USB Power Delivery for Your Next Project
Since its introduction in the late 90s, the USB standard has never ceased to grow in popularity. Multiple revisions of the standard have kept it relevant to the growing demands of data delivery, which has made it the ubiquitous choice for electronic devices. There has been a growing trend toward USB being a power delivery interface with data, rather than a data interface that can supply power, as the 1.0 specification originally intended. To supply the increasing thirst for power over USB, the USB 3.0 Spec with Type-C began implementing the Power Delivery standard, which you should consider using for your next electronics project.
There are several projects I’m working on which need far more power than the old USB 2.0 standard (with 2.5W of power) can deliver. I’m charging battery packs or running higher demand loads which need more voltage. The USB Power Delivery standard was designed to handle situations like this, where you might need to run your laptop or power intensive tablet device over USB which, requires far more power than 500mA of 5V, and often at higher voltages that 5V USB with power delivery the sink device can negotiate with the source (such as a wall charger) for up to 20V and 5A of current to give a whopping 100W of power to your device. For many applications, this allows you to do away with barrel jacks, specialized power packs or building AC power into the device, cutting down on waste and reducing your costs.
The USB Type-C® connector, released in 2015, has been on the fast-track to becoming one of the most prolific standards on the market, having a projected adoption of 5 Billion units by 2021.
We do not have to look far to understand the technical reasons behind the success. The connector is relatively cheap to manufacture, offers extensive backward compatibility to USB 2 and USB 3 standards, increases the bandwidth available from the USB 2 connector through the addition of 4 extra super-speed data lanes (two more than the two super-speed lanes on the USB 3.0 Type-A connector), offers alternative modes such as HDMI and DisplayPort, is reversible, durable (with often 10x the cycles of micro-USB connectors), and slim.
I recently upgraded my phone from one with a USB Micro B connector to USB Type-C, and it’s easy to see why it’s popular from an end user point of view, not just a technical point of view. Trying to plug the polarized Micro-B into your phone in the dark was somewhat of a challenge, plugging in Type-C is a breeze due to its non-polarized shape. The connector is far more robust as well, which is a big bonus when designing consumer devices that get abused or dropped while still plugged in.
As mentioned earlier, as an engineer one of the essential features of the USB-C connector is the support for USB Power Delivery standard.
USB-PD: USB Power Delivery
You may already have extensive familiarity with the USB Power Delivery standard as a consumer. Between its widespread adoption in smartphones, tablets, laptops, and monitors, the USB Power Delivery standard is an unavoidable part of contemporary technology.
The specifications diverge substantially from previous USB charging technology, introducing the following features:
- Up to 100W of power over a single USB cable,
- multiple voltages,
- bi-directional current flow (each device, if able, can act as a source or sink),
- smart active negotiation of voltage and current,
- communication of errors such as overcurrent events,
- communication outside of the primary USB protocol.
The high wattage and wide voltage range that can be negotiated for, offers more possibilities for the devices you are designing. Rather than taking a 5V supply and boosting it to a more practical voltage for your circuit, you can simply request that higher voltage be delivered directly to your device. Error communication, rather than just having your power cut off, is also very convenient.
The USB-PD standard is commercially available in two revisions, USB-PD 2.0 and USB-PD 3.0. USB-PD 1.0, the first revision of the standard meant to operate through USB-A and USB-B connectors never received widespread commercial adoption and is now obsolete. In this article, we address the differences between the most recent two revisions.
The Separation Between Power and Data
The first fundamental concept to grasp in understanding the intricacies of the USB-PD protocol is out-of-band communication.
In a USB 2.0 Connection, the lines D- and D+ carry the data, designated as pins A/B7 and A/B6 on the USB-C connector. In a USB 3 connection, the data is carried by 2/4 data lanes on differential pairs A/B2, A/B3, A/B9, A/B11.
