I use HT7333-1 for my CH32X035 board. (https://github.com/ch32-rs/ch32x0-hal/issues/1)

The LDO can operate successfully at 20V (USB PD 3.0). However, it fails when attempting to use 28V VBUS(USB PD 3.1 EPR).

It should be a DCDC.   

Thanks for the info! I also think that with over 20V VBUS the voltage difference is simply too large for an LDO, I don't even want to talk about the inefficiency. A DC/DC converter is the better option in this case.

You've got a 78L05 between Vbus and Vcc. Won't that not regulate but cause a couple of volts difference between the two buses? Or did you actually install a LDO regulator providing 3.3V like the AMS1117-3.3?

It works because the chips operates in the voltage range of 2.0V - 5.5V without any problems. 78L05 is cheap and good enough, that's why I chose it. A better solution (but more expensive) would be the HT7333-1. The AMS1117 is not suitable, because its maximum input voltage is only 18V.

Yes, but does this have implications for the high level of the GPIO pins? Say the regulator drops 2V, then you get 3V for high which might affect the noise margin with 3.3V peripherals. I wonder what the other USB-C dev boards (including other MCUs) do, whether they are limited to 15V power supply.

I just looked at the specs of the Xiao RP2040 and ESP32-C3. They take 5V USB-C. I think the USB-C power sink has to request higher voltages from the power source, and with no negotiation the voltage provided will be 5V.

At 5V input you have about 3.3V, at 7V and above 5V. I think the official WCH development board uses the HT7333. However, if you request 20V the LDO might get quite hot if you connect peripherals to it, that draw some current.

I think it's better to have a known 3.3 V Vcc and just not request higher voltages from the power supply (just don't ask for it and it will default to 5 V), otherwise you're relying on a regulator that doesn't actually regulate within a range of input voltages to drop the voltage to somewhere between 3 and 5 V, requiring peripherals that work on 3.3 V but are 5 V tolerant.

This may be true if you don't intend to use USB PD and higher voltages. But then there's no need for the CH32X035. The CH32X033 is cheaper, has all features of the X035 except USB PD and has even more memory. That's why this board is designed for experimenting with USB PD in the first place.

Ok, so it's designed to be used for experimenting with a PD supply that can provide at least 7 V, negotiated, and using 5 V peripherala. Or with 5 V tolerant 3.3 V peripherals if PD is not used.

It's designed to work with peripherals that need up to 20V without additional boost converters oder a second power supply. In these cases the board provides stable 5V at the VCC pin and the negotiated voltage (e.g. 20V) on the VBUS pin. As I said before, in my opinion this MCU only makes sense if higher voltages are needed. You can of course replace the 78L05 with a pin-compatible 78l03 if you need 3.3V. I chose 5V because of the smaller voltage difference and the better handling of MOSFETs to control the "high" voltage peripherals.

Fair enough. I was hoping the likes of Elecrow would pick up your design and manufacture it, but I suppose there are a lot of cheap CH32X dev boards out there.

dev need screen, buttons and power for mobility

Maybe, but that's not the purpose of the board.