Bringing innovation to power delivery in the USB ecosystem
03 April 2017
The potential for USB 3.1 and the Type C connector/cable specification to revolutionise the electronics industry should not be underestimated. But it will require innovative, single-chip solutions to enable OEMs to meet their customers’ high expectations.
Far from just being a simple way to connect low power peripherals to a computer, the Universal Serial Bus truly does have universal popularity. Over a relatively short period of time it has become a dominant and in effect the only ‘must have’ interface for portable electronic devices that require a wired communications port.
Part of that popularity, however, has to be attributed to its ability to simplify power delivery. Effectively, any portable device with a USB interface that requires power from an external source can receive that power from a USB Host. This alone has revolutionised the portable market; the adoption by most of the world’s mobile phone makers of the Micro-USB connector, as the ‘one charger fits all’ power port for mobile phones, indicates this.
The ability for a USB port to deliver power along with data has always been a fundamental part of the specification, but with the introduction of USB Type C it has recently taken on a whole new perspective.
Removing orientation frustrations
One of the main benefits of USB Type C is its physical flexibility; both the connector and cable orientation are universal, making it much more convenient when plugging things together. Another, widely reported, benefit of the USB Type C port is its ability to both deliver power, as well as accept it. This means that devices, and not just power supplies, will be able to meet a wider range of the power requirements of other devices; variable levels of power can be provided and accepted in both directions.
This really does put USB Type C in a new class of connectivity, but it is important to appreciate that USB Type C is the specification for the cable and connector arrangement only; the ability to deliver power comes under a different specification, known as USB Power Delivery (or PD).
USB PD has been around for much longer than USB Type C and is responsible for the way we charge our portable devices today. However, it is undeniable that the introduction of Type C connectors and cables will propel USB PD to meet its full potential.
Devices that are able to source power are defined by USB PD under six profiles; 0 to 5. Profile 0 is reserved, while Profile 1 is the ‘default’ for most devices, delivering 5V, 2A (10W). This extends in steps to Profile 5, which can deliver a maximum of 100W.
Before USB Type C, devices were defined as either a source or sink port, making power delivery relatively simple. With the introduction of Type C, the designations of source and sink become less relevant. It also introduces a new level of interrogation for devices that will be equipped to both sink and source power, as they will be required to deliver power at the right level for the device requesting it.
For example, a laptop driving a desktop monitor over a USB Type C connection could be receiving its power (and even be charging) from the monitor, but the reverse scenario is also valid; the laptop could power the monitor (as along as its requirements are within the 100W maximum). In addition, a mobile phone also connected to the laptop could itself be being charged.
While the consumer’s experience is clearly going to improve, the OEMs making these devices need to consider the implications of this flexibility and understand the most efficient way to implement these new features.
A new power paradigm
The Power Delivery (PD) protocol allows hosts and peripherals (devices which, under PD, change roles) to negotiate for the amount (and direction) of power provided/delivered. This would typically be handled by a microcontroller which, in turn, would handle the power management, perhaps with the help of a PMIC (Power Management IC).
This new power paradigm will demand a new power management solution, one that is able to handle not only bidirectional power requests but variable demands. In effect, a device equipped with a USB Type C interface and implementing the USB standard for Power Delivery will need to behave like a conventional power supply or mains-fed adapter. This fundamentally changes the power circuit in a device; as it no longer simply needs to manage the power it receives, but deliver the power other devices need, too.
In a conventional topology, a battery-powered device would use a buck/boost converter to convert the voltage provided by its battery cells (which could vary over a relatively wide range around 5V) to the 5V/2A (10W) required. The same device could now need to be capable of delivering up to 20V at 5A (Profile 5).
This will also impose changes on the design of conventional USB power adapters. Instead of only providing 5V to a device, they will need to negotiate with the device to deliver the power it requests using the PD protocol.
This isn’t as simple as standardising on 20V @ 5A and leaving the device to step it down, as the standard allows connected devices to request ‘just’ the amount of power they need. This feature also requires a single device to power multiple peripherals or devices, at different power levels.
While a combination of buck and boost circuits could be used to meet the specification’s requirements, the industry will naturally need to implement these new features in the most efficient way possible. Using multiple buck/boost devices would introduce costs in both power efficiency and Bill of Materials (BoM).
The solution now being introduced by specialists in power management is to integrate both buck and boost circuits in a single device; one that is flexible enough to meet all the demands of the USB PD specification as implemented using the USB Type C interface. This is effectively a new class of power management device that has been designed specifically to meet the demands of USB Type C in all its forms.
The industry’s first single chip solution to USB Type C power delivery supports the entire USB Type C ecosystem, capable of operating in buck mode, boost mode or buck-boost mode in both forward and reverse directions. This can remove the need for a second device and associated inductor, lowering the overall BoM by up to 40%.
Developed by Intersil, the ISL9238 represents its 3rd Generation of buck-boost charge technology and uses the company’s patented Robust Ripple Regulator (R3) modulation technology. This is a technology that allows both the PWM switching frequency and duty cycle to be modulated simultaneously in response to the input voltage and output load.
While R3 has been developed to deliver high light-load efficiency and fast transient response, the ISL9238 uses this technology to target USB PD. It achieves this by driving an external N-Channel MOSFET bridge that uses one transistor pair in a buck arrangement and the second pair in a boost configuration (Figure 1). The centre tap of each pair is connected using a single inductor, L1.
In buck mode, Q3 remains off while Q4 remains on; Q1 and Q2 are alternately turned on and off. In this mode, Q1 is the control FET and Q2 is the sync FET. In boost mode, Q1 remains on, Q2 remains off, while Q3 and Q4 are pulsed. In this mode, the control FET is Q3 and the sync FET is Q4.
When operating in buck-boost mode all four transistors are pulsed on and off in pairs; Q1 and Q3, followed by Q2 and Q4. In this mode both Q1 and Q3 are the control FETs, while Q2 and Q4 are the sync FETs. A similar but reversed pattern is used to achieve OTG (On The Go) buck, boost and buck-boost operation. The control and sync FETs are also reversed. Figure 2 shows the operation modes.
As shown in Figure 3, the ISL9238 is able to transition between buck, boost and buck-boost mode seamlessly, based on the input voltage. This means the device can automatically switch between using the adaptor, a battery, or both as the source of system power; dedicated outputs drive P-Channel MOSFETs to connect and disconnect the battery and adapter voltages from the system.
The SMBus interface allows all modes to be controlled from a host microcontroller or PMIC, which include its comprehensive battery charging features. The device provides over-voltage protection for the adapter, battery and system voltages. Over-current and over-temperature outputs are also integrated.
Figure 1: an external N-Channel MOSFET bridge that uses one transistor pair in a buck arrangement and the second pair in a boost configuration
Figure 2: the operation modes of ISL9238 the industry’s first single chip solution to USB Type C power delivery
Figure 3: the ISL9238 is able to transition between buck, boost and buck-boost mode seamlessly, based on the input voltage
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