Bringing the features of USB PD to power bank design

By Satoshi Terada, senior marketing manager and Hideo Kondo, product marketing manager, ON Semiconductor

Standards are good but sometimes you need a little more. For a perfect example of this, take a look at USB Type-C; its intention is to create a standard interface for high speed data transfer and power delivery, replacing the need for multiple cables with just one. This makes sense, partly because electronic device manufacturers would rather not supply a cable (or cables) with every device they ship, and also because consumers are approaching a critical mass of ‘spare’ cables that are left unused in draws, cupboards and on desktops.

But standardising on one specification doesn’t have to mean that all devices are created equal. The protocol that enables USB 3.x Power Delivery also allows vendors to communicate with devices over the Configuration Channel (CC), using what is called Vendor Defined Message mode, or VDM. Using VDM, Vendors can exchange information beyond that defined by the USB 3.0 Power Delivery specification. It can be used for various things but it also means manufacturers can communicate directly with their own devices. The equipment appears as standard to any other device, but brings added value to customers using a wall adapter designed specifically for their phone, for example.

Access to VDM happens in the USB controller and will further add to the appeal of USB Type-C connectivity, by enabling features such as faster charging or putting the device into Alternate or Accessory mode to repurpose some of the connections to accept non-USB protocols.

These features will create new applications for USB, not least around the area of Power Delivery. The continued drive towards ultra-low power processing means more can be done with less power and as the total amount of power available from a USB connection can now be as much as 100W (Figure 1), this opens up a wide range of possibilities. But realising that potential will require a USB controller capable of delivering on the promise.

Figure 1: USB Type-C power options

Fast charge

The Power Delivery specification works alongside the existing Battery Charging specification but brings additional benefits when using VDM. Most notably today, it enables Qualcomm’s Quick Charge feature, which delivers higher power to deliver faster charge cycles for smart phones based on select Snapdragon processors. The technology is also available to licence by other manufacturers. Several charge protocols are now in use, such as Motorola’s TurboPower charging technology, Pump Express from MediaTek, and Samsung’s Adaptive Fast Charging technology.

When not using a compliant power adapter the smart phone will default to a lower power setting, but when it is used with the manufacturer’s own/approved charger, users will receive a faster charge. Although specifications vary between manufacturers, the USB 3.1 PD specification is capable of delivering up to 27W at between 5V and 12V.

To help consumers choose the right solution, the USB Implementers Forum (USB IF) has extended its USB Charger Compliance and Logo programme to include USB Fast Chargers. Chargers certified under this programme support the Programmable Power Supply (PPS) features of the USB PD 3.0 specification.

Part of the appeal of PD is that it will allow devices powered over USB to negotiate with the power source, to supply only the amount of power necessary. This includes devices that require less power than the default, and it will mean that power sources can provide power to more devices, share power between devices and deliver more than just power to a device. For example, a display may provide both power and a communications channel to a laptop, as well as an external hard drive, allowing the laptop to access the hard drive through the display.

Power banks

The ubiquitous nature of the Battery Charging feature provided by the USB interface has enabled manufacturers of power banks for many years. While power banks provide a convenient way of accessing extra power when away from an outlet, the migration from Battery Charging to Power Delivery will demand a new generation of power banks. Supporting the variable power provided by PD introduces a level of complexity that is incompatible with existing power bank designs, which typically use a simple, generic power management IC or regulator.

To support all of the features of PD, including Fast Charging using VDM, power bank manufacturers will need to move to an intelligent controller designed to provide the features offered through USB PD. An example is the LC709501F from ON Semiconductor. This highly integrated Flash-based configurable device brings together all of the functions needed to manage the power provided by a Lithium-Ion battery through multiple USB interfaces. Figure 2 shows a typical application example.

Figure 2: A typical Power Bank application using the LC709501F

Configured through firmware developed by ON Semiconductor, it can also be extended to use VDM mode, so manufacturers can add extra value such as implementing fast charge technology.

It directly drives external MOSFETs to provide scalable power, while its switching controller produces the necessary voltage, from 5V to 12V. By selecting the appropriate external MOSFET, all of the PD features can be supported, including Quick Charge 3.0 High Voltage Dedicated Charging Port (HVDCP) Class A. The Buck/Boost topology enables power banks with USB Type-C DRP and USB BC1.2, and it also includes a battery status feature allowing the battery level to be represented using four LEDs. Using the features of USB 3.x, a Smart Phone application can provide further information about the power bank, such as charge status, cycles and time to charge/discharge.

An evaluation board is available to help manufacturers to design-in the LC709501F and select the most appropriate firmware release. For example, FW02 supports a Micro-B input, with Type-A output on two channels, Quick Charge 3.0 HVDCP, Boost auto start-up and an external boost IC for a further power output, while FW05 supports a Type-C with DRP and a Type-A output, as we as Boost auto start-up and an external boost IC. Further releases of firmware are being developed to support other configurations. Figure 3 is a pictorial representation of the functions provided by FW05.

Figure 3: An illustration of the features provided by FW05 for the LC709501F

By directly supporting four LEDs to display the battery level, manufacturers do not need to add an additional microcontroller, saving BoM costs and design complexity. The evaluation board (part number LC709501EVA05GEV) implements FW05 and supports four modes, as shown in Figure 4.

Figure 4: The four modes of FW05

A new kind of controller

The introduction of USB Type-C and PD will deliver significant benefits to consumers and manufacturers, however it comes with a period of adjustment, as developers learn how to access the new features, with the demands that puts on engineering teams.

Developing a power bank that fully supports PD, including multiple outputs and fast charging, will require a new kind of controller, one that has been developed with USB 3.x in mind. The LC709501F is part of a family of devices aimed at making USB PD simpler and more accessible to developers, and it offers a level of integration that is hard to beat. Its ultra-low quiescent current of just 15µA, coupled with features that eliminate the need for an additional microcontroller, mean it can provide all these features while consuming the least amount of power in the battery.

The ability to configure the controller through the Vendor Defined Message mode is an additional advantage over less capable controllers, giving manufacturers the ability to differentiate their products while still complying fully with the USB IF specification.

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