While that is true for some of the "phone charger" type power supplies, the types of power supply linked to by bmatic are intended for use inside equipment and will be capable of supplying their rated max current without their output voltage drooping significantly.
The RPi4B does not sense what the PSU is capable of delivering. It has no means of sensing that.
The RPi 4 can detect what value the USB-C CC termination resistor Rp is which indicates the maximum current available. It doesn't appear to do anything with it though.
There is no resistor value for 5A and the RPi 4 only wants a maximum 3A anyway. Without USB PD, you get the choice of default USB power, 1.5A or 3A. The RPi 4 can detect the resistor value but doesn't appear to do anything with it.
No official documentation for it (yet). If you look at the schematics, you'll see the CC line is connected to one of the PMIC analog input pins. You can read the voltage on that pin using "vcgencmd measure_volts usb_pd"
Code: Select all
$ sudo vcgencmd measure_volts usb_pd volt=0.4100V $ echo 5 * 5100 / 0.400 - 5100 | bc 58650