USB charging, part 1: requirements
USB, the Universal Serial Bus, was primarily designed to transfer data to and from peripherals, with a secondary function of providing power to those peripherals so that they don’t need to be independently powered. This secondary function has gained importance over the years, so that today it is sometimes the primary or only function. Many smartphone owners regularly use USB to charge their device but rarely, if ever, transfer data over USB. When Linux is running on that smartphone it needs to be able to optimize the use of whatever power is available over the bus — a task that is neither straightforward nor cleanly implemented in mainline Linux. We start this two-part series by looking at how USB communicates power availability information and will conclude in the second part by looking at how Linux does, or more often doesn’t, make use of this information.
To begin, it will be helpful to be clear about some terminology. USB is an asymmetric bus — the two connected peers each play different roles. One peer is the master, or initiator, and controls all the data flow; it is known as the host and has an A-series connector (or receptacle) or, possibly, a B-series plug. The other peer is the slave, or responder, that can only send or receive data when the host tells it to. This is known as a USB device though, since device is an overly generic term, it is sometimes referred to as a gadget. A USB gadget has a B-series connector, of which there are a range of sizes, or an A-series plug.
A USB cable connecting a host to a gadget typically has 4 wires. Two carry power, VBUS and GND, and two carry data, DP and DM, also known as D and D-. Power normally flows from the host to the gadget, but this is not universal as we shall see later. USB 3.0 adds extra wires and pins to the A-series and B-series connectors to carry SuperSpeed data, but only changes power delivery in that power flowing in different directions can flow over different wires. The USB 3.0 C-series cable is bidirectional and does add extra power signaling options, but the details of that would not particularly help the present discussion.
Some USB peers can serve as either a host or a gadget using a specification known as USB On-The-Go — USB-OTG. These devices have a B-series connector with a fifth pin called ID. The port will act as a gadget port unless a cable is plugged that connects the ID pin to GND, possibly through a resistor. When that happens, the device will switch the port to host mode so it can control an attached gadget.
From the perspective of a mobile battery-powered Linux device with a B-series port, the important question is: how much power can be drained from the bus and used in the device? As the voltage is fixed at 5V ±5% this is equivalent to a question of how much current can be drained, and the answer is usually given in milliamps (mA). The device will typically have power-management circuitry that can limit the current used, and other circuitry that will divert some to charging the battery when that is appropriate, but those details are not really important at this stage. For now, we only care about a number.
The USB Implementers Forum provides several multi-page specifications describing how to get that number, in particular the Battery Charging v1.2 document (BC-1.2, which is my primary source and comes from the Class Specification page) and the newer USB Power Delivery spec. In practice, there are two classes of answers.
Current from a Standard Downstream Port
The first class of answers applies when the device is connected by a standard cable to a standard A-series host port such as on a notebook or desktop computer. A USB host provides a Standard Downstream Port (SDP). These ports must provide 100mA for at least the first second after attachment and can be configured to deliver more — up to 500mA in USB-2 — after enumeration has completed. If no enumeration happens, the port is expected to become suspended after 1 second at which point only 2.5mA is available.
Enumeration involves the host asking the gadget (in this case, our mobile Linux device) about its configuration options, and then requesting that some specific configuration be activated. details on enumeration can be found in an earlier article on the USB composite framework. A configuration includes the amount of current that will be required, which may be close to zero for a separately powered device, or may be the maximum supported for something that is power hungry and fully bus-powered. The host knows what it can provide and will ignore any configuration that requires too much power.
This protocol is quite suitable for a gadget that is dependent on bus power and needs a certain amount of current or else it cannot reliably function. It is less suitable for a battery-powered gadget like a smartphone that can function with no bus-power at all, but would be happy to receive as much as is available. Such a device can present two (or more) distinct configurations to the host: one that claims to require 500mA and a second one that requires zero. A host with power to spare should activate the first one. A host that cannot provide this power should reject the first and accept the second.
Current from other port types
There are a variety of other port types that a USB gadget can find itself connected to. A Dedicated Charging Port (DCP) provides power, at least 500mA, but no data. A Charging Downstream Port (CDP) provides data access much like an SDP, but also provides at least 1.5A, and as much as 5A, that is available even before bus enumeration. USB C-series connectors introduce more options with the same maximum current of 5A, though there is the possibility of increasing the voltage up to 20V, which would yield 100W of power.
For USB-OTG there is an extra port type, the Accessory Charger Adapter (ACA) as well as an extended form: the ACA-Dock. The ACA switches the OTG port into host mode, but also provides power to it, rather than requiring power from it. The ACA-Dock provides power and can switch the OTG port between host and gadget mode, presumably based on what else is plugged into the dock. An ACA-Dock will provide at least 1.5A, while a simple ACA can provide as little as 500mA.
