Type C solution needed

[blueskull]

Hi all,

I'm working on a USB type C design that takes adapter power and while powering up the device itself, also powers the computer it connected to. So basically my device has 2 type C ports, one being power port with PD2.0 support, and the other being data port.

Now, I want my device to have three features:
1. Powered by type C
2. Can charge computer on data port
3. Can use computer's charger

There are lots of cheap type C dongles that simply connects upstream CC pins to downstream CC pins and tap power from VBUS, but I can't do this.
My device consumes up to 30W, and hence tapping from VBUS without letting DFP and UFP know that is stupid and will risk overloading the PSU and/or cable.
Therefore, I need a type C translator that tells the device it can provide input max power minus 30W.
For this part, I think I can do with 2 CC transceivers, one talking to DFP and one talking to UFP, and an MCU bridges them with Po=Pi-30W logic.

As for the power path, how should I handle this? I have 3 plans as of now:
1. Use no converters -- pass all voltage options from the PSU to the PC, and let the PC to dictate which voltage it wants to use, in case it wants 5V, my device asks for highest power profile and generates a local 5V for the PC
2. Use a buck converter -- get the maximum power profile from PSU, and tell the PC it can supply a wide range of voltages not higher than PSU voltage, and give PC whatever voltage it wants
3. Use a buck-boost converter -- get the maximum power profile from PSU, and tell the PC it can supply a wide range of voltages, and give PC whatever voltage it wants

Of course, if the PC is not a PD compliant device, my device will not dump any voltage to VBUS and just ask power from PSU to power itself only.

Which one do you think is better, and why?

Personally I would not choose method 1, since there are a lot of computers that can't take 20V, so no matter how high power my adapter is, it will never be able to charge the PC at full speed. Say, a PC can take 12V 5A input, and my adapter can supply 20V 5A. Since there's no buck converter in between, my PSU has to output 12V, therefore after 30W taken form my device, the PC can only charge at 30W.

I would choose between method 2 and 3. So the real question is, is method 3 anyway better? I can't seem to find a good use of it. An extreme case would be a laptop capable of taking in 3A max at 20V, and the charger can only do 16V 5A, so with a buck-boost converter, I can charge it at full power. But how many devices support 20V but not 5A?

Any ideas are welcomed.

Also, if you think option 3 is making sense, please suggest me a buck-boost solution that is small. Space is important, and I don't care about cost. My device has more than enough profit margin to accommodate an LTM8055 or something better.

[ebclr]

Your answer starts here

http://www.cypress.com/products/usb-type-c-and-power-delivery

[krho]

I'm going to kidnap the topic by the following question.

How does one handle USB PD and voltages higher than 5V when the MCU that's actually connected onto the USB lines e.g D+- requires VBUS connection, which of course should't be larger than 5V. Two resistors in series don't work as the voltage will be to low, putting a resistor end 4V7 zener for that probably won't work and will dissipate precious power at 20V. 5V regulator won't work when PD voltage is 5V.
What's most interesting is that none of the MCU providers that have the Application notes in connecting usb-c to legacy aka usb 2.0 MCU doesn't handle this use case.

@blueskull: Yeah that's what I have chosen too for next project.

[amitchell]

Hi

ST just came out with the STusb4710 standalone UBS-PD controller. Might be worth a look.

http://www.st.com/en/interfaces-and-transceivers/stusb4710.html

[krho]

ST's solution is only for DFP. One usually needs both. Or at least one still needs an analog path to pass UART through and for that you need also UFP path and that also requires to enable at least 5V on UFP.

Although they do have certified PD firmware only for F0 series for STUSB1602 AFAIR you have to sign NDA for you to see/get the sources. But the pricing for that one is TWO times more that for FTUSB302B @ 1000 pcs
Form what I've read cypress gives you all that for free (unless I missed something).