I need to know how much current the 5v Pro Mini's regulator can supply. I think I may have made a miscalculation when I was originally determining how much current I had available to power one of my circuits.
Because I'm just about to assemble the circuit, I decided to redo my current calculations to make sure I was running everything within suitable parameters. However, the numbers I've come up with are way lower than what I originally calculated.
Here's my current requirements:
1) The microcontroller itself, from what information I could find on the forums, seems to draw between 27-50mA.
2) I have two green leds which I want to power with 20mA becuase they're kinda dim. They're wired in series, so the total current draw for the two would be 20mA.
3) I have a piezo speaker. I've put a 1000ohm resistor on this to ensure that it only draws 5mA. Though it may be drawing much less than this.
4) I have a red power led, and a green mode LED on illuminated pushbuttons. Each of these will draw 10mA.
5) Lastly, I have a multiplexed 8x8 array of LEDs. I'd planned to allocate 160mA to this array. (20mA per LED per row, one row lit at a time, current sunk by darlington array connected to GND.)
The problem is that last one. It now looks like I may not have enough current to spare to run the array without making the LEDs really dim.
This, I've heard, is the datasheet for the Pro Mini's voltage regulator: http://www.micrel.com/_PDF/mic5205.pdf
With a 9v input, a 5v output, and a 0.7 voltage drop across a diode I believe may be in front of it, the regulator needs to drop 3.3v.
And according to the datasheet, the power dissipation can be calculated as follows: Pd = (125 degrees C - 40 degrees C) / 220 degrees C/W Which gives: Pd = 386mW
(I did the calculation based on an ambient temperature of 105 degrees Farenheit. I figure nobody'll be using the prop on a day which is hotter than 90 or so. Also, it's based on the minumum heat sink.)
So, if we take 386mW, and we divide it by the 3.3 volt drop across the regulator, we arrive at 116mA as the maximum current the regulator can supply if the ambient temperature is 40 degrees celsius.
Even if I assume the ambient temperature is less than that, say, 20 degrees celsius or 70 degrees farenheit, the picture doesn't get a whole lot better.
So... 116mA. That's not much. Subtracting the atmega's current requirements from that leaves me with 66mA. 20mA for the two green leds, and 5mA for the piezo eats up another 25mA leaving me with 41mA. And supplying 10mA to each of the illuminated pushbuttons leaves me with only 21mA.
21mA to drive an array of 64 LEDs. If that's all I've really got, I might as well have not even included the darlington array to sink the current. And I've got a big problem because the bargraphs I planned to use for the array only have 16mcd LEDs.
So what do you think? Are my calculations correct? When I originally designed this thing I thought the Pro Mini reuglator could handle 335mA, but now I wonder if there wasn't a mixup somewhere along the way, and what was actually meant was 335mW.
If I can't get more current out of this thing, my only option may be to buy 64 rectangular LEDs which are way brighter than the ones in these arrays.