Each of these peers can be detected directly by the USB PHY — the circuitry responsible for the physical interface with the bus. This detection happens without needing to enter enumeration negotiations, so if power is available it can be accessed quickly.
USB connection negotiations between a host and a gadget start with the host providing a 5V level on VBUS and the gadget PHY detecting this voltage. The PHY then advertises its existence to the host by pulling one of DP or DM up to the same level as VBUS, the choice of pin giving some indication of supported bus speed. At this point the host starts enumeration. Before it applies the full VBUS voltage, the PHY can send other signals at a lower voltage and check the response. The simplest case involves setting a low voltage (e.g. 2V) on DP and checking if it is echoed back on DM. Seeing the same 2V on DM strongly implies that the DP and DM lines are shorted together, which is how a dedicated charger (DCP) can be detected. A similar but more complex signalling will detect a CDP.
For a USB-OTG port, the ID pin is supplied with 5V and, when there is no cable plugged in or when a normal 4-pin B-series plug is in place, it will stay at 5V drawing no current. As mentioned, an OTG cable will pull this pin down to GND and if a resistor is used to pull it down the resistance indicates the type of device. If ID is a short-circuit to GND, or shows a resistance of at most 1kOhm, then a simple gadget that doesn’t provide any power is being attached. The OTG must provide 5V to VBUS and cannot expect anything back. If a resistance is measured between 1kOhm and 200kOhm (the minimum to be considered open-circuit), then some sort of ACA is implied and the specific resistance indicates the type of ACA.
The USB PHY controller in the mobile device can usually perform these various tests in hardware, possibly with some software support, the moment that a voltage change is detected, and can make the results available to the Linux driver. The Linux driver then just needs to tell the power-management driver how much current to draw.
A range of options
When connected to an SDP, and after a configuration has been negotiated, a simple number is available to the Linux device so it knows how much current it can draw — the number that was requested in the configuration that was activated. In theory, it should be safe to draw that much current at 5V. When attached to other port types, it isn’t quite so simple.
According to BC-1.2, the current provided by a dedicated charger, DCP, is IDCP, which has a minimum of 500mA and a maximum of 5A. Similarly, a CDP provides ICDP, which ranges from 1.5A to 5A. ACA configurations have ranges too, the lower ends of which were mentioned earlier. Setting a current limiter in the portable device to a single number is normally quite simple. If we only have a range of allowable values, it isn’t immediately clear what number we should use.
The intended meaning of these ranges is that the power source must provide at least the minimum listed current at a voltage within 5% of the 5V target, so at least 4.75V. Demands for higher current may cause the voltage to drop and, at some point, the supply may cut out completely. The supply should not cut out while the current is below the maximum, unless the voltage has first dropped below 2V.
The graph at right (click for larger, animated version) shows hypothetical power curves that could be presented by chargers, which would be within specifications. As the load increases, the current provided increases, and then eventually the voltage starts to drop. The key requirements of the specification are that the trace must never enter the grey areas and that the charger must not shut down (depicted by a black disk) before the trace enters a pink area. This blog post shows some power curves presented by real devices.
Pulling all of this together, the negotiations on the bus will provide an upper and lower bound for the amount of current that can be drawn. The device can safely request the minimum and if that is less than the maximum it can then slowly increase the requested current until the maximum is reached, or until the supplied voltage drops below some device-specific threshold that must be greater than 2V and at most 4.75V. Each voltage level should be averaged over 250ms to avoid being confused by transients.
While this procedure should be safe, it is not unheard of for charging hardware to be less than perfect, and some cables can introduce more resistance than one would like. So a truly cautious driver would start requesting current well below the negotiated minimum and advance slowly with large margins for error.
The basics
Before we dive into the weeds, let’s start with the fundamentals.
Every smartphone has a battery, and every battery delivers power in more or less the same way.
- You’ll have to shell out a few bucks to take advantage — Apple doesn’t include USB-PD-compatible chargers in the box for many of its phones.
The USB Power Delivery (USB-PD) standard was developed by the USB Implementers Forum (USB-IF), and it’s a standard that any manufacturer can use on any device with a USB port. It’s capable of delivering up to 100W, so it’s suitable for use with all kinds of devices beyond smartphones, including some laptops, provided they have a USB-C port. USB-PD brings other benefits, too. The direction of the power is not fixed, so you will find portable battery chargers, for example, that have a USB-C port that can be used both to charge another device or to charge the battery pack itself.
Apple iPhones from the iPhone 8 through the iPhone 12 models implement USB-PD, the same industry standard used by the iPad Pro, the 12-inch MacBook, Google’s Chromebook Pixel, and Lenovo’s X1 Carbon. Intercompatibility is its biggest advantage — USB-PD doesn’t require any special cables or wall adapters.
