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I'm new to USB Type-C and trying to design a USB-C powered device. After investing some time reading the specification and some application notes, I'm still having problems understanding the big picture.

My requirement is to be able to draw 1A at 5V. The USB connection will be used for power only. There will be no USB data transfer. With a CC-Controller and compatible wall adapter this should not be a problem as the current capability will be announced via CC, right? And PD would not be required.

However, I want the user to be able to use any old USB wall adapter (>=1A). But the specification defines C-to-legacy cables to have a CC pullup to VBUS, limiting the current to the default USB 500mA/900mA. As far as I understand it, 2.0 chargers/adapters use BC 1.2 via D+/D- to supply higher current to devices (not totally sure on this one).

So in total, does this mean I need a controller that supports CC as well as D+/D- legacy current negotiation for delivering power only? Does something like that exist?

mauseschnauz
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  • The spec on legacy power delivery is 500mA as you say however most dedicated USB chargers can be capable of much higher current (phone chargers, etc.). USBC PD is quite complex to implement and seems like a lot of extra work if you just want a 1A/5V supply. I'd use a USB 2.x/3.x connector and just specify a particular wall adapter or spec. – Brendan Simpson Nov 11 '21 at 14:55
  • @BrendanSimpson Thank you! The USB-C is fixed for me, so I can't go around that. I'm just wondering if there is any way to know for my device that it is connected to a legacy supply that can't deliver more than 500 mA. Because even if it was connected to a charger that supports more, CC would limit it to 500 mA – mauseschnauz Nov 12 '21 at 08:30
  • I think the short version is: A USB C UFP connected to a USB-A DFP cannot negotiate for power delivery under the UBS PD spec. This answer sums up power negotiation on legacy devices: https://electronics.stackexchange.com/questions/5498/how-to-get-more-than-100ma-from-a-usb-port My conclusion is that you use a PD controller on your device, to handle talking with USB-C chargers, and you implement the 1.8A detection on the D+/D- if you want to detect capable USB-A chargers. You will probably need to read the spec – Brendan Simpson Nov 12 '21 at 16:53

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