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switching to 3v supply of battery from LM317.

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mandar_mahajan

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lm317

Hi Everybody,
I am using NXP make 89lpc936 for my project. I am currently giving supply from LM317(3V). I found that my circuit current consumption is 25mA.
I am using the internal crystal of Ic(7.37Mhz).
I want to use battery(AAA size or Lithium) for my 3v supply. can anyone suggest me how to do that? because the current capacity for this batteries is very less. & I also want a longer battery life.

Any kind of help is welcomed...

Regards
Mandar
 

ctownsend

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3v lm317

This document may be of some help to you.
 

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pauloynski

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1- The voltage of AA alkaline batteries is 1,5V only when they are fully charged. Near end of life they fall down to 1,2V. Lithium cells show 3,3V when fully charged. They fall down to 2,2V when fully discharged.
2- If you want a no so complicated circuit (like the one posted above) you´ll have to protect the battery by adding a reverse blocking diode, which will further decrease the available voltage. That´s why schottky diodes are used for this purpose. Your circuit must be designed to work with such low voltages.
3- A very good solution is to use a single battery in conjunction with a boost converter like the MAX1724. It has the ability to extract all the power from the battery while keeping a well regulated output voltage.
4- You have to consider decreasing the power consumption (put the CPU into sleep mode or decrease clock speed maybe?) when using batteries OR you´ll have to choose batteries with higher ratings. It all depends on how much time you want them to last.
 

mandar_mahajan

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The problem is that when i connect the lithium battery to my ckt, the voltage drops considerably to a level in which my 89lpc936 is un-operational.
This is because of the IC current consumption.
If i use the external crystal, will this be possible........
 

FvM

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Although only loosely specified by NXP, current consumption in internal oscillator operation should be considerably lower than 25 mA. Thus I think, at least half of it is caused by other loads or a badly operated processor (e.g. with floating input pins).

As said, power managed modes offer many options to reduce the power consumption to levels needed for long term battery operation. You didn't however specify your requirements, so it's difficult to make detailed suggestions. Before selecting a battery type and supply method (e.g. regulated, unregulated, boost converter) you should know intended operation time and possible average current reduction in power managed mode.

Li cells have specified voltage/current characteristics, 1/2 AA or AA Li should work at least.

In my opinion, the processor is suited for unregulated operation with 2 standard or 1 Li cell(s) by it's 2.4-3.6 V supply range.
 

mandar_mahajan

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my requirements are , to make a load cell based weighing scale. in which i am using mcp604 rail-to-rail opamp(current consumption 2.5mA).
If i use external crystal,will it reduce my current consumption & serve my intended purpose.
 

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pauloynski

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Thus I think, at least half of it is caused by other loads....

FvM is right assuming there are other loads to be considered. The load cell acts like a 350Ω load (resistive) thus drawing some 9mA plus 2,5mA from the op-amp and another circuits (like a display). I´ve seen many battery powered weighing scales and all of them turn power off after a few seconds to decrease consumption. Furthermore, usually, they use 2 lithium cells to achieve a more reasonable voltage.
 

mandar_mahajan

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If i use external crystalfor controller,will it reduce my current consumption ..........
 

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pauloynski

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Current consumption (of the CPU) is mainly a function of frequency. It doesn´t matter if you´re using an internall or external oscilator. So, if you really intend to use this CPU you should consider decreasing the frequency.
I just took a small cell powered weighing scale and measured the strain gauge resistance: 1150Ω. (Powerpack model PW-390). This means the current to the cell is only one third compared to a normal 350Ω conventional one.
 

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If i use external crystalfor controller,will it reduce my current consumption
Yes, if the crystal frequency is considerably lower than 8 MHz. But the comparison is only valid for continously running processor, which is actually the least intelligent way to operate a battery powered system.

I'm under the impression, that you didn't even notice the purpose of power managed processor operating modes. It doesn't make much sense to discuss the topic this way.
 

mandar_mahajan

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I am planning to use 32KHz external crystal.
& FVM, thanks for focussing towards this point, I do know this modes(IDLE mode, WAKE-UP mode,etc) but not used it yet.
Once my opamp side gets finished I will move towards this & will seek ur help if needed....
 

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