Supported outputs, cables, and adapters
You’ll have to shell out for accessories if you want to take advantage of the iPhone’s USB-PD compatibility because Apple hasn’t packed USB-C cables or adapters in iPhone boxes until recently. You’ll also need to buy a Lightning to USB-C cable that supports USB-PD — if you use a standard Lightning cable with a USB-C to USB-A adapter, the charger will default to the lowest wattage.
- Apple 18W, 20W, 29W, 30W, 61W, 87W, or 96W USB-C Power Adapter.
- A comparable third-party USB-C power adapter that supports USB Power Delivery (USB-PD).
Charging speed
No matter which USB-C charger you buy, you’ll have to put up with hard-coded safety limits in your iPhone. Fast charge kicks in when the capacity is between 0% and 79% but stops when it reaches 80%.
If you don’t mind forking over a few extra dollars for charging accessories, you get much faster charging than you would otherwise. The fast charging iPhones can charge from 0% to 50% in 30 minutes using USB-PD.
Google fast charging via USB-PD
- The same USB-PD kit will work with a number of devices.
- Google supplies a fast charging charger and cable in the box.
Every Google Pixel phone, from the original to the Pixel 4a, is capable of fast wired charging up to 18W using the USB-PD standard. It doesn’t require any special chargers or cables.
Supported outputs, cables, and adapters
You can use the charger and cable in the box with your Pixel phone to enjoy maximum charging speeds. If you go for third-party accessories, use any USB-C with USB 2.0 power adapters and cables. If you use a USB-C to USB-A cable, then it will charge your Pixel more slowly, regardless of the power adapter you connect to.
Charging speed
A Pixel phone can charge rapidly from 0% up to around 80% but will slow down beyond that point. You’ll find the same limitations with most phones.
Using the cable and 18W adapter supplied or any decent third-party one with a similar or higher rating, you can get seven hours of use from a 15-minute charge.
Qualcomm Quick Charge
- One of the most widely implemented charging standards.
- Backward compatible with older versions of Quick Charge.
- Built-in safety features prevent overheating and short-circuiting.
Chipmaker Qualcomm’s Quick Charge is one of the most widely implemented charging standards on the market. That’s no mistake — it’s an optional feature of Qualcomm system-on-chip technology, like the Snapdragon 855, 845, 835, 820, 620, 618, 617, 430, and others, which powers phones like the Samsung Galaxy S10, Google Pixel 3, and LG V40 ThinQ. But the technology isn’t tied to Qualcomm’s Snapdragon processors — any smartphone manufacturer is free to license Quick Charge’s power controller technology.
Tech specs and compatible adapters
Quick Charge achieves fast charging by upping the charging voltage, which in turn boosts the wattage. Announced in July 2020, Quick Charge 5 is the latest standard in the series, promising the ability to recharge phones to 50% in five minutes. While support is currently limited to recent phones such as the Samsung Galaxy S20, Quick Charge 4.0 support is growing in popularity. You can find it in phones like the LG G8 ThinQ, Razer Phone 2, and Xiaomi Mi Mix 3, and it can deliver up to 27W of power.
Quick Charge 4 and newer have the added bonus of compatibility with USB-PD chargers, but Quick Charge 3.0 and older only work with Quick Charge-certified accessories. Still, Quick Charge’s ubiquity means there are plenty to choose from. Qualcomm’s website has a partial list of the most popular options.
Charging speed
Qualcomm claims that Quick Charge 5 can fully recharge smartphones in 15 minutes and take a phone to 50% in just five minutes.
In our testing, Quick Charge 4 took the Razer Phone 2’s big 4,000mAh battery from 18% to 90% in just over an hour.
Safety measures
Quick Charge’s intelligent thermal balancing moves current via the coolest path, and device sensors monitor the case and connector temperatures to prevent overheating and short-circuiting.
Enter USB 3.0 and USB Battery Charging
The launch of USB 3.0 in 2008 was a notable milestone in USB charging technology. It not only improved data transfer speeds to 5 Gbps but also increased power output to 900mA at 5V, providing up to 4.5W of power. This rise in power output broadened the range of USB-chargeable devices to include tablets and some smaller laptops.
At the same time, the USB Implementers Forum (USB-IF) released the USB Battery Charging specification. It further increased the power output to 1.5A at 5V, or 7.5W of power, and introduced the Charging Downstream Port (CDP), allowing faster charging and data transfer simultaneously.
USB Type-C, USB Power Delivery, and Enerlites USB Receptacles
In 2014, the USB Type-C connector was introduced. Its reversible design was a significant upgrade over its predecessors, offering a user-friendly connector that could be inserted in any orientation. But more importantly, it was designed to support the USB Power Delivery (USB PD) specification, heralding a new era in USB charging technology.
