circuit analysisschematics
Can someone help me figure out what the little letters and numbers mean on the pins of this ic?
They're used throughout various ics in this iPhone 6 schematic. Are they indicating sequence?
For example A1 happens first then A2 etc or? 
Sorry that it's showing up blurry. It's crystal clear on my phone. Not sure why it's compressed like that
R1 and D1 will not limit the voltage to 20V. The resistor would need to be in series with the +24V supply rail for this to happen.
In any case, why do you want to limit the voltage to 20V if you are limiting the current through the resistors? The current limiting circuit will adjust the voltage across the resistors to the same value either way.
ZA and RA just limit the gate voltage to 5V (or not - I didn't look at the values, Steven is right there), in case you want to interface with a microcontroller according to the Instructable. The only effect this will have on the current limiting is setting an upper limit to the MOSFET turn on (which may be a good thing, but not necessary)
The circuit will limit the current through the LEDs just fine like this.
To show what happens with the circuit over a range of supply voltages, here is a quick circuit I created, without the unnecessary bits:
Simulation:
I set the current for around 20mA using 0.7V / 0.02A = 35Ω.
The X axis is V+ (the supply voltage) being swept from 10V to 30V. The green trace is the voltage across the 3 LEDs, and the blue trace is the current through them.
You can see how once the voltage rises high above the LEDs combined Vf, any further increase has very little effect on the voltage across them or current through them (less than 1mA variance over 20V)
Looks good at a quick look through. Nothing obviously bad.
BSS138 are especially wimpy MOSFETS. Ok for playing but do not do justice to the rest of the circuit. Not up to quad copter use, but, you know that.
DTR is intended to reset the processor I assume.
To pass the Olin schematic test you'd want (amongst other things)
To move text strings off lines (eg PS3 PS4 by MOSFETs and rotate them to horizontal where sensibly possible. PWM3 label is overwritten by MOSFET name etc. VCC label in eg FTDI box turn 90 degrees.
Have connectors from FETs vertically above FETs rather than bent at 90 degrees. Vbatt could be above or to one side.
And similar
Aim is to not 'twist brain' to follow obvious flow.
Some people will read diagrams with small points as above and find them less than comfortable but bearable. Others will essentially refuse to look at them. The latter reaction is somewhat extreme but reflects an underlying semi-reality - a really tidy diagram with attention to layout, text and component orientation, consistent labelling and more SUGGESTS an underlying technical competence, and a messy diagram tends to suggest the onbious. Neither of these impressions may be correct, but they are often a useful starting guide. Also, arguably, a tidy diagram reflects respect for those who you expect to use it (and the validity and relevance this has been argued at length :-) ) and is more likely to allow you to spot technical errors.
NB: DO NOT ever take my diagrams as examples of how to do things well. I am a "classic messy" who attempts to force the world's mold on my output with varying success. Judgements based on normal standards may mislead one wrt the value of a classic messy's offerings - and may not :-). Equally a chronic super-tidy (who can in their own way be just as bad as a messy) may produce superb copy but not always match it with underlying competence. FWIW.
Best Answer
You've asked two questions:
As Mike F. has mentioned, my answer to this previous question also answers this part of your question - those are the BGA ball numbers on the IC.
They are indicating the order (layout) of the balls on the IC, but not what happens functionally. For example, across the top of IC
U1401in your image, the pinsA2B2D2C2are shown as connected together, so they are all used at the same time.Note that there are a variety of different BGA layouts, so you should check the manufacturer's datasheet to confirm the specific ball numbering for an IC. Based on available photos, the device shown as
U1401might be manufactured by Texas Instruments, however its datasheet does not seem to be publically available.Just as an example, here is a diagram from Texas Instruments showing the magnified layout for one of their other BGA packages (larger than
U1401in your schematic):Notice that the circles for each "ball" are not complete; that is a convention indicating that this is the top (not the bottom) view of the package (confirmed by the text also saying "top view"). You can see that the upper row of the grid layout is row
Aand the left column is column1. Then the row letters and the column numbers increment, producing the "sequence" likeA1A2etc. that you noticed on that schematic.