The USB-PD protocol instead travels through a single bidirectional “CC” line, that depending on the polarity of the connection may end up being in either the A5 pin (CC1) or B5 pin (CC2). This communication is entirely independent of the primary USB protocol; many USB-PD devices, chargers for example, only implement the USB-PD standard with unconnected data lines. This CC line dares to communicate with devices no other line would dare to, such as the USB cable itself, in case of digitally marked for current-carrying capacities from 3 to 5 amps.
The Power Flow
The direction of the power flow, in previous USB standards was mainly determined by the type of connector (USB-A or USB-B), can now be actively negotiated through the USB-PD protocol. USB Type C connectors can be used for the host or client device, so each device may choose a role, either Source (the provider) or Sink (the consumer) when it comes to power.
The role may change during operation, depending on the specific application requirements; a representative example can be identified in popular USB hubs with the USB-PD “passthrough” feature: when plugged in the computer, the USB hub initially acts as a Sink. Subsequently, on the connection of an external power supply to the hub, a new power contract is negotiated between the devices, taking into consideration the capabilities of the power supply. Finally, computer and hub swap their Source and Sink roles, with the computer now getting charged through the hub.
The role swap has received further improvements with revision 3.0 of the USB-PD protocol, following the introduction of the Fast Role Swap feature. Fast Role Swap allows devices such as laptops to take over the source role from HUBs in case of an AC power loss in under 150 microseconds, keeping hard drives and other sensible devices powered.
Depending on the revision of the USB-PD protocol adopted, either 2.0 or 3.0, sources can offer two modes of operation once the power contract is in place:
- Fixed Supply (USB-PD 2.0 and 3.0)
- Programmable Power Supply (PPS) (USB-PD 3.0 only)
In fixed supply mode, the source provides a fixed voltage of either 5V, 9V, 15V, or 20V; and a maximum current based on the power capability of the device (Up to 5A and 100W) and the max current supported by the cable, from a standard baseline of 3A up to 5A.
In Programmable Power Supply mode, the source acts as a CV-CC (constant-voltage / constant-current) generator with configurable steps down to 20mV and 50mA intervals under direct control of the Sink device. This leading-edge feature allows moving battery charging power circuitry outside of the sink device and into the source device, increasing efficiency, reducing cost and dimension of the sink device. In addition to battery charging applications, it also allows a great deal of flexibility in your device—potentially allowing you to remove one or more voltage regulators from your device, offering cost savings in components as well as board space.
New in USB-PD 3.0
Additionally to the aforementioned Programmable Power Supply mode and Fast Role Swap, USB-PD 3.0 includes support for USB4, introduces communication of battery information between devices, introduces the USB-Type C Authentication protocol, and supports the newly announced USB PD Firmware Update standard. The latter may be of great interest to many electronic engineers, allowing for firmware updates through the USB-C connection in a similar way to the USB DFU (Device Firmware Update) specifications, but without the need for a complete USB device, and the complication of having to enter DFU mode through user interaction such as pressing a button for 10 seconds.
Together with other capabilities of the USB-C connector, such as USB-C Debug Accessory Mode, USB-C may be the only connector you need for both user interaction and product testing. This unification and expanded capability allows you to make further cost and space savings within your devices, as well as simplifying your production further lowering costs.
Do you need USB-PD?
The question haunting the dreams of every engineer adding a USB-C: do I need to add USB-PD support to my device, increasing the BOM lines with a USB-PD controller and satellite components? The answer may surprise you.
If all you need is a power source for your device, and want to adopt a USB-C connector, you should consider your requirements in voltage, current and power.
USB-PD is the only official USB-IF (USB Implementers Forum) standard that will allow you to get voltages higher than 5V from a USB port. Qualcomm’s proprietary Quick Charge® standards are mainly relegated to the smartphone world, adopted by devices running on Qualcomm SoCs. However, the latest revision of Qualcomm’s Quick Charge® is fully USB-PD 3.0 compatible, which leaves little justification for a Quick Charge implementation over, or in addition to, USB-PD unless you are using a Qualcomm processor which already has full support for Quick Charge.