USB PD significantly expanded the scope of power delivery, supporting higher power output up to 100W. This was achieved through an adjustable power output of up to 20V and 5A. Now, a larger range of devices, including laptops, monitors, and even printers, can be powered and charged via USB.
Enerlites USB Receptacles have been a key player in this evolution. These receptacles integrate USB ports with conventional AC outlets, allowing users to charge their devices directly from the wall without needing an adapter. Enerlites USB Receptacles have fully embraced the USB PD specifications, offering charging capabilities that can power a wide range of devices efficiently and safely.
over, USB PD introduced power direction negotiation, allowing two connected USB PD-enabled devices to negotiate which device should send power and which should receive it. This has enabled new use cases, like a laptop charging a phone, or a phone powering a USB fan.
The Future
The future of USB charging, particularly as it pertains to Enerlites USB Charger Receptacles, is geared towards enhancing convenience, efficiency, and versatility. Here are a few potential developments we might see:
Support for Higher Power Outputs: As USB technology continues to evolve, we can expect Enerlites to adapt and produce receptacles that support even higher power outputs. This would allow users to charge power-hungry devices such as laptops, or even small appliances, directly from their Enerlites USB receptacles.
Integration of USB4 and Thunderbolt: As the latest USB standard, USB4 integrates the capabilities of Thunderbolt 3, providing data transfer speeds of up to 40 Gbps. While this doesn’t directly affect charging, its adoption could lead to more devices using USB-C for both data and power. Enerlites USB Charger Receptacles could adapt to this change, offering full USB4 support.
Improved Efficiency and Safety Features: Enerlites will likely continue to improve the efficiency of its USB Charger Receptacles, reducing the power consumed when no devices are connected or when a device is fully charged. Additionally, we can expect further advancements in safety features, such as overcharge protection, to protect both the devices being charged and the electrical system of the home.
Multiple USB-C Ports: Given the increasing adoption of USB-C, future Enerlites USB Charger Receptacles may feature multiple USB-C ports alongside the traditional USB-A ports. This would allow users to charge a wider range of devices from a single receptacle.
In conclusion, the future of USB charging with Enerlites USB Charger Receptacles is set to bring even more convenience and versatility to homes and businesses. By staying at the forefront of USB technology and integrating with other digital trends like Smart home technology, Enerlites is well-positioned to continue providing innovative solutions for our charging needs.
For more information on our new line of USB PD receptacles, visit our website or give us a call at 877-372-6447 to talk to a representative.
MP2760 and MP2651
The MP2760 is a 20V, 6A, buck-boost charger with NVDC power path for battery packs with 1 to 4 cells in series. The device features 4 integrated switching FETs, ADC, and I 2 C interface for an extremely compact solution. It can operate in boost mode, buck mode, and buck-boost mode, depending on the input and battery voltages; by operating in reverse, this device can power the input from the battery in source mode. The MP2760 battery charger is compliant with USB PD DRP operation across the full range of SPR voltage and current levels.
The MP2651 is pin-to-pin compatible with the MP2760, but is optimized for applications that do not require NVDC power path, such as power banks, power tools, and battery backup products.
Both devices are implemented with the CCG3PA USB PD controller to create a complete USB PD solution (see Figure 7). This solution contains a DRP USB Type-C port for bidirectional operation. If an adapter is inserted, the port can act as a sink and charge the battery. If a sink is inserted, the port can act as source to power the USB using the battery. For additional flexibility, both parts can be configured via an I 2 C interface. This design is ideal for applications such as speakers, tablets, medical devices, point-of-sales (POS) systems, drones, and cameras.
Figure 7: USB PD Solution with the MP2760
Conclusion
USB Type-C connectors (also called USB-C connectors) are versatile, incredibly small compared to older connectors, backwards compatible with older USB standards, and can deliver high power up to 240W. By following the specifications for USB Type-C and USB PD and optimizations discussed in this article, manufacturers help maintain interoperability between products, provide protections that can extend the product’s lifetime, and contribute to the EU’s goal for e-waste reduction.
Although the common charger mandate in the EU is a recent development, MPS has been creating charger ICs and battery management devices designed to cover a wide range of USB Type-C and USB PD applications. Buyers and manufacturers for portable applications can benefit from our extensive portfolio as USB Type-C connectors become more and more common worldwide.
Technical Forum
Hello, I used MP2759 Chip on PCB board. 4S2P lithium-ion battery is being charged. As if the battery was almost charged, the charging current graduall.
Hello! I’m working on a project to provide power to a device and wanted to use the MP2731 as the battery manager but I have a few questions that are.
Dear forum members and experts, a few weeks ago I started to design a battery charger with MP2759/MP2759A. My main concern is that the system which wi.