> interesting: the power supply on a USB interface is fixed at 5V... why not just a 'buck' supply?
> [...]
> The power supply is a buck-boost from Richtek. Interesting choice as it allows the assembly to be powered from a voltage as low as 1.8V (battery use, perhaps??)
That'd be consistent with how I've used the original Pi Pico in projects, yep. The Pico (and Pico 2) has VBUS and VSYS pins - VBUS being the USB port's 5V, and VSYS being intended for non-USB power sources, including batteries. The Pico 2 datasheet (https://datasheets.raspberrypi.com/pico/pico-2-datasheet.pdf) has some sample schematics for three use cases:
1. A simple case where you connect the external power source to VSYS through a Shottky diode - in which case the Pico will pull from whichever of USB or VSYS has the higher voltage
2. Based on #1, except with a MOSFET instead of a Shottky diode, and with the gate connected to VBUS - in which case the Pico will not pull from VSYS when receiving USB power
3. Based on #2, except there's a battery charger between the external power source and the MOSFET, and VBUS is also connected to the charger's input - in which case the Pico will charge the battery when receiving USB power, in addition to the behavior of #2
(The datasheet also mentions an additional use case: if you're using the Pico as a USB host, you'd want to supply 5V to VBUS in order to power both the Pico and the attached device)
I ain't an EE, either, but I'm pretty sure the answer boils down to "that's just how transistors work".
So you have:
- Some external power source into the MOSFET's source pin
- VBUS into the MOSFET's gate pin
- The MOSFET's drain pin into VSYS
And by some transistor magic that I as a lowly layman ain't privy to, the MOSFET only allows power to flow into the input/gate pins and out of the output pin, and prevents power from flowing back out of the input/gate pins. The datasheet does specify a P-channel MOSFET, so that might have something to do with it (the differences between P-channel and N-channel MOSFETs are a bit over my head).
If you wanted extra assurance that power won't flow back into VBUS, you could probably stick a diode between VBUS and the MOSFET's gate pin.
> [...]
> The power supply is a buck-boost from Richtek. Interesting choice as it allows the assembly to be powered from a voltage as low as 1.8V (battery use, perhaps??)
That'd be consistent with how I've used the original Pi Pico in projects, yep. The Pico (and Pico 2) has VBUS and VSYS pins - VBUS being the USB port's 5V, and VSYS being intended for non-USB power sources, including batteries. The Pico 2 datasheet (https://datasheets.raspberrypi.com/pico/pico-2-datasheet.pdf) has some sample schematics for three use cases:
1. A simple case where you connect the external power source to VSYS through a Shottky diode - in which case the Pico will pull from whichever of USB or VSYS has the higher voltage
2. Based on #1, except with a MOSFET instead of a Shottky diode, and with the gate connected to VBUS - in which case the Pico will not pull from VSYS when receiving USB power
3. Based on #2, except there's a battery charger between the external power source and the MOSFET, and VBUS is also connected to the charger's input - in which case the Pico will charge the battery when receiving USB power, in addition to the behavior of #2
(The datasheet also mentions an additional use case: if you're using the Pico as a USB host, you'd want to supply 5V to VBUS in order to power both the Pico and the attached device)