How to Enable USB-C Power Exchanges With and Without Power Delivery
In this article, we'll discuss the main features required to have a USB-C power exchange with and without PD.
USB Type-C enables a flippable connector with a small form factor. It extends the power capabilities to 15 W, allowing the source to dynamically manage current from 0.5 A to 3 A. Data speeds have been increased to 10 Gbps.
Besides, USB-C supports USB power delivery (PD)—a single-wire protocol that lets devices create a contract to deliver the optimum power level for each application. With PD, USB-C enables negotiable power up to 100 W.
In this article, we’ll look at the main features required to have a USB-C power exchange with and without PD. We’ll also take a look at the MAX77958, which is a recently-released USB-C PD controller from Maxim Integrated.
Power Exchange Without PD
To have power transfer over a USB-C connection, we first need to specify the power role of the ports. A port can be a source, sink, or a dual-role power (DRP). As the name suggests, a source provides power over the VBUS line and a sink receives power from this line.
A DRP can act as either a source or a sink. To specify the power role, a source pulls up its CC lines with Rp resistors while a sink pulls these lines down with Rd. A USB-C source-to-sink connection without PD is depicted below.
Block diagram of a USB Type-C source to sink without power delivery. Image used courtesy of Richtek
Note the pull-up and pull-down resistors at the source and sink ports.
We also need to specify the power level that the source can provide. Without PD, VBUS has a fixed voltage of 5 V. However, depending on the USB version employed, different current levels including 500 mA, 900 mA, 1.5 A, and 3 A, are possible. Hence, the maximum power level without PD is 15 W.
While the pull-down resistor has a fixed value, different values of pull-up resistors are employed to specify the current level that the source can provide. The sink monitors the CC line voltage to detect the current level advertised by the source.
The USB-C provides the 5 V on the VBUS line only after a source-to-sink connection is detected (cold-plugging). Therefore, as shown above, MOSFET switches are required for the 5-V rail. Additionally, the source should include overcurrent protection circuitry to prevent the sink from drawing a current higher than what the source can deliver.
Another feature that should be included is the VBUS discharge mechanism. There are circumstances that we need to rapidly discharge the VBUS line, e.g. when the cable is disconnected or the sink requests a lower voltage on the VBUS (the latter might be needed when implementing USB-C with PD). The USB-C specification requires a source-to-discharge VBUS line within 650 ms after a sink is removed.
Note that we can have USB-C non-PD applications with DRP ports. In this case, the port should be able to connect the CC lines to either pull-up or pull-down resistors.
Power Exchange With PD
USB power delivery (PD) is a power exchange protocol designed to meet the growing needs of power-hungry applications. USB PD allows the VBUS voltage to increase up to 20 V at a maximum current of 5 A (maximum power of 100 W). Different power levels can be provided based on the following power profiles from the PD specification.
PD profiles. Image used courtesy of Texas Instruments
With PD, four different bus voltages—including 5 V, 9 V, 15 V, and 20 V—are supported. Up to 60 W, the PD specification sets a maximum current of 3 A and allows the source to increase the power level only by increasing the VBUS voltage. However, beyond 60 W, the source is allowed to increase the current level by 5 A if required.
Also, note that the source should be able to deliver all of the voltages and power levels below its maximum power level. For example, if a source advertises a maximum power of 45 W, it should be able to deliver 15 V at 3 A; 9 V at 3 A; and 5 V at 3 A. This rule is introduced to ensure that a high power source can support lower power sinks as well.
A USB-C source-to-sink connection with PD is depicted below.
Block diagram of a USB Type-C source to sink with power delivery. Image used courtesy of Richtek
A voltage regulator is incorporated to deliver the required VBUS voltages. With PD, higher levels of power are transferred over the VBUS line. This requires a higher power MOSFET switch with a dedicated gate driver for the VBUS line.
The PD specification requires communication between the source and sink to choose the appropriate power level. These communications start with a request from the sink for the source power capabilities.
After the source provides a list of its supported power levels, the sink requests a suitable power level. Then the source adjusts the VBUS voltage to the requested value and sends a "power supply ready" signal back to the sink. All these PD communications are performed over the CC lines as shown in the above block diagram.
Another feature that should be included in PD applications is protection against a short between the connector pins. The USB-C with PD incorporates a higher pin density along with higher power levels. As a result, a short between pins is more likely to happen and can have more catastrophic consequences.
To address this problem, many USB PD controllers employ short-to-VBUS protection on certain pins of the interface.
The Maxim Integrated’s New USB PD Controller
One example of a USB PD controller that provides protection against shorts between connector pins in Maxim Integrated's recently-released MAX77958. This USB PD controller supports sink, source, and DRP configurations. The device's default firmware supports USB-C version 1.3 but it can be updated for future specification revisions. A typical application diagram of the MAX77958 is shown below.
Simplified block diagram of the MAX77958. Image used courtesy of Maxim Integrated
The MAX77958 is PD 3.0-compliant and supports customizable actions on events. It can detect moisture and prevent corrosion on the USB-C connector. The new device supports high voltage VBUS (28 V) and short-to-VBUS protection on CC pins (22 V). It employs programmable power supply (PPS) and fast role swap (FRS) features.
The IC is available in a 3.10 mm x 2.65 mm, 0.5 mm pitch, wafer-level package (WLP).
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