The PD negotiation occurs with the sink looking for the source PDO which matches the voltage according to its own capabilities, starting from the highest PDO. Depending on the current in the source PDO, two things can happen:
Section 184.108.40.206 of the Power Delivery specification R3 states as follows:
Capability Mismatch occurs when the sink cannot satisfy its power requirements from the capabilities offered by the source. In this case, the sink shall make a valid request from the offered capabilities and shall set the “Capability Mismatch” bit.
When a sink returns a Request Data Object in response to the advertised capabilities with this bit set, it indicates that the sink wants power that the Source cannot provide. This can be due to either a voltage that is not available or the amount of available current. At this point, the source can use the information in the request message combined with the contents of the Sink_Capabilities message to verify the voltage and current required by the sink for full operation.
When these operations are performed, a valid Request Message means the following:
4. If the GiveBack flag is set to ‘1’, i.e., there is a Minimum Operating Current/Power field:
This example requires the following:
Block diagram of the setup:
Source 1 PDO available: 5V@3A and 10V@ 3A
Figure 1. Source 1 PDO capabilities
BCR sink requirement: 12V
Because the available Source PDOs were 5V and 10V, the negotiated voltage was 10V which is according to the PD specification.
Figure 2. BCR Sink capability mismatch and PDO request
Source 2 PDO available: 5V@ 3A and 13V@ 3A
Figure 3. Source 2 PDO capabilities
BCR sink requirement: 15V@ 3A.
Because the available Source PDOs were 5V and 13V, the negotiated voltage is 13V, which is according to the PD specification.
Figure 4. BCR Sink capability mismatch and PDO request