I studied
this official schematic diagram. It shows that all chips are powered from an onboard voltage regulator chip RG2 for 3.3 V. This means that the 5 V input voltage should not be very critical. The only other use of the input 5V is VDD_BAT1 to 4, which could be some sort of voltage measurement used to display the thunderbolt if the input voltage drops below 4.65 V. The chip used for RG2 is a NCP-1117-3.3V, with
this datasheet. This ic has a voltage drop of min. 1.2V, so the input should be at least 3.3 + 1.2 = 4.5 V. Add a little margin and 4.65 V is a reasonable min. voltage to guarantee that all chips onboard of the RPi will function correctly. I do not see a problem of input voltages higher than 5 V. Of course, if the input voltage gets much higher, the dissipation of RG2 will increase, but there is no hard limit of e.g. 5 V + 5 % = 5.25 V. So e.g. 5.30 should give no problems.
This does NOT mean that every usb power supply would work. To charge a phone battery, it would not be a problem at all if the usb power supply has voltage dips. But for powering the chips inside the RPi, any voltage dip on the 3.3 V rail would be fatal, no matter how short this voltage dip would be. Translated to the 5 V usb input, this would mean that that input should NEVER drop below 4.65 V. So the official Raspberry Pi power adapter is not a regular USB charger; it is designed to NEVER drop below 4.65 V. It also has a slightly higher voltage, the nameplate voltage is 5.1 V, and I measured 5.25 V. In practice, there is always some loss of voltage in the cable and micro usb connector, so by starting at a slight overvoltage, the risk of ever getting below 4.65 V is minimal. Again, a slight overvoltage is not a problem at all, as all chips are at the 3.3 V level, and the overvoltage is dealt with by the RG2 voltage regulator.
For designing your own power supply, I got good results with a cheap 3 ampere switching regulator set to an output of 5.25 V and directly wired to the 5V and GND pins on the 20-pin RPi headers to eliminate voltage drop over a usb connector. In fact, the 5V header pins are directly connected to the usb power connector, except that the usb connector has a polyfuse, intended to avoid damage in case of a short circuit in a usb device fed from the RPi.
I am still puzzled about the exact power voltage measurement used to show the lightning bolt in case of undervoltage. I just guess that the VDD_BAT1 to 4 pins are used to measure the input voltage. I
read about a APX803 (the power-monitoring device used on Pi 3) and GPIO26, but could not find anything about this on the schematic diagram. Any insight from more knowledgable people welcome.
