Andy_123

Can't find any info, so decided to ask:

I have battery powered device that needs 80mA @5V and 120mA 3.3V
I am planning to use 4 AA batteries that gives me about 6V if fresh.
Device will be powered for about 10 minutes at the time so I expect relatively long batttery life.
I can easily get 3.3V from 5V (7833), but what will be easiest and cheap way to get stable 5V +0.2V/-0.5V from the battery.
Any special ICs or circuit designs?
Also I would like to have reverse polarity protection.

I understand that getting 3.3V from 5 using linear regulator is not the best way, so I may need to generate 3.3V from battery directly.

5V side used for PIC18F and LCD display.

Thanks

stevez

One thing I'd encourage is that you understand the lower limits of voltage from the AA cells - the point at which you might consider changing them. I don't know what that is - but let's say it's 1.25 volts per cell. That suggest you have no headroom for a linear regulator - leaving a switching regulator as a possible solution. If by some chance the lower limit per cell was 1.5 volts that leaves 1 volt - possibly enough for a low dropout regulator however that seems awfully close.

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stevez

Andy_123

That is why I posted this:

Technically I want to have 2 switchng regulators that can give me 5V and 3.3V with minimum energy loss and ability to work with low voltage AA cells.

I just thought someone can recomend regulators that specifically designed for battery circuits

Hero999

Use a low dropout regulator for the 5V supply, you also might want to consider using a switching regulator for the 3.3V supply.

There are ICs around that will do what you want, check Maxim's and Linear's websites.

You can build also it from descrete components but it won't be as straight forward, I can post some links and schmatics if you like.

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Andy_123

Since I posted this question I realized that I should use switching regulators with boost-up to minimize energy loss

I will try TPS60121 for 3.3V and TPS60140 for 5V
In this case I can use only 2 AA cells

I actually know my requiremens much closer now:
3.3V @ 160mA
5V @ 40mA

5V may go down to 3mA if I will eliminate LCD backlight and one LED

Attached Images

	tps121.jpg (22.4 KB, 6 views)
	tps140.jpg (26.4 KB, 10 views)