Bench Talk

(Source: Alexander - stock.adobe.com)

Everybody's charging every day, all the time, on multiple devices, everywhere you look. As more designs opt for using USB Type-C^® connections rather than USB Type-A, this type of connection needs to be thoroughly understood by the design engineer. USB Type-C does have some significant advantages, with a couple of complications. The release of USB Type-C adds more capabilities to these connections than just charging. Data is also being exchanged at very high speeds.

Though the designations for various USB-C versions and types can be confusing, understanding that USB-C is a game changer is essential for several reasons delineated in this blog.

Since its introduction in 1996, the USB has revolutionized how we connect our computers to peripheral devices, replacing a multitude of outdated serial and parallel ports. But that was just the beginning. With the release of USB 2.0 in 2000, speeds soared to an impressive 480Mbps, and the power capabilities were increased to 500mA, making it possible to charge cell phones and other devices.

Improvements didn't stop there. USB 3.0, or USB 3.1 Gen 1, can handle data speeds of up to 10Gbps, while USB 3.1 Gen 2 doubles that to a mind-blowing 20Gbps. The latest USB-C connectors, introduced in 2014 (Figure 1), are quickly taking over the world, and with a recent EU mandate requiring their use in chargers and devices, it's clear that USB-C is the way of the future. But, with different formats and revision levels, design engineers must take care to ensure compatibility.

Figure 1: The USB-C plug. (Source: prima91- stock.adobe.com)

A USB-C connection includes a configuration channel (CC) (Figure 2) that is used when a device is attached to determine the orientation (flip) of the cable and to establish a power delivery "contract" passively or actively between the source and the load (and the cable, as we shall see). The load (downstream port) can detect the source capability by sensing the value of a resistor presented by the source. Table 1 below shows the various possibilities.

Figure 2: This USB-C connector pinout drawing shows its reversibility. (Source: © Chindi.ap, BY-SA 4, Wikimedia Commons)

When a USB-C cable is first plugged in, the device (load) end checks a resistor divider connected on both the host and device side using the CC connector pins (see Figure 2). That divider has three levels and indicates if the host is a basic 5V/0.5A or is capable of a bit more at 1.5A or 3.0A. If the proper level is not found, the system will stay at the lowest data and power levels but continue operations.

The basic CC line interpretation can up the power available to 3.0A but cannot raise the voltage above 5V. If the host and load start talking over the digital (D+ and D-) lines, the connection can be upgraded to use PD multi-voltage operation.

Whereas USB was only a mover of data with limited power capabilities in the past, the latest versions offer significant power capabilities. USB PD describes the power delivery protocol that operates across this interface. USB source to load communication is multi-layered. USB PD is a specific version that emphasizes power delivery and often has data communication. PD communicates power capabilities and needs between the source and the load.

The USB-C Power Delivery (PD) standard allows the voltage to be 5, 9, 15, or 20V at power levels up to 100W (Table 1). The current is a maximum of 5A, but a voltage of 5, 9, 15, or 20V allows much more power. "Fully featured” USB-C PD cables have a chip in their cable connector body called a channel configuration or port controller. These chips produce an e-Marker that tells the other end what power it can send and receive. Cables with a transmission current of 3A or below do not need a channel configuration chip.

Table 1: Evolution of USB power levels. (Source: CUI)

PD messages are in 196-bit blocks transmitted at 300KHz +/- 10 percent over the CC line. At the end of the packet, a 32-bit CRC is sent, followed by a 4-bit end-of-packet (EOP) token that completes the message.

In a PD setup, a chip in each cable connector senses the type of power connection at both ends and adjusts the voltage supply. The device is powered/charged and can request intermediate voltages between 5V and the maximum available fixed voltage of the charger.

USB-C PD cables deserve some caution. 'Basic" cables may not meet USB wire size or signaling requirements, so double-check the specifications. If your power source is capable and you plug everything in the right, but you are not getting the power level or signal speed, which is often hard to tell, it’s likely because of the cable.

USB PD power transmission offers several advantages. One of the key benefits is the higher power available, which is sufficient for powering laptops or monitors and fast-charging compatible smartphones. The charging process can be managed by the phone, which can instruct the charger to slow down or speed up depending on the current requirement. USB PD is also highly versatile, accommodating a wide range of devices, from smartwatches to laptops. Additionally, wall plug power adapters (Figure 3) can now safely charge almost any device with high efficiency and optimized rates, simplifying hardware and reducing costs by being single direction from the adapter.

Figure 3: The SWC45-N series wall plug is a USB C PD power adapter from CUI Inc. (Source: CUI)

In 2021 the USB Implementers Forum (USB-IF) released updated Certified USB Type-C Cable Power Rating logos to specify the USB-C cables’ power capabilities for consumers. Certified USB Type-C Cables now feature logos (Figure 4) highlighting support for 60W or 240W of power, as defined by the recently published USB Power Delivery.

Figure 4: The USB-IF Implementers Forum Association USB-C Cable Logo. (Source: USB Implementers Forum)

International standardization of USB performance for data and power has now been formalized in IEC 62680.

In conclusion, USB-C is becoming more popular and has significant advantages over its predecessor, USB Type-A. USB-C connections are not just for charging, they are also for data transfer, with very high-speed data transfer capability. USB-C connections have a configuration channel that determines the orientation of the cable and establishes a power delivery "contract" passively or actively between the source and the load. The latest USB-C connectors can handle data speeds of up to 20Gbps, and USB-C PD standard allows power delivery up to 100W at power levels up to 5A.

However, caution must be exercised when using USB-C PD cables, and it is important to ensure that the cables meet USB wire size and signaling requirements. Despite the complexity of their connections, USB-C is the way of the future.