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Posted on 01 June 2019

USB On-The-Go

 

 

USB for portable systems

The ever increasing number of battery powered portable devices in use, and performance driven demand for a high-speed data interface resulted in development and consequent wide adoption of the point-to-point USB protocol known as USB On-The-Go.

By Thomas Schaeffner, Systems Engineer Power Management Units, Texas Instruments

 

The initial specification for the Universal Serial Bus (USB) is dated back to 1996 when printers were connected to a PC using bulky parallel cables. Modems were connected using a serial RS232 interface. At that time, people were looking for an easy way to use serial interface in a PC environment. The USB defines a system that allows easy connection of multiple peripheral devices and easy peripheral driver installation. The PC acts as the host and the devices connected to it are always peripherals.

The USB is the host centric system that does not allow connecting two hosts or two peripherals to each other. The ever increasing number of battery powered portable devices in use, and performance driven demand for a high-speed data interface resulted in development and consequent wide adoption of the point-to-point USB protocol known as USB On-The-Go. It is a host centric system in which one of the peripherals acts as USB Host. To qualify as a USB Host the peripheral has to:

  • Support USB Power Management (source the current and regulate USB Supply voltage)
  • Implement USB Command parser and support different, but limited, USB peripheral classes
  • Have series "A" host connector receptacle

For portable devices, it is not practical to have these strict requirements for a host. In many cases, they are not needed for portable equipment.

In response to this demand, in 2001, a supplement to the USB 2.0 specification was introduced. USB On-The-Go (OTG) defines a rule set to overcome the strict limitations to qualify as a host. USB.OTG defines a minimal set of changes to the USB 2.0 specification, such that portable USB applications are enabled. These features include smaller connectors and the ability to connect devices directly to each other. Furthermore, USBOTG allows to turn off the interface while not in use in order to save power. As the USB standard defines two wires that are used to provide power via the USB cable to the peripheral, the requirements for the host to provide power to the bus was reduced to a current of 8mA minimum. The USB 2.0 specification requires that host systems source 500mA to the bus in order to enable the connection of bus-powered devices (like USB FLASH devices). A USB hub is also required to source power on to the bus, either 500 mA for a self-powered hub or 100mA for a bus-powered hub.

The supported data rates for USB-OTG did not change.

  • Low speed (USB 1.0) : 1 MBit/s
  • Full speed (USB 1.1) : 12 MBit/s
  • High speed (USB 2.0) : 480 MBit/s

In order to enable direct communication with each other, different classes of devices were introduced: A-devices, B-devices and dual role AB-devices, also called OTG devices.

A-Device:

  • Uses a standard-A
  • Supplies power to Vbus
  • Must source at least 8mA (can be more)
  • Is host (master) at the start of the session
  • Can relinquish the role of host to a dualrole device

 B-Device:

  • Uses a Standard-B, Mini-B, or Micro-B plug (ID pin floating)
  • Is peripheral (slave) at the start of session
  • May consume up to 8mA if bus-powered

 Dual-role Device:

  • Has limited host capability
  • Must operate as a standard USB peripheral
  • Targeted peripheral list
  • Session request protocol
  • Host negotiation protocol
  • One, and only one, Micro-AB receptacle
  • Need to source minimum 8mA on Vbus
  • Means for communicating messages to the user

With USB OTG, the device must act as a standard peripheral when connected to a standard host (such as a PC), and when in host mode, can service non-OTG peripherals if on the targeted peripheral list. Therefore a mobile phone must be peripheral when connected to a PC to synchronize the phone book, while it can be host when connected to a printer or mass storage device. The devices take care of all host/peripheral negotiation by themselves as soon as a connection is established. The cable connection defines which device will be the default host (A-device) or default peripheral (B-device). The OTG cable shorts the ID pin on the Aside connection and leaves it floating on the B-side. USB OTG allows a device to be both, host and peripheral (dual-role) and to switch roles on demand based on targeted peripheral list support. This is called host negotiation protocol.

Low power consumption is a key feature in portable equipment. For standard USB devices, the Vbus voltage of 5V, which is provided by the host, is always turned on. In OTG devices, the Vbus voltage as well as the transceivers can be turned off. A "session request" protocol was introduced to power up the devices again and to start the session. Session request protocol allows Bdevice to request A-device to turn on Vbus and its transceivers. The B-device therefore pulses D+ and Vbus and the A-device responds to one of them. The Vbus voltage is defined to be in the range between 4.4V and 5.25V for devices that can not source a higher current than 100mA - minimum 8mA.

The OTG host is usually supplied by a single LiIon cell with a voltage range of 3.0V to 4.2V. This voltage needs to be boosted to 5V nominal. There are two possible solutions: Inductive boost converters or charge pumps.

For an output current up to 100mA, the charge pump solution can be implemented with small external components. For a higher output current, it makes sense to use an inductive boost converter. One challenge in designing a converter to provide the Vbus voltage is output capacitance. While a standard USB host has a minimum of 96uF, an OTG device's output capacitance must be between 1uF and 6.5uF. Such a low value makes it very challenging to design a stable control loop for the power management device. This can only be achieved by boost converters with a high switching frequency or with charge pumps. Usually ceramic output capacitors are used in these kind of applications because they offer good performance at low cost and small size, however, they show a so called DC-bias effect. When a voltage is applied to a ceramic capacitor its actual capacitance drops with voltage applied. A 4.7uF ceramic capacitor may drop to 50% of its nominal value when 5V are applied across it. The strict requirements on output capacitance on an OTG-device is Vbus pulsing, when the A-device drives Vbus, charging the capacitors connected to the bus, the B-device monