The USB Type-C™ specification introduces new options for delivering scalable power delivery over USB, but the specification is intricate and developers face safety and layout issues.
This article will introduce USB Type-C (also known as USB-C) receptacle solutions and guide developers on how to integrate and layout these USB-C receptacle connectors into a new design to safely supply scalable USB power to external devices.
The original USB 1.1 standard specified a maximum current of 500 milliamps (mA) @ 5 volts (2.5 watts), and USB 2.0 allowed the same maximum. This changed with the USB 3.1 specification that allows a maximum current of 900 mA. All of this is using the familiar rectangular USB connector. However, as USB became ubiquitous, its applications and the demands put upon it increased in terms of connector compatibility and power delivery capability.
These demands led to the development of the USB Type-C™ standard. USB-C is not a data delivery specification, but a new standard for a miniature USB connector. Throughout its history USB has been plagued by problems with connector compatibility. Inserting the standard rectangular USB Type-A connector has forever been plagued by Murphy’s Law: no matter how the user inserts the polarized connector, it is always upside-down (Figure 1). Even when inserted in the correct orientation, the connector may not seat properly, resulting in the connector being inverted and inserted again, and again, and again.
Figure 1: The many types of USB connectors have plagued developers and users since USB 1.1. The most common connector found on consumer computers is the USB Type-A connector, used for USB 1.1, 2.0, 3.0, and 3.1. (Image source: Wikipedia)
Given the large size of the polarized Type-A connector, the smaller polarized keystone micro and mini connector types were developed for easier integration on small consumer devices. Even these had the same orientation problems for developers and users as the Type-A.
The new USB-C connector (lower-right of Figure 1) is only slightly larger than the USB micro-B connector found on Android smartphones and Internet of things (IoT) devices. It replaces both the computer (Host) and the Device connector, resulting in the replacement of multiple cable types with a single type. Also, the USB-C connector is not keyed and has no preferred orientation, allowing for a solid connection regardless of how the connector is inserted.
USB-C connector pinout and power levels
The USB-C connector supports both USB 2.0 and USB 3.1. When used for USB 3.1, the standard requires that it also support backwards compatibility with USB 2.0, and this is the recommended use for new designs. However, for low data rate designs the connector can also be used for USB 2.0 only.
Figure 2: The USB-C 24-pin receptacle connector is non-polarized and reversible, allowing a plug to be easily connected no matter which way it is inserted. (Image source: STMicroelectronics)
Looking at the USB-C connector receptacle pinout, the four ground pins (GND) are arranged at the outside of the connector (Figure 2). This helps with noise immunity, and also allows for easy connection to the metal grounded connector shell. The standard USB 2.0 bidirectional data pins D+ and D- are duplicated in the center and are mandatory for all USB-C data transmission applications. USB 3.1 has separate high-speed transmit and receive data paths, with the receive pins RX1+ and RX1- doubled up with RX2+ and RX2-. The USB 3.1 transmit data paths are the same with TX1+ and TX1-, and duplicate TX2+ and TX2-.
The USB-C connector standard also supports video transmission, including DisplayPort and HDMI. The standard calls this Alternate Mode and will not be covered in this article.
What’s important in this context is that the USB-C connector standard specifies a maximum current delivery of up to 3.0 amps at 5 volts for up to 15 watts of power. Taking this further is the USB Power Delivery Standard v2.0, which specifies that a USB-C connector supporting USB 3.1 may source up to 100 watts of power (20 volts at 5 amps). This power is sourced at the four VBUS pins. This takes the USB interface from being an auxiliary source of power to being a primary power source.
Implementing USB-C connector designs can be tricky
Supporting up to 100 watts of power in a project requires careful board layout procedures to ensure safety for the user as well as the developer. Most projects won’t need to source that much power; for example, a very high current smartphone charger might be rated at 3.0 amps. However, a common sweet spot for most commercial USB-C connectors is 5.0 amps between VBUS and GND pins. This is supported by the USB 3.1 10137062-00021LF Gen 1 right-angle USB-C connector from Amphenol FCI (Figure 3).
Figure 3: The Amphenol FCI 10137062-00021LF USB-C connector is a right-angle top-mount short body connector, and can be through-hole mounted or surface mounted. (Image source: Amphenol FCI)
This USB-C receptacle connector supports a maximum of 5 amps, so to source 100 watts would require 20 volts DC. For most projects, however, 25 watts (5 volts at 5 amps) is sufficient and safe. This USB-C connector supports the USB 3.1 Gen 1 data rate of 5 gigabits per second (Gbits/s), and the maximum voltage rating is 100 volts DC or AC, which could source a maximum of 1 amp, per the specification’s 100 watt maximum power.
