Hello all,

I am trying my first USB C circuit for sink and have some questions with how the power delivery is negotiated.
I have understood the basic concept of negotiation once the CC pins are connected as follows:
1. first by default 5V is provided and based on the resistors on CC pins the source will deliver the (500ma to 3A) current.
2. Source controller advertises the available voltage/current to sink controller
3. Sink requests from one of the voltage / current.
(but this is where is got stuck and also could not find clear answer on specification).
For example, if the sink needs 15Watts, it could use (5V3A, 15V 1A or 9V 1.6A). Which setting does this request?
I assume from efficiency point of view higher voltage lower current is preferred but how to explicitly define this or is this done automatically.
In most datasheet there are settings to define minimum and maximum voltage and current but if multiple options are available, how does it choose to request specific voltage / current .

So, basically how is the priority of voltage / current defined when the source can provide multiple options as stated above?

 

In Step 3 the sink tells the source the needed voltage/current.

The sink has to know, how much voltage it can handle (not all devices can handel more than 5V) and requests the best voltage for it

 

In Step 3 the sink tells the source the needed voltage/current.

The sink has to know, how much voltage it can handle (not all devices can handel more than 5V) and requests the best voltage for it

On datasheet I did not see how sink can request these. For example if the sink needs 15W, i had options to define minimum and maximum voltage and maximum current settings but i could not define priority. So, how is the priority of voltage (5V3A) or (15V1A) defined? (is this defined in standard USB C specification)?
(for example TPS25730 )

 

when I was looking into using usb c power delivery I found a micro controller and pd management chip rolled into one and it handled the negotiation. You set the desired voltages and currents in fuse bits and it handled the communication on the cc lines to negotiate that with the provider. If you're looking for the specific time in the state machine that it negotiates that, you'll probably have to look in the power delivery 3.0 standard to find that out. I cant remember the specific chip I was looking at, but st micro has some more info. https://www.st.com/content/st_com/en/ecosystems/stm32-usb-c.html

To answer you're question, there is data transfer between the devices to negotiate that. Or I think there is a way to static chose a specific voltage to use but I can't remember where in the state machine that is.