Design Calculations for the Seasoned EE

I'm a professional electrical engineer who routinely designs new circuits for volume production, and have been for over 35 years.

Yes, I frequently do calculations to determine the exact part specs. There are also many cases where experience and intuition are good enough and the requirements loose enough that I just pick a value. Don't confuse that with a random value, though.

For example, for a pulldown resistor on the MISO line of a SPI bus, I'll just spec 100 kΩ and be done with it. 10 kΩ would work fine too, and someone else picking that wouldn't be wrong either. If I'm using a 20 kΩ resistor elsewhere, then I might spec another one on the MISO line to avoid adding another part to the BOM. The point is sometimes you have a lot of leeway, and intuition and experience are good enough.

On the other hand, looking at the schematic of my latest design, which I'm in the middle of bringing up first boards of now, I see a case where I spent some time not only specifying the part value but calculating the result of variance on the rest of the system. There were three cases of two resistors used in the feedback to a switching power supply. Here is the problem worded like homework:

A powersupply chip feedback input threshold is 800 mV ±2%. You are using three instances of this chip, to make the 12 V, 5 V, and 3.3 V power supplies. You have previously decided to use around 10 kΩ for the bottom resistor of each voltage divider. Determine the full resistor specs in each case, and determine the min/max resulting nominal supply voltage. Stick to readily available resistor values. Use 1% if suitable and spec accordingly.

That's a genuine real world problem that took a few minutes with a calculator. By the way, I determined that 1% resistors were good enough. That's actually what I expected, but did the calculations anyway to make sure. I also noted the full nominal range for each supply right on the schematic. Not only might this be useful to refer to later, but it also shows that this issue was considered and the calculations done. I or someone else won't have to wonder a year later what the tolerance of the 3.3 V supply is, for example, and re-do the calculations.

Here is a snippet from the schematic showing the case described above:

I just picked R2, R4, and R6, but did the calculations to determine R1, R3, and R5, and the resulting power supply nominal ranges.

Added about the SHx parts (response to comment)

The SH parts are what I call "shorts". These are just copper on the board. Their purpose is to allow a single physical net to be broken into two logical nets in the software, which is Eagle in this case. In all three cases above, the SH parts connect the local ground of a switching power supply to the board-wide ground plane.

Switching power supplies can have significant currents running across their grounds, and these currents can have high frequency components.

Much of this current just circulates locally. By making the local ground a separate net connected to the main ground in only one place, these circulating currents stay in a small local net and do not cross the main ground plane. The small local ground net radiates far less, and the currents don't cause offsets in the main ground.

Eventually power has to flow out of a power supply and return via the ground. However, that current can be filtered much more than the high frequency internal currents of a switching power supply. If done right, only the well behaved output current of the switcher makes it out of the immediate vicinity to other parts of the overall circuit.

You really want to keep local high frequency currents off the main ground plane. Not only does that avoid the ground voltage offsets those currents can cause, but it prevents the main ground from becoming a patch antenna. Fortunately, many of the nasty ground currents are also local. That means they can be kept local by connecting the local ground net to the main ground in only one spot.

Good examples of this include the path between the ground side of a bypass cap and the ground pin of the IC it is bypassing. That's exactly what you don't want running across the main ground. Don't just connect the ground side of a bypass cap to the main ground thru a via. Connect it back to the IC ground via its own track or local ground, then connect that to the main ground in one place.

I do mainly low volume commercial & industrial market stuff, so this may be different elsewhere.

At least 75% of a typical schematic is usually building block sort of engineering, "I need a 5V rail at 3A, 5% tol, I have 15V", there is nearly no point in designing that, when Ti/Linear/Micrel have all got perfectly good designs in their datasheets, it is just a case of picking one (And the choice usually does not much matter). I can of course design from first principles, but that is not what I get paid for.

Same thing applies for many other subsystems.

Then there are the "It just needs to have the right order of magnitude" cases, pull up and pull down for cmos, series resistors for indicator LEDs, stuff like that. My usual practise here is to leave figuring these out until I see what values I need in those few places it really matters, then pick something from those values if at all possible. "Power on LED, green, 12V rail? Ok, led will drop a couple of volts more or less, and I probably want somewhere in the 1 - 10mA or so range, so anywhere in the couple of K region will be fine, oh look I needed a 3k9 resistor for that filter, one of those will get it done".

The real trick is knowing when that 'finger in the air' guess is NOT going to cut it, usually things like filters, matching networks and timing circuits, pll and other feedback things involving significant phase shifts are likely bad places for guessing. Places like that where you actually do need to get your math on (Usually matlab/scilab/ads will get the job done, no need to actually remember much of the standard tables of integrals beyond the very basic trig).

It is actually quite rare (and very nice when it happens) to end up in that place where the electronics meets the physics meets the maths, sure it happens, path loss calcs, noise calcs when doing analogue, that sort of thing, but that is maybe 10% of a design, the rest is usually cookie cutter stuff.

Particularly when using an analog IC, there will typically be one or more suggested application circuits in the datasheet. For example, I am currently designing a Qi receiver for a project. The capacitors in the inductive loop are dependent on a number of variables, and the datasheet provides some equations for determining their values:

So it is just a matter of plugging in the numbers, breadboarding the circuit and trying it out.