What is a USB Type-C charge-through dongle (CTD)?
Enabling a multifunction dongle with charge-through support is one of the major achievements of USB Type-C. Through a single USB Type-C port, one can have USB3.0 data, DisplayPort video, and enough power for most systems. Charge-through dongles are a particularly convenient as they can operate both on host power and via the power supplied via an external USB Type-C charger.
Figure 1: Charge-Through Dongle Block Diagram
Charge-through dongles will have one or two additional port namely USB 3.0 and/or Display. This system cannot have more than one USB 3.0 port because in the absence of the external Type-C charger, the notebook power is not sufficient for more than one USB 3.0 port. The Display port can be DisplayPort or HDMI/DVI/VGA. However, in the case of an HDMI/DVI/VGA port, the system will require a DP-to-HDMI/DVI/VGA protocol convertor.
Power flow in the system
Case 1: External USB Type-C charger not connected
The power source is from the notebook (typically 5 V/1.5 A), which is sufficient for one USB3.0 downstream port and a display (DisplayPort/HDMI/DVI/VGA) port. The charge-through dongle system cannot support more than one USB3.0 data downstream port due to insufficient power from the notebook (host). The power delivery (PD) controller on the Type-C plug will act as a sink for power
Case 2: External USB Type-C charger connected
The power source is from the external charger. Some part of the power is allocated to the CTD system (including the USB3.0 and display port) and the remaining power is delivered to the notebook. The PD controller on the Type-C plug will act as a source for power.
Power loss issue in USB PD2.0
One tricky case that the USB Power Delivery 2.0 spec does not cover is as follows:
When an external Type-C charger is attached to the CTD, the CTD takes power from the charger to enable whatever external ports are available on the CTD, and passes the remainder to the host. In the event of a disconnect of the external charger, there is no longer any power available for the external ports, potentially resulting in data loss. The host will detect a disconnect event due to loss of power on the VBUS and will start sourcing VBUS to the CTD because the PD controller on the Type-C plug will act as a sink for power after the external charger was disconnected. This VBUS turn-around time is long enough for the power to cycle on the external device is a particularly poor user experience, because the user never detached the accessory or the external device, and yet it is reset. There is no easy solution to the power loss issue as per the USB Power Delivery 2.0 specification. A standard power role swap is far too slow to be used in such an emergency situation because there would have to be enough capacitance on the device to hold up the external power until the swap is completed.
Power loss issue resolution in USB PD3.0
USB Power Delivery 3.0 spec implements a standard way to perform an emergency quickswap known as Fast Role Swap (FR_Swap) to restore power to the CTD within a well-defined time after charger disconnection or failure. Fast Role Swap is a process of exchanging the Source and Sink roles between Port Partners rapidly due to the disconnection of an external power supply. CTD can then be designed with a minimized amount of capacitance to maintain power. As the emergency Fast Role Swap is followed up by proper PD messaging to resolve the state, Fast Role Swap is intended to complement, not replace the normal power role swap.
For more details, see USB Power Delivery spec 3.0, section 6.3.17 and section 7.3.15.
Cypress’s USB PD 3.0 solution
The EZ-PD™ CCG3/CCG4 fully supports the latest Power Delivery 3.0 specification, especially the Fast Role Swap feature. Fast Role Swap enables an externally powered dongle to rapidly switch to bus power when their external power supply is removed. This solution is designed to be fully interoperable with revision 3.0 of the USB Power Delivery specification as well as revision 2.0 of the USB Power Delivery specification.
Cypress has developed CCG3-based Charge-Through Dongle reference design that will enable customers to speed up their design and development cycle. More details about the reference design can be found: