Most of my MCU circuits to date have been using in the order of a few 100mW. For those circuits the MCU module onboard regulator has surficed for 90% of things. Else ways, you just through any jelly bean 3.3V regulator on there and as the most jelly bean linear 3.3V regulators "claim" 800mA to 1Amp, you don't need to worry much about the power.
The trouble I am now facing is that my MCU dev board is pulling half an amp at 3.3V. This has identified that the claimed 880mA at 12V the AS1117 claims, is assuming it has adequate heat sinking etc. It IS providing the 3.3V at 500mA from a 5.8V supply, but it's doing it while at maximum package temperature of 80*C.... and it's my third in the this project. The first one died due to inrush current. The second one (on a module) died from simply overheating and releasing it's magic smoke.
Now, first approach. "Bigger hammer". I ordered a few LM317T 3.3V regulators in To-220 package. I have some TO220 heatsinks. The 317 claims 2.2A. A small TO220 with heatsink can dissipate about what to air? 5W? For 0.5Amps linear drop 5W is only 10V. So, yes, it will probably be "ok" doing 0.5Amps.
The trouble is... I'm not finished. I am about to add a second board to the project which itself will pull 0.5Amp or more. So I need a different solution. 3.3V at 1Amp just seems wrong. It's too small a voltage to be dealing with voltage drop on rails. You can't even afford to drop 10% and (as I found out the hardware) boosting the 3.3 to 3.5 or 3.7 will fry ICs.
Am I heading in the right direction with a hierarchical power distribution network, rather than a central power supply?
Say, for example, my input is raw "automotive class 12V". So that's going to be, in my case, anything from about 10V (rare) through (13V nominal) to 15V with 15 amps of current in the extreme.
If I diode/ferrite ring, inductor and bulk cap that input voltage. Use an LM8712 to regulate that to 12V (max), distribute the 12V to each sub board. Each sub board then linearly drops that to 5V which is distributed to each individual module to regulate down to 3.3V.
Damn. That's not going to work. Linear regulation is still going to just cost a lot of heat. 13V down to 3.3V at 1 Amp is 11.7 watts of heat, all I am doing above is dividing out who drops what parts of the heat.
I suppose I don't really have a choice but to go switch mode.
Twists and turns. I want to avoid switch mode for "Audio noise" reasons. I was hoping the DC power supply was to be provided by a battery, a bit like a UPS upside down. It disconnects from the charger when in use and reconnects to the charger when idle. By making that battery a high voltage and providing a center tap in the pack I can create a true floating -12V 0V +12V linear supply for the headphone amp.
So it seems as though, while that will work, the issue is in supplying those hungry 3.3V rails from +12V (or 24V if that way inclined to keep the pack balanced).
So, I think I have answered my own question. If I am to use a battery pack with a centre tap the power hungry 3.3V rail will absolutely need to be supplied by the full pack voltage to keep it balanced. I am NOT linearly dropping an amp at 24V down to 3.3V. No sir. I'd like this to be useful in summer also.
Hmmm. Now I need to find a convenient and quiet! DC-DC converter in module/daughter board form. Such that I am not involved in the analouge calculations required there in
I still need to answer if 3.3V 0.5 amps is resonable input requirements for a PCB and if it really should be 12V+ at 0.1A and locally step it down with additional DC/DCs.