FT230 in a USB-PD self-powered application - Page 1
Forum section: electronics / projects

author: timbos
author_rank: Contributor
timestamp: May 23, 2025, 04:24:00 pm
content: Hi,I'm designing an application that's going to be powered over a USB-C port at 20V. I'm using a dedicated USB-PD chip to negotiate the power contracts between the source and the sink (an STUSB4500, if it matters) and am looking to add an FT230XS to handle the data transfer (some low bandwidth comms). FTDI's data sheet is not amazing, and there's contradictions around whether I should use VBUS_sense or not (my thought is not because there's no alternative power source to the USB connection so it can never push power back out of the USB port).I've got a 3v3 bus that's derived from the bus voltage that will come over the USB port, so the order of business at connection is:1) default 5v arrive on the bus2) 3v3 is available for the FT230XS, and the microcontroller3) STUSB4500 receives power and negotiates 20V budget4) VBUS raises to 20V assuming that the source can provide it5) microcontroller starts to talk over UART through the FT230Does this look sensible in terms of a schematic? What am I missing?

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 23, 2025, 06:03:39 pm
content: The datasheet explains what the VBUS_SENSE does. What is probably confusing in your case is that you have something that's effectively bus-powered but sort of acts as if it were self-powered due to the fact you can't power the FT230X directly from VBUS.If I understand correctly what you want to do (assuming that your VDD is generated from VBUS via a regulator), this is a bus powered configuration, but indirectly.How do you generate VDD, to make sure?From what I understand (haven't personally used a FT230X in self-powered configurations, but only in bus-powered, so I can't speak from experience about this particular point), it's recommended to connect VBUS_SENSE so that the chip can handle USB suspend properly. In a classic config with VBUS = 5V (which they show as an example), there's a voltage divider from VBUS because the VBUS_SENSE pin is actually just a configured IO and thus is in the VCCIO power domain (typ 3V3). But in your case, the difficulty is that VBUS can change in a rather wide range so a simple voltage divider won't work.If my assumption above is correct though (you generate VDD from VBUS), you don't need to bother IMO anyway, because if the device is disconnected (and so VBUS drops), the FTDI chip won't be powered anyway, so you don't care about what it does in that case as it won't do anything.Just a small warning about that: if you have significant capacitance at the output of your VDD regulator, the FTDI chip could remain powered for a short while when disconnecting the chip, but I don't think it really matters.If, OTOH, VDD comes from a separate power source than VBUS, this is another matter.Final note: FTDI actually now has chips with integrated PD controllers. In your case, you could achieve the same thing with a single chip, such as the FT232HP.So unless the latter is hard to source, or the combination you want to use (ST chip + FTDI) turns out cheaper, you may as well go for the single-chip solution.

author: timbos
author_rank: Contributor
timestamp: May 27, 2025, 10:38:49 am
content: You're right that VDD is generated from VBUS, so my device is effectively bus-powered. I also contacted FTDI support, and their recommendation was that VBUS_SENSE was not required in this application because there's no power when the cable is unplugged. For future reference to anyone finding this thread, the schematic that I went with is attached. 3v3OUT becomes an input when the chip is powered with 3v3, so it needs to be connected to VDD/VCC.Thanks also for the suggestion regarding the FT232HP. Will look into that.