USB Power Delivery

USB has several methods of distributing power between Hosts (Downstream Facing Ports (DFPs)) and Devices (Upstream Facing Ports (UFPs)).

The USB specification also provides for a Battery Charging specification to allow a USB Host the ability to charge the battery of a Device.

Default Power

The USB specification calls for 2.0 compliant Hosts to deliver 500 mA at 5 V to an enumerated Device. USB 3.0 and 3.1 Hosts offer 5 V at 900 mA. Product manufacturers must design their applications to be compliant with these specifications. USB certified Hosts must be capable of delivering this amount of power. Certified USB Devices must likewise limit their power draw from the Host to these limits.

Bus-powered devices will begin to enumerate at a power level lower than the default power. During the enumeration process, the Device informs the Host of how much power it would consume on a configuration-by configuration basis. The host analyzes the available power before the enumeration process is completed and a Device configuration is activated. It is the Device's responsibility to control its current usage and keep it in compliance with the Host's capabilities. A key restriction of USB compliance is that a Device will not consume more power than it has told the Host during enumeration.

Type C Current Mode

Type-C systems are capable of supplying more power than the default power of 2.0 / 3.x systems. The amount of power delivered by Type-C systems can vary from system to system. Using the pull-up resistors (Rp) connected to the CC1 and CC2 signals, Type-C DFPs advertise to UFPs their ability to supply current in excess of the USB default power.

DFP Capability Rp for 5 V signal Rp for 3.3 V Current source
1.5 A @ 5 V 22 kΩ 12 kΩ 180 μA
3.0 A @ 5 V 10 kΩ 4.7 kΩ 330 μA

It is the responsibility of the UFP to monitor the voltage on the CC line to determine how much power is available.

Power Delivery Protocol

USB Power Delivery 2.0 (PD) refers to the protocol used between a DFP, an Electronically Marked cable, and a UFP. PD commands are transmitted through a cable's CC line. Despite its name, PD is not limited to just power negotiations. Alternate mode and Data Role switching (swapping of UFP and DFP roles) are achieved through PD.

At start-up, the DFP is the Power Provider and the UFP is the Power Consumer.

PD provides for the Power Provider to inform the Power Consumer what current and voltage levels can be provided. The Power Consumer will inform the Power Provider of which profile it wants to use and which voltage level is required. The Power Provider then sets the appropriate voltage level. There are five standard protocol profiles available from the PD:

Profile # ~5 V ~12 V ~20 V
1 2 A (10 W) n/a n/a
2 2 A (10 W) 1.5 A (18 W) n/a
3 2 A (10 W) 3.0 A (36 W) n/a
4 2 A (10 W) 3.0 A (36 W) 3 A (60 W)
4 2 A (10 W) 5.0 A (60 W) 5 A (100 W)

PD allows the Provider and the Consumer roles to be switched at any time.

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