 3.6V. We want as few components on this board as possible. Would you use a regulator or 2 diodes in series to drop the voltage? This board only has a custom lcd, 4 pushbuttons and the batteries. Thanks.

[haklesup]

Use a regulator. Any dropping diode/resistor combination would assume the source voltage and current is constant. Batteries are not constant voltage, the voltage diminishes as it dies. The rate at which the voltage decays through use can usually be gotten from the battery spec sheet online.<p>A zener diode in parallel would regulate but at the cost of wasted battery power.<p>Check the MSP spec sheet, the device may tolerate a small variation of Vdd as long as the input voltages track accordingly (and they will if you are using the same source)<p>[ September 21, 2004: Message edited by: haklesup ]</p>

[jwax]

Silly question, but why not run the thing on two (2) AAA cells?

[bodgy]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by haklesup:
Use a regulator. <p>[ September 21, 2004: Message edited by: haklesup ]<hr></blockquote><p>Make sure it is an LDO type regulator they will work down to 100mV across the regulator or less. They also use less current for themselves than standard regulators. Price difference not that much.<p>Colin

[ljbeng]

I should have said 3 AAA batteries (typo in previous post) The customer wants the batteries to add weight to the device. Voltage is still the same as AA. I am planning on using a TPS76933 3.3V regulator.

[jwax]

Alright then, two (2) AA cells.

[haklesup]

"Alright then, two (2) AA cells. "<p>2 Alkaline cells would add up to 3.0V fully charged (and forget it if someone tries to use rechargables instead - which they will - only 2.8V) which is less than 3.3V operating voltage right off. just as well, even a little discharge of the batteries would reduce the voltage further leading to sooner shutdown of the chip and poor contrast on the display. 3 batteries solves the voltage problem and adds considerable charge capacity which in turn leads to longer life between battery changes.<p>The only difference between AAA and AA other than the obvious size/weight is that AA have a lot more capacity (mA-Hour) and last longer for the same job.

[ljbeng]

Thanks everyone. I bought a demo board from www.softbaugh.com (BLCDA4) that I am using to do the programming. It is very similar to what our final product will look like so I can test battery life and regulators with that board. Currently, it is drawing 99uA while displaying data on the lcd. We will have a couple of led lamps so current on our board will be higher but only during short periods of time where the unit is in use. Thanks again.

[ljbeng]

I got a suggestion to use a DC/DC step up converter like LM2703 or MAX1674. Anyone with any experience with these? It looks like the LM2703 will use more battery current when the processor is asleep. MAX1674 looks best but I hear bad things about supply/delivery/price.

[jwax]

I was going by the spec sheet which indicates an operating voltage range of 1.8 - 3.6 for the device, with an "Active" operating voltage of 2.2v. Two (2) AA's are considered very dead by then. The MSP430F412 has a low voltage indicator to boot.
BTW, products that require 3 volts simply say, "DO NOT USE RECHARGABLE BATTERIES!"
If the other devices require 3.3v, by all means use a regulator and 3 batteries.