Design World – Power Transmission Reference Guide June 2019

eeworldonline.com | designworldonline.com 6 • 2019 DESIGN WORLD — EE NETWORK 31

Teledyne LeCroy Voyager M310P test platform

https://teledynelecroy.com/protocolanalyzer/usb/voyager-m310p

source make voltage changes in small steps. As a result, charging

completes in less time while keeping heat under control, which

can also help extend the battery’s lifespan. Once the source

reaches predetermined voltage level, the sink re-negotiates by

requesting a constant voltage.

• New PD 3.0 terminology for PPS mode: Many different
  devices might be plugged into a given PD charger, but PD
  aspires to universality. Thus, the PPS approach demands
  that the sink side be smarter, given that the sink knows
  how much power it needs. This gives rise to the new term
  Augmented PDO (APDO) to define chargers that can
  deliver a range of voltages. Yet, a maximum current still
  must be defined. PPS sinks will now use what are termed
  Request Data Objects (RDOs) to make these real-time
  dynamic adjustments to power.


• Fast Role Swap (FRS) Hub: This new PD 3.0 feature
  specifically targets USB hubs and docking stations.
  Typically, users plug their docks into a wall outlet; thus, it
  is the dock that is powering a laptop and any other USB-
  powered accessories plugged into the dock. If power is
  lost by, say, the cord being unplugged, users run the risk
  of losing data being saved to a USB device attached to the
  dock at that moment.


• FRS hubs detect the loss of ac power and quickly perform
  a “power role swap.” Upon sensing loss of power, the
  dock speculatively signals FRS by pulling VCC to ground
  for 60-120 μsec. The dock must have hold capacitors that
  can maintain Vbus at 5 V for at least 150 μsec after Vbus falls
  below 5 V. In this manner, the unpowered dock continues
  to source Vbus on its downstream ports while the Fast Role
  Swap takes place.

After a power loss, the sink must be monitoring VCC. When VCC

goes low for 30-60 μsec, that constitutes a Fast Role Swap

request, and the sink then becomes the source by supplying 5 V

to the bus. The key here is that this FRS capability is relevant only

for Dual-Role Power (DRP) ports that can alternate between sink

and source roles.

• Extended Messages: PD 3.0 offers an extended payload
  size for larger data transfers with packets of up to 260
  bytes. Initially, these larger packets were added to allow
  firmware updates delivered over the PD channel, but
  their usage was extended into arenas such as security
  (authentication), battery information, and other data.

As described by the USB Implementors

Forum, Fast Role Swapping is a new PD 3.0

feature for USB hubs and docking stations.

POWER DELIVERY 3.0

Power

USB PD

capable host

USB PD

capable hub

Power Power

Bus powered

accessory

Power source

DRP DRP DFP

UFP

Fast role swapping sequence

LeCroy — Test and Measurement HB 06-19.indd 31 6/7/19 1:39 PM