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Author Topic: Best way to regulate 4.2v to 3.3v (question)  (Read 4481 times)
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Anchorage, AK
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With a built-in regulator, how would you know when the battery was low?  It completely removes any ability to fuel-gauge it.  Though I guess there could be a supplementary connector with raw voltage for that reason.
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I'm not at all familiar with Lithium-Polymer batteries - this little 11mAh one is the first one I've used but don't some of them have a third wire anyway?  From memory, I think the batteries in my camera and phone have four.

John.
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You can use a micropower regulator such as http://uk.farnell.com/microchip/mcp1702-3302e-to/ic-v-reg-ldo-250ma-to-92-3/dp/1331485, which has only 2uA quiescent current. I use the 5V version to power the mcu in designs that run from a 9V battery and have no on/off switch.

I had another think about this later and came to the conclusion that, as I'm stuck with the small "4.2v" LiPo (due to size limitations), the differential required by this regulator (625mV) would make it's behaviour much like a diode in this application. So, I'm back to square one.

I've ordered some of these anyway as they seem much better than the ones I normally use in projects in which I can use a decent-sized 9v battery.

Given that 3v3 is the electronics standard and 4v2 is the fully-charged Li-Po standard, I have to say  I'm surprised that there isn't a simple 3-pin  IC that will "bridge" the two!  smiley

John.
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I had another think about this later and came to the conclusion that, as I'm stuck with the small "4.2v" LiPo (due to size limitations), the differential required by this regulator (625mV) would make it's behaviour much like a diode in this application.

Only if you are drawing the full rated current (250mA) from it. Look at figs. 2-14 and 2-15 on the datasheet.
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Quote
I'm back to square one.

You want to rethink about if you need to regulate.

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I had another think about this later and came to the conclusion that, as I'm stuck with the small "4.2v" LiPo (due to size limitations), the differential required by this regulator (625mV) would make it's behaviour much like a diode in this application.

Only if you are drawing the full rated current (250mA) from it. Look at figs. 2-14 and 2-15 on the datasheet.


dc42 is correct. See the charts here form the datasheet. The top is regulating at 2.8V. the bottom is regulating at 5V. Notice that it appears you will be needing well less than 0.1V of overhead to regulate at 3.3V based on the currents you are quoting.



* Regulator DropOut.png (49.32 KB, 493x759 - viewed 35 times.)
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Ah right, yes. I didn't study those graphs in that detail - I just saw the "headline" quoted differential.  Most of my projects tend to be very low current so I'm glad I ordered 10 of them!

As it turns out, I bit the bullet yesterday and gradually wound up my power supply to 4.2v and the display has been running 24 hours none stop. Makes the whole project a lot easier and I can now even make sensible readings of the true battery voltage.

Thanks for all your comments and suggestions.

If anyone is interested, here's the draft, not finalized, article on my website. http://vwlowen.co.uk/arduino/camera/camera.htm
There are comments relating to the power supply at the foot of page 3. It's not "live" on my website yet because I've yet to add credits and references.



John.
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To measure battery voltage on the system I referred to earlier (power from 9V battery, MCP1702 5V regulator, no on/off switch), I use a voltage divider across the battery made from two 4M7 resistors, feeding an analog input. Also a 0.1uF capacitor to ground to avoid noise pickup. As the input resistance of the ADC is quoted as 100M typical, this introduces an error of around 2%, but that was acceptable. Using such high value resistors means the the additional current draw due to the voltage divider is only 1uA.
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One problem I have is I don't have a spare analog input. In fact, I don't have a spare pin at all. I even had to use A0 to A4 as digital outputs.

I'm using the function shown here http://hacking.majenko.co.uk/node/57 . I had to tweak the 1125300L slightly to make it agree with my DVM but it's accurate over the range I'm interested in. 

Now that I'm measuring the actual battery voltage, I can get the ATmega328 to go into Power Down if the Li-Po battery gets dangerously low - just in case it's own internal circuitry fails.

John.
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OK so you're using the internal 1.1V reference to measure supply voltage. That 1.1 reference isn't very accurate, so you need to calibrate it - as you have done.
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Nice project John.  Well done!
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Thanks.
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