This connector supports surface mount or through-hole assembly and sits on top of the pc board. The stainless steel connector shell is more rugged than aluminum, and is electrically connected to the GND pins.
The shell must be grounded using four narrow tabs that slip into slots in the pc board, two on each side of the connector. Be sure to solder these tabs to the pc power ground plane using a generous amount of solder to insure a solid connection.
USB-C connector signal routing
The USB 3.1 high-speed differential signals must be carefully routed so that they are adjacent to each other and at the exact same length. Keep the traces for the differential signals as short as possible to minimize EMI. For best noise immunity, place the differential signals on an inner layer of the pc board. If routed on an outer pc board layer, isolate the signals from other data lines by surrounding the differential pair traces with ground traces. Also, always route differential signals over a solid ground plane to minimize EMI.
Design the pc board so that the differential trace impedance is 90 ohms ±10% in order to match the USB cable differential impedance. In addition, route each trace so that the single-ended impedance of each pair is the same. As a rule of thumb, in this situation, the impedance of a differential pair is twice the impedance of one of the pairs. As such, the traces should be routed so that each single-ended impedance is at or near 45 ohms ±10%.
How to safely route USB-C power signals
Routing the power signals is more critical. Safely sourcing 5 amps must be done carefully to prevent accidental shorts to the project casing or the user. The 5 amps can be routed on a top or bottom layer of the pc board, but it should not be too close to the edge of the pc board. This will help prevent accidental connections to the project enclosure caused by a shock or damage to the enclosure.
To safely source 5 amps on a pc board over copper with a thickness of two ounces per square foot requires a trace width of 44.6 mils. A safer method is to insulate the current from any external influence by routing the 5 amps on an inner pc board layer, which would require a trace width of 116 mils with the same copper density (calculations based on IPC-2221 profile). Route as much copper as possible near the VBUS connector pins to prevent current loss.
Vertical mounted USB-C connectors
If pc board space is at a premium, the USB-C receptacle connector can be mounted vertically. For this, Amphenol FCI has the USB 3.1 10132328-10011LF vertical mount USB-C connector.
Figure 4: This vertical mount USB-C connector from Amphenol FCI has a small pc board footprint and can be used to save board space. (Image source: Amphenol FCI)
This vertical USB-C connector supports the USB 3.1 Gen 2 data standard of 10 Gbits/s. It also supports power delivery of 100 watts with its maximum voltage rating of 100 volts DC or AC, and ability to source up to 5 amps. It has the same stainless steel shell construction as the right-angle connector. Like the right-angle connector, ensure the four tabs on the housing are safely grounded through holes in the pc board with a generous amount of solder.
Unlike the right-angle receptacle, it is only surface mount on the small end of the connector, making the VBUS power contacts closer to the signal contacts. Careful routing of the power contacts away from the signal contacts is a must. Given the cramped space, the safest method is to place the data pairs and the VBUS power contacts on different pc board layers.
When sourcing power to the above receptacle connectors, there is a brief handshaking protocol between the USB Host and the Device that decides how much power to source. There are ICs that handle the USB sink-to-source connections, making the process transparent to the developer.
A good example is STMicroelectronics’ STUSB1700 USB-C source controller. This safely manages 5 volt USB-C Host to Device connections. When sourcing power, the STUSB1700 can detect and protect against power short circuits, current draw above a programmed limit, overheating above 145°C, under and overvoltage conditions, and reverse current and reverse voltage conditions. This greatly simplifies the safe design of a USB-C system while reducing the complexity for the developer.
Figure 5: The STUSB1700 in this circuit is sourcing 3 amps of power and can operate independently. If managed by an optional microcontroller with an I2C interface, pullup resistors R3 through R10 must be added. (Image source: STMicroelectronics)
The STUSB1700 is used for USB-C Host connectors and can detect a new connection between the Host and a Device. It can determine the power needs of the Device and source the necessary current. It also determines if the device is a digital audio accessory, so it can assert a signal to the microcontroller to supply digital audio through the USB-C port. It can negotiate with the USB Device to decide if the power needs to be the USB default (up to 900 mA), USB medium (up to 1.5 amps) or USB high current (up to 3.0 amps).
The new USB-C standard makes it easy to safely supply power up to 100 watts to suitably designed devices. With all smartphones, digital cameras, computers, and electronic accessories standardized on one easy to use connector, developers don’t have to worry about which size and type of connector to use, which also future-proofs designs.