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[Reply #15 on:]

In retrospect, I guess that makes sense.

[Reply #16 on:]

I've been trying to decipher exactly what fuse settings are made in the bootloaders. I found the
following in "Makefile" in the IDE optiboot directory,

atmega328_isp: atmega328
atmega328_isp: TARGET = atmega328
atmega328_isp: MCU_TARGET = atmega328p
# 512 byte boot, SPIEN
atmega328_isp: HFUSE = DE
# Low power xtal (16MHz) 16KCK/14CK+65ms
atmega328_isp: LFUSE = FF
# 2.7V brownout
atmega328_isp: EFUSE = 05
atmega328_isp: isp

This would explain why I can run the chips at 3.3V without the brownout triggering.

I've not yet found whether the watchdog is enabled or not. Takes a lot of searching to nail this stuff
down.

[Reply #17 on:]

I've not yet found whether the watchdog is enabled or not.

[Reply #18 on:]

If you haven't tried digging through the zillions and zillions of Arduino IDE subdirectories to try and
find a smidgeon of information, then an entire new universe awaits you, LOL.

[Reply #19 on:]

I rigged up an experiment today.
I did a load test using a 7805 vs an LM2904-5 (an LDO lin reg).

Code:

        no load   60Ω    47Ω
*7805*
Vin      Vout    Vout    Vout
4.0      3.23    2.62
4.5      3.72    3.07
5.0      4.2     3.58
6.0      4.99    4.56
7.0      4.99    4.99

*2940*
4.0      4.0     3.9     3.9
4.5      4.3     4.42    4.42
5.0      4.95    4.93    4.89
6.0      4.97    4.97    4.97
7.0      4.98    4.98    4.99

The 7805 blows at this low V_in thing and the LDO seems to hold up,
but I'm not convinced that there isn't a "gotcha" in there.
Anyway, such are my findings.

[Reply #20 on:]

That's only 80-100 mA load, try it with 500 mA.

[Reply #21 on:]

Well, the OP said his max expectation was for "100 mA",
but I may take a look at that.
The 2940 output with 47Ω showed some difference
from 60Ω, I checked and re-checked it.

[Reply #22 on:]

Results for the LM2940 were interesting, I was just wondering how it would go for heavy loads.

[Reply #23 on:]

Late to this party, questions for the OP. How long to the AA cells need to last? What is the source of the 12V supply?

[Reply #24 on:]

ISorry I've been away lately.

Runaway Pancake, thanks for your chart there. It's nice to back up with experimentation what everyone was saying would happen.

Late to this party, questions for the OP. How long to the AA cells need to last? What is the source of the 12V supply?

I'm planning a timelapse controller. It probably would only be run for 4 hours maximum at a time. The 12V would be from a SLA or Li-ion battery or whatever the user wants to take into the field for power.

[Reply #25 on:]

A buck-boost regulator comes to mind. Haven't actually done one myself, but I've had good results with a small boost converter (MCP1640) in a circuit that requires right around 100mA at 5V. It can use two or three alkaline or NiMH AA cells. Haven't done a real good test yet, but it looks like two fresh alkaline cells should run it for at least 8 hours, three should give half again that. Makes a lot of sense to boost a lower voltage when it can be done at 90%+ efficiency, as opposed to knocking it down with a linear regulator and wasting power. A lot of the switching regulators these days don't require complex circuitry. Build it on a PC board, keep things close and traces short and you're off to the races.

I definitely would try to avoid the scenario of running a regulator (or most anything) out of spec.

[Reply #26 on:]

That's a fantastic solution. I will definitely look into it. Based on the datasheet, the additional circuitry for the MCP1640 is minimal, and they even have a recommended PCB layout.

The only problem is this eliminates the external 12V option, but I can still have a linear regulator for that.
Or would anyone have an example of a switching regulator with a 2-12V input and 5V output?

Edit: Anyone have any comments about the MIC2570?
http://www.digikey.ca/product-detail/en/MIC2570-1YM/576-2190-ND/1028727

[Reply #27 on:]

Took a quick look at the MIC2570 datasheet, it's a boost regulator as is the MCP1640, I'm not 100% sure, but my initial assumption would be that the input voltage cannot exceed the output voltage (same as MCP1640). Other than that, it looks fine. TI makes some interesting buck-boost regulators, they're in very small surface mount packages though, if that's an issue.

Using a linear regulator on the 12V input also occurred to me. Feels a bit kludgey, but if the low efficiency is not an issue (and it might not be for a SLA, etc.) then that could work.

I have a small PC board which I use as a breadboard power supply that uses the MCP1640. Happy to share, if you're interested, PM me.

[Reply #28 on:]

^Do you think you could post a picture of your PCB layout?

I am a bit lost finding a suitable inductor. I don't understand saturation current or ESR. Could you link a suitable inductor that digikey supplies? I would like an SMD part, but I will be soldering by hand, so the leads can't be on the bottom.

For the capacitors would these work? http://www.digikey.ca/product-detail/en/C2012Y5V1A106Z/445-1371-1-ND/567608