USB-C PDO Negotiations when the Source Offers Higher Power than Source Needs

[skent96]

Hi All - 

I'm not 100% certain that this is the right location for this discussion, so my apologies if it is not.

I'm trying to understand how the USB PD specification requires the handling of a situation where a PD Source offers a power rating in one of its PDOs that is higher than a PD Sink requires.   

For example, in my application I'm considering configuring my CCG2 with the following Sink PDOs: 

• 5V@3A  [15W], 
• 9V@2.8A  [25.2W], 
• 15V@1.8A  [27W].

My application uses a charger that would likely draw 2.64A@9V  or 1.66A@15V .   At 5V my charger would like to draw 3.6A, but the charger is limited to an input current of 3.3A, so I think a 3A PDO limit is fine as I'll software limit the charger.

My confusion is how the CCG2 will negotiate with various off-the-shelf chargers that are available on the market.   For example, the Nintendo Switch and Apple MacBook chargers offer the following PDOs:

Switch:

• 5V@1.5A [7.5W]
• 15V@2.6A [39W]

MacBook:

• 5.2V@2.4A [12.5W]
• 9.0V@3A [27W]
• 14.5V@2A [29W]

Now, when I plug my device into the Switch charger, which of my configured PDOs will it default to?  My highest PDO power draw is 27W, so will the Switch output my requested 15V@1.8A or will it provide me with 9V@3A which seems to be a standard 27W supply... 

The same question happens for the MacBook charger.   If I plug that into my device, then which PDO will be negotiated?

Let's also assume that my highest PDO option was actually 15V@2A instead of 1.8A for a power draw of 30W.  In that case, would the Switch negotiate 15V@2A even though that isn't one of the listed PDO that it offers?  And the MacBook charger... What would that output?

Hopefully this information will be useful to others as well.  I can't seem to find clear examples of these negotiations anywhere else.

-Shane

[Sananya_14 · Accepted solution]

Hi Shane,

The PD negotiation will occur with the rule that the sink will look for the source PDO which matches the voltage as per its own capabilities, starting from the highest PDO. If the current in that source PDO is more than the operating current of the sink PDO, it will send a request with its own operating current and draw the same. If the current in that source PDO is less than the operating current if the sink PDO, it will send a request with the operating current of the source mentioned and the "capability mismatch" bit set. Please refer to section 6.4.2.3 in the USB PD 2.0 specification for a detailed explanation.

So, for the sink PDOs mentioned above, the request should be for 15V, 1.8A (27W) for the Nintendo Switch charger and 9V, 2.8A (25.2W) for the Macbook charger.

If you change the sink PDO to 15V, 2A now, the request would change for the Nintendo Switch charger with the current increased to 2A but there should be no change for the Macbook charger.

Best Regards,

Sananya