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I have a question about power supplies and Power Delivery on USB-C. I'm trying to understand the USB-C specification, Power Delivery, UFP and DFP, DRP... but this is a nightmare. There are not clarifying examples on the USB-C Specification, only a ton of schemes with 1200 configurations with different specs but i find it hard to understand.

I have the next USB-C connector, configured as 5V 3A supplier to my board. My question is, What happens if I connect a 5V-9V-12V-20V supply? How it will be the negotiation between power supply and my board? I suppose that with the CC resistors is a standard USB connection, and it will be chosen 5V. Is that correct?

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Juanma
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    *What happens if I connect a 5V-9V-12V-20V supply?* Then you will get 5 V and 5 V only. For more than 5 V to be delivered from the power supply, negotiation is needed. Think about what would happen if that wasn't the case. If an (older?) USB-C device that can only support 5 V would get 9 V or worse: 20 V, what could be the consequences? So that's why the complex negotiation is needed, to make sure the higher voltage is OK. – Bimpelrekkie Jul 06 '21 at 08:43
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    Yes, the resistors put the supply into the default mode, which is 5V (without them, you get nothing from a USB-C port). Anything else requires negotiation, which is a two way communication through the CC line(s). – Finbarr Jul 06 '21 at 08:49
  • You might find [this link](https://www.silabs.com/community/mcu/8-bit/knowledge-base.entry.html/2016/09/26/what_s_the_role_ofc-kQYe) helpful – Finbarr Jul 06 '21 at 08:57
  • Thank you so much for your answers. Can you write your comments as answers to choose them as a answer to the question? – Juanma Jul 06 '21 at 09:16

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By default, you get nothing out of the Vbus pins of a USB-C port until you plug something into it and the port detects that through resistors attached to the CC pin(s).

To get the standard 5V Vbus supply, all that is needed is a pulldown resistor in your device. This combines with the pullup resistor to 5V in the host port to detect that a device has been connected and advertise the amount of current available from the port at the default output of 5V:

Image from https://www.silabs.com/community/mcu/8-bit/knowledge-base.entry.html/2016/09/26/what_s_the_role_ofc-kQYe

(Image from https://www.silabs.com/community/mcu/8-bit/knowledge-base.entry.html/2016/09/26/what_s_the_role_ofc-kQYe)

The value of Rp identifies the current available at 5V as follows:

  • 56kΩ: Default (500mA or 900mA)
  • 22kΩ: 1.5A
  • 10kΩ: 3.0A

As Rd is defined as 5.1kΩ it's possible that Rp may be implemented as a current source to get the same effect rather than a fixed resistor.

Any higher voltage from Vbus requires negotiation through the CC pin, which means that any device can assume it will get a supply of 5V without it. This is essential to ensure that legacy devices will not be damaged by supplying them with higher voltages than they can accept.

If you do need that higher voltage, there are plenty of devices on the market you can build into your design to do the negotiation for you without having to worry about the details.

Finbarr
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