I've sorted through the confusion of the dual packages in the MEGA reference schematic, and got over my surprise at the positive ground of the populated voltage regulator, IC4, a MC33269D-5.0 from On Semiconductor. Neither Mouser or Digikey supply this chip, and that particular version is already on last shipments from OnSemi ....
So I'm wondering does anyone out there know a voltage regulator they think is appropriate for a MEGA clone?
MIC2937A-5.0WT looks nice and offers 750ma, but is a bit pricey at $2.27
It's cousin the 2940 offers 1.2ma but isn't quite as efficient.
There don't seem to be a lot of choices in the SOT-220 3 pin form factor, and most regulators either supply much less than the 800ma of the MEGA's or much more. I'm a little bewildered.
I'm switching to a two pin connector for the power supply and that opens up enough room for a TO-220 package. Anything wrong with a "standard" 7805? I'm not sure what is meant by a low-dropout regulator (sounds good though!)
A 7805 should work ok at the expensive of some efficiency loss. Also the fact that it not a LDO regulator might require some change to the two 10k voltage divider resistors feeding the op amp comparator so that the voltage switch over to Vin power happens at the correct value (don't forget to also consider the voltage drop loss from the reverse voltage protection diode).
The loss of efficiency does not really effect much if the Vin power is coming from a AC/DC wall wart power module, however it is a loss if Vin is from a battery power source.
Anything you can share about what your Mega clone will look like?
I'm not sure what is meant by a low-dropout regulator
A linear regulator requires some "overhead" voltage to work. A 7805 (one "standard" linear regulator) requires at least 7V in Vin to give a reliably regulated 5V out. This difference (7V - 5V = 2V) is the dropout voltage.
A low dropout (LDO) regulator's dropout voltage is less than the 2V of the 7805. Some are in the 400mV - 600mV range. I saw one description that called a regulator LDO when the dropout voltage was 1.4V - that's a bit high IMO, but it is less than 2...
I am wanting battery power for my application. I've got some second thoughts about the TO-220 size too.
This is for the Madhuino board I posted a couple weeks ago.
I created two versions to make it 4 times harder to work out all the tweaks
Files are at http://sites.google.com/site/madhuino
I hadn't thought about the power switch-over circuit. Now you've got me thinking about using a manual switch like the Seeeduino to cut down on components. Hmm, I just looked and couldn't figure out what they are using for the regulator on that.
I've been able to source everything else easy enough (it sure is a lot of work though!) and once I get the power supply issues sorted out I'm eager to get the first boards made.
A low dropout regulator is only any good if you are going to run it at a low voltage. If you are going to feed it say 12v for a 5v regulator then it makes no difference if it is low drop out or not.
So yes, LDO is just an extra feature if you plan on having vss>=9v or so, but efficiency is always nice for battery operation. Most spec sheets give a quiescent current, which I take to be the static base current level when no load is present, most are just a few milliamps, but there is about a factor of 2x difference. Then there are ground pin currents usually graphed as a function of supply voltage and current output or something like that, it gets hard to compare if you know what I mean. What are the key parameters to look for that indicate efficiency, especially at nominal usage levels?
The power consumed/dissipated by a linear regulator is: Preg = (Iquiesenct * Vin) + ((Vin - Vout) * Iload)
And of course the power consumed by the load is: Pload = Vout * Iload
"Efficiency" is then: E = Pload / (Pload + Preg)
A simple regulator like a 7805 has Iq of a couple milliamps, which is absolutely terrible if you have a microprocessor that spends a lot of its time in low-power modes consuming fractions of a milliamp; you'd like the circuit to be nearly OFF under such circumstances, and there it is still consuming several mA.
But for an operating circuit consuming some 10s of mA (an arduino is "about 50mA"), the dominating factor is that ((Vin - Vout) * Iload) term, and that's NOT dependent on any characteristic of the regulator itself; it's pure physics ("ye canna break the laws of physics!")
A low-dropout regulator will permit you to use a lower Vin to increase efficiency, but that's frequently a factor not under your control (batteries coming in fixed voltages), so they frequently don't help much in the real world.
Thanks for the physics lesson Westfw. That's a harsh reality. If I'm supplying 12v (from a battery), then I'm wasting more power than I'm using. Lets say about 40% efficiency for a linear regulator converting 12v to 5v with around 100ma, a switching dc-dc converter (at about $15 each) would give about 60% efficiency, which is about a 50% change in efficiency, or about 2/3 of the current consumed by the linear option. It seems like the laws of economics are as harsh as those of physics!
You should be able to find or build a 12V-5V switching regulator for much less than $15.
Check thrift stores for car phone chargers. A lot of these are 12V to (approximately) 5V switching regulators. Or you might find a 12V to 8V regulator which would power the input to a 5V linear regulator for a nice overall efficiency boost. Most of these are based on the mc34063 switchmode chip, which is less than $1 (for just the chip.) So you can consider building your own. (or import modules from china: http://www.sureelectronics.net/goods.php?id=365 ?)
Consider discreet circuits like "The Black Regulator" http://www.romanblack.com/smps/smps.htm
Somewhere I have a freeduino board design with a Black Regulator replacing the linear, but I've never built or tested it (nor even a plain Black Regulator), so it's "not ready for prime time." It does fit, more or less.
60% is really poor for a switchmode converter, BTW. Most are 80% plus...
But the efficiency difference IS pretty dramatic. Back in the good old days, large computers had these HUGH linear power supplies. One mainframe I was familiar with, during its decline into dotage, had someone do a mod where "modern switching power supplies" were added to replace the linear monsters. The supplies were small enough to fit in the chassis without removing the old supplies, and the increased efficiency allowed the mod to pay for itself VERY quickly, once you took into account both less power into the computer AND less power used by the air conditioning...
Wow, those are some great tips Westfw. I'm going to have to try a couple of those Black Regulator circuits. I get distracted by all the candy when I go to Sure Electronics!
BTW the regulator (which I won't bother to find again!) that I was looking at did get up to 85+% efficiency around 400ma, but had quite a curve and only got 60+ around 100-150ma.
L4940V5 Low Drop Voltage Regulators/Drivers 5.0V 1.5A Positive
Dropout Voltage (Max): 0.4 V @ 500 mA
1.5 A current
0.5 V dropout at 1.5A (0.2V at 0.5A)
reverse polarity protection
thermal shutdown
for Qty 10: @USD $1.69
