SLVAFG6B December   2022  – November 2024 TPS25762-Q1 , TPS25772-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 Terms and Abbreviations
  5. 2 SPM Engine Overview
  6. 3Multi-Port Power Allocation Policy
    1. 3.1 Assured Capacity Policy
    2. 3.2 Shared Capacity Policy
      1. 3.2.1 Fair Share Power Policy
        1. 3.2.1.1 Blind Sink Support
    3. 3.3 Hybrid Mode
  7. 4Power Foldback Policy
    1. 4.1 Thermal Foldback Operation
    2. 4.2 Engine On/Off Transition Management
      1. 4.2.1 Engine On/Off Power Foldback Operation Example
  8. 5Multi-Port Power Allocation Policy Examples
    1. 5.1 Assured Capacity Policy Example
    2. 5.2 Fair Share Power Policy Example
      1. 5.2.1 FSP Policy Example – Blind Sink Support
      2. 5.2.2 FSP Policy Example – Blind Sink Support With Max Power
    3. 5.3 Hybrid Mode Example
  9. 6References
  10. 7Revision History

3.2.1 Fair Share Power Policy

Fair Share Power (FSP) Policy is the Shared Capacity Policy's Power Distribution Rule. When using FSP, Port Min Power is guaranteed for each Source port and the remaining power in Power Reserve is intelligently distributed to ports according to their Port Partners' Sink Capabilities.

The FSP parameters shown in Table 3-2 are used in the power distribution method and to achieve FSP between two Sinks.

Table 3-2 Fair Share Power Policy Parameters
Fair Share Power Policy Parameters Description
Port Min Power Configurable minimum VBUS power for each port
Equally Divided Shared Power Equally Shared Power from Power Reserve that can be distributed to each port.
Power Reserve / Total number of ports.
Per Port Guaranteed FSP Guaranteed Shared Power for each port.
Equally Divided Shared Power + Minimum Port Power

The typical flow of the Fair Share Power Policy power distribution is as follows:

  1. The first connected Source port initially advertises Port Min Power and sends a Get Sink Capabilities request to its Port Partner.
  2. The Sink responds by returning a Sink Capabilities message and receives power accordingly.
  3. The remaining power is stored in Power Reserve.
  4. The second connected Source port initially advertises Port Min Power and sends a Get Sink Capabilities request to its Port Partner. After receiving the Sink Capabilities, the SPM begins the power distribution process. During the power distribution process, power is harvested from or contributed to Power Reserve to fulfill the Sinks' requirements as shown in Table 3-3.
Table 3-3 Example Scenarios and Corresponding Power Distribution
Example Scenario Corresponding Power Distribution
1 Both Port Partners' Sink Capabilities are less than the Per Port Guaranteed FSP. The SPM distributes power to meet the Sink Capabilities and stores the remaining power to Power Reserve.
2 Both Port Partners' Sink Capabilities are higher than the Per Port Guaranteed FSP. The SPM distributes the Per Port Guaranteed FSP to each Sink.
3 One Port Partner's Sink Capabilities is higher than the Per Port Guaranteed FSP and the other Port Partner's Sink Capabilities is less than the Per Port Guaranteed FSP. The SPM distributes power to the Sink requesting lower power and the remaining power to the other Sink.
Note: It is required per USB PD specification for Sink devices to report a Capability Mismatch if the power offered by the Source does not satisfy its power requirements. Once the Source receives the request with a Capability Mismatch flag from the Sink, it requests the Sink Capabilities by sending a Get Sink Capabilities message. Some Sinks, however, are unable to report a Capability Mismatch. Therefore, although a Sink may report a Capability Mismatch or present limitations to accurately communicate its full power needs, the TPS25772-Q1 always attempts to fetch the Sink Capabilities from the Port Partner by sending a Get Sink Capabilities message to better fulfill the Sink's power requirements.

For example scenarios of FSP to better understand this policy's power distribution methods, see Section 5.2.