If your device only has a limited voltage requirement of 5V however, alternative standards may propose a viable solution, such as USB BC (Battery Charging) 1.2, which allows up to 2.4A draw. USB-PD allows your device to be fully USB BC 1.2 compatible if you wish to support both standards.
If you need the full 3A offered by USB-PD, but do not need voltages over 5V, TUSB321 from Texas Instruments may be the IC most indicated for your need at the time this article was written.
Available commercial solutions
Implementing all the USB-C physical layer and protocol requirements from scratch would be a herculean feat, and not something many engineers would be interested in doing unless their production volumes were incredibly high, to justify the engineering cost. Luckily, many silicon companies are offering a broad portfolio of USB-PD ICs, with varying levels of integration and feature support.
Through Octopart, search features we have found the following manufacturers offering USB-PD silicon, in alphabetical order:
- Analog Devices,
- Cypress Semiconductors,
- Dialog Semiconductor,
- Diodes inc,
- NXP Semiconductors,
- On Semiconductor,
- ST Microelectronics,
- Texas Instrument.
Undoubtedly, this list is non-exhaustive, with manufacturers launching USB-PD products at an ever-increasing speed. With the new 3.0 standard, updated and additional products are being released at a rapid rate.
Here are a few USB-PD integrated circuits that have caught my interest recently. I bought dev kits for several of these for further evaluation of how they can be used in my upcoming projects:
- STUSB4500: This IC is a popular and widely adopted solution for converting existing devices into USB-PD compatible sink. The IC does not require an external MCU, but you must use the I2C communication at least once during the first programming if you need a different voltage than 5V. A software called STSW-STUSB002 allows achieving this without writing a single line of code. If you want to connect it to an MCU, many open-source libraries are available on GitHub, such as ardnew/STUSB4500
- STUSB4700: Source-only USB-PD controller, similar to STUSB4500.
- STUSB1600: Base controller offer from ST. As the STUSB4500 and STUSB4700, it does not require an MCU.
- TPS65983B: Flagship USB-PD controller from Texas Instruments. This IC includes countless features out-of-the-box, including PD3.0 Fast Role Swap support, both Source and Sink support. While, at 120 pages, the datasheet may look intimidating, it sure is a stimulating read that reflects the profound contribution of Texas Instruments to the standard.
- FUSB302: If you have an I2C line to spare in your design, this minimalistic and affordable IC, part of ON Semiconductor portfolio, could be just what you need to add USB-PD source or sink capabilities to your design. It takes care of all the low-level communication.
- TUSB422: This USB-C controller from Texas Instruments has just 9 pins in a BGA package, perfect for space-constrained applications.
- STM32G0: STM32 Cortex M0+ MCUs that include a USB-PD controller. MCUs with integrated data and power have been announced for the second half of 2020.
- TUSB321: In case your USB-PD device needs only 5V (and thus, max 15W and 3A) this is the IC to choose. Doesn’t require an MCU and can be configured with just a few external resistors, keeping your BOM cost down.
Regardless of your IC choice, remember always to double-check you have added all the necessary TVS Diodes. They’re easy to forget, and a USB connector is probably the most likely place for an ESD event in your product!
Whether you are working on consumer products or bespoke equipment, the USB-PD standard might be just what you need to add versatile power capabilities to your devices. For me, USB Power Delivery 3.0’s ability to negotiate a very wide range of voltages allows me to build schematics that are powered by USB, despite requiring unconventional voltages.
The enormous wattage that is available on request can facilitate rapid charging of even large multi-cell lithium polymer battery packs without the need for a separate battery charger. You’ll see a new project from me related to this soon, which utilizes USB Power Delivery for rapidly charging a cinematography battery without an external charger.
While the implementation is undoubtedly more intricate than a modest DC connector, customers are surely going to appreciate using your product through the same connector they use for everything else. In addition to the cost savings from potentially not needing to supply a power adapter for your device, it's becoming expected that a device can plug into a USB port for power, even if not for communications. Using USB Power Deliver could offer a level of future proofing for your device, as well as convenience.
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