What are the Maximum Power Output and Data Transfer Rates for the USB Standards?

June 8, 2020 Cadence PCB Solutions

Variations of USB cable

Over the last decade, the USB port has been the most recognized component in electronics, aside, of course, from the smartphone. Some end-users even base their desktop and laptop purchases on whether there are enough USB ports.

The USB port has bridged the gap of communication between a myriad of devices that are otherwise physically incompatible. The fact that we are in our third series generation for the component speaks volumes to its popularity and widespread use.

The USB port is an indispensable component and thus requires the understanding that not all USB ports are the same. This becomes especially important when the device you wish to charge or use requires a USB 3.0 port, and the device you want to use only has a USB 2.0 port.

Brief History of the Universal Serial Bus

The first standard for USB, called USB 1.0, dates back to the 1990s. Although by today’s standards, a maximum transfer rate of 1.5 Mbps (USB 1.0) and 12 Mbps (USB 1.1) is nearly unfathomable, back then, these standards set the bar. So much so that it persuaded PC manufacturers to implement them into their board designs.

To put it mildly, USB 1.1 was a game-changer in the field of computers and peripherals alike. During this time, the number of different types of ports on a PC’s motherboard was staggering. However, with USB 1.1, this all changed, and even the peripheral manufacturers added USB connections to their devices.

Like all things in the field of electronics, the USB standard progressed and evolved, thus ushering in the standard USB 2.0. In the spring of 2000, the USB 2.0 standard supplants USB 1.1 as the defacto USB standard. Also, with the newer standard came increased data rates. The specifications for USB 2.0 include a data rate of 480 Mbps, and this increase afforded the use of external hard drives.

USB Multiport adaptor

USB has come a long way since its inception; however, remains a standard. 

The USB 3.0 Series Standard

The USB 3.0 standard is, of course, the next iteration in the universal serial bus standard. Moreover, it is being called the one-port revolution since USB 3.0, and USB 3.1 are physically identical. USB 3.1 (10 Gps) has an increase in data rate speed compared to USB 3.0 (5 Gbps). Also, with the increase in data rate comes an increase in bus power, i.e., from 500 mW (USB 1.1 and USB 2.0.) to 900 mW (USB 3.0).

The USB 3.0 series standard, which includes USB 3.1 and USB 3.2, is the current de facto standard in USB. However, like its predecessor, USB 3.0 is backward compatible with USB 2.0, albeit the USB 2.0-speed limitations still govern you.

The five overall main speeds of the USB standards are as follows:

  • USB 1.0: 1.5 Mbps

  • USB 1.1: 12 Mbps

  • USB 2.0: 480 Mbps

  • USB 3.0 and USB 3.1: 5 Gbps

  • USB 3.2 gen 2: 10 Gbps

The Effects of USB Power Output and Data Rates

In every instance, when dealing with power and electronic devices, power matters. This includes USB standards, and it is evident by the increase in functionality as the standards evolved. For example, with USB 1.1, it was functionally compatible with keyboards, mice, and printers as a connection option. However, with USB 2.0, it afforded the use of flash drives and other external hard drives.

These increases in functionality correlate with the increase in data rates and the improvements in output power. Now, with the USB 3.0 series standard, power output nearly doubled, and the data rate increased by a multiple of 10. This, of course, affords USB a myriad of new uses, including system backups, increased gigabit Ethernet adapter performance, and the ability to handle large video files.

USB Maximum Power Output

In any USB network, there is one host and at least one device. In nearly every instance, a computer is the host, whereas a tablet, camera, or smartphone is the device. Moreover, the flow of power is always from host to device, but data can flow in either direction. Take, for example, connecting your smartphone to your laptop to transfer music files or photos.

In terms of USB power specifications, a standard USB 1.0 or 2.0 port contains four pins, and the applicable USB cable has four wires. The inner pins carry data (D+ and D-), and the outer pins provide a 5-volt power supply. However, USB 3.0 ports include an additional row of five pins; therefore, USB 3.0 compatible cables will have nine wires.

With regards to the actual current (mA or milliamps), there are three types of USB ports we reference by its current specifications. They include a dedicated charging port, a charging downstream port, and a standard downstream port. Moreover, the dedicated charging port is what we call a wall charger. Also, the charging downstream and standard downstream ports, you will find on a PC or laptop.

USB Maximum Power Output Standards

In general, the specifications for a USB 1.0 and 2.0 standard downstream port, delivers up to 500 mA or 0.5A. Also, with a USB 3.0, it can provide up to 900mA or 0.9A, which translates into 4.5 watts. These power output specifications are a rating based on the 5 volts from each standard output. However, the USB 3.0 dedicated charging and charging downstream ports provide up to 1,500 mA or 1.5A, which translates into 7.5 watts.

As I am sure you are aware, the universal serial bus is not only a method of transferring data but power as well. Except for mobile devices manufactured by Apple, all current smartphone models utilize a USB port for charging, data transfer, and even sharing their network connection (tethering).

Some devices only use USB as a method of power transfer, for example, power banks. Plenty of devices that transfer no data at all still use USB for charging. This also means that there are some cables strictly designed for power transfer only and lack the wiring for data transfer. Moreover, in some cases, it is preferable to use USB cables that can only transfer power since it prevents hackers and malicious software from infecting your device.

USB Power and Charging Guidelines

USB ports, in terms of power, provide a wide variety of outputs. Keep in mind that when using a standard USB port, it will only draw as much power as the port can supply or as much energy as it needs, whichever is lowest.

A USB’s maximum power output depends on the standard of the host and the device as well as the cable in use. In other words, USB will provide power in accordance with the hardware’s generation standard.

For example, you may see a rating of 2.1 A on a device’s charger and USB 3.1. Since USB 3.1 can output 3 A, the device’s software will negotiate its requirements to limit the output to 2.1 A. Scenarios such as this are due to a device’s irregular or nonstandard power requirements.

USB being plugged into a port on a laptop computer

Making sure you don’t accidentally design a supercharged USB is important for maintaining longevity. 

The advent of the USB standard changed the electronic and computer landscape almost overnight. The ability to interface with devices that otherwise was not possible is the electronics revolution that has ripple effects that will extend into the unseen future. However, with the various standards and available cables, it is prudent that we be mindful of their capabilities. Simply put, when dealing with power and electronic devices, power matters.

USB design and layout, as well as more generally applicable power supply design, has never been easier with the suite of design and analysis tools available from Cadence. The layout solutions offered through OrCAD PCB Designer are paramount for working through designs with USB ports and other various power and data transfer needs. 

If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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