Posted on 23 July 2019

A Truly Connected Mobile Experience

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The Push for USB OTG

The beauty is to charge a battery per the latest USB Battery Charging Specs, using a high efficiency buck topology converter that utilizes a chip inductor for minimized space and cost, but can then run the buck controller in reverse as an effective boost controller requiring no additional components.

By Shadi Hawawini, Applications Engineer, Summit Microelectronics


The USB On-The-Go supplement, or USB OTG has been available for quite some time now, and yet we see very few devices that actually support this new technology. This new technology truly offers limitless connection potential and can aid in creating a mobile world that is truly mobile. As things are now, the world is very PC centric; end users take pictures with their camera, but what do you do with those pictures after? Most users take them home, and then download them onto their PC where they can then upload them to the web, email, or print the pictures. But this really defeats the purpose of the camera being mobile because ultimately you have to bring it home again before you can share the pictures. With USB OTG, it is possible to share pictures, without having to come home to the PC. To upload them to the web, a user can simply connect their camera to their USB OTG enabled cell phone and upload them to the web directly, or to share the pictures just taken with the friends in the picture, if they have a USB memory stick, you can plug it directly into a USB OTG enabled camera and copy the pictures directly to the memory stick.

Block Diagram of connection from Cell Phone to Digital Camera

Distinguishing the host and peripheral device

The fundamental idea for the USB OTG supplement is to allow the ability for a portable device to assume the role of standard peripheral, as well as host. Traditionally, the only device that would act as a host was the PC, and that would handle the more complicated communication protocols, allowing peripherals to be developed far more simply. Of course with mobile technology processing power increasing, the feasibility of the peripheral device to assume the host role is no problem. USB OTG devices are differentiated by the fact that they are the only devices allowed to use the USB Micro-AB connector. The connector nomenclature is very intuitive and is consistent with the nomenclature for traditional USB connectivity, namely, an A female connector is used for an embedded host, and a b female connector is used for a device that is a peripheral device. It follows, that when a male A connector is plugged into a USB Micro-AB connector, that device initially assumes the host role, whereas if a male B connector where plugged into a USB Micro-AB connector, the OTG device assumes the role of a peripheral. It is important to note at this point, that an OTG A-device or OTG B-device only refers to the orientation of cable insertion, and it will be clear later, that is does not necessarily indicate which OTG device is the host and which one is the peripheral.

The distinction from a system perspective between a male A connector being inserted, or a male B connector inserted, is made by first monitoring the ID pin of the USB cable, and monitoring VBUS for voltage, which in a general manner ensures the cable is physically connected.

For a male A-connector, the ID pin is connected to GND, whereas for the male B connector, the pin is left floating. This intuitively makes sense because as an example, for the traditional use case of a peripheral device being connected to a PC, the PC only has a USB A connector with 4 pins, which means any 5th pin would automatically be floating, which is the case for a B male connector being plugged into a receptacle.

Host Negotiation Protocol

The USB micro supplement makes a point to clearly label the connector ends with either an A or B to distinguish to the user which direction the cable should be plugged in to determine who will be the host and who will be the peripheral when two OTG devices may be connected together. It was realized that most consumers would not have a clue how to differential a host from a peripheral, and frankly may not care. Even a person who is very familiar with the spec may not be able to properly read the marking indicator; after all, it is a micro connector. To resolve this problem, the USB OTG spec provides the means for the devices to communicate with one another and decide logically, based on what the user is trying to do, whether to assume a host role or peripheral device role. The ability for the host and peripheral to switch roles is defined by the Host Negotiation Protocol, HNP. The HNP describes that the device that was originally defined as the A device (USB A male plug inserted into USB-AB connector), defaults to the host role and is always required to supply the voltage for VBUS, regardless of whether it is the host or the peripheral. The HNP can switch the role of host between the A and B device many times, but this should all be transparent to the user.

Session Request Protocol

Another concern that was addressed by the OTG supplement is that of power consumption. Typically, OTG devices will be battery powered, so conserving battery life is of the utmost importance. If the OTG device is to be the A device, then it must supply Vbus during the entire time the USB cable is plugged in, regardless of whether communication is taking place or not, which can drain the battery considerably. This is not ideal, so to alleviate this, the A device is allowed to leave VBUS turned off when there is no communication occurring. When the B device wishes to request a session, it initiates the SRP by pulsing the data line. Data-line Pulsing works by the Bdevice wait for VBUS, D+, and D- lines to be low for a prescribed time and then turns on its D+ pull-up resistor. The A-device then detects this, and supplies VBUS to begin a session.

Providing Power To VBUS

The requirements for the A-device to supply VBUS are that is must provide a voltage of 4.4V – 5.25V and supply a minimum current of 8mA. Unfortunately, 8mA is good for basic OTG interchangeability, however, there are many more potential applications would require more current. One potential application is for charging the battery of a Bluetooth headset from a mobile phone that has a battery at least 10 times the capacity. Of course, charging at 8mA would be excruciatingly slow, and practically, a user would only collapse the bus and not be able to charge at all. This application would make best use of being able to provide at least 100mA to the output and this cannot be achieved using a charge-pump. The only route to go then would be to use a boost regulator to boost from the battery voltage to the VBUS voltage. This of course is not ideal, because adding an additional control IC with FETs, inductor and capacitors increase cost, complexity, and board size. The best solution then is to purchase a USB compliant battery charger IC. The battery charger of course is required in any application, and if the device is an OTG device, it is then best suited to be USB Battery Charging Spec compatible as well. The beauty of such an IC is that it can charge a battery per the latest USB Battery Charging Specs, using a high efficiency buck topology converter that utilizes a chip inductor for minimized space and cost, but can then run the buck controller in reverse as an effective boost controller requiring no additional components. This essentially provides the means to support USB OTG for free, without any additional components, and the user is able to benefit from OTG connectivity, and USB charging flexibility. One such IC, the SMB339, manufactured by Summit Microelectronics, is the next-generation follow- up to the hugely successful SMB138, which is at the forefront of USB Battery Charging, and providing USB OTG power.


Moving away from a PC centric world is the dream of anyone who wishes to have a mobile experience that is not cumbersome, but instead powerful, fast, simple, and efficient. With so many potential use cases, and a simple, yet powerful, connectivity scheme based on the already widely used USB spec, the USB OTG supplement has the potential to create a truly connected mobile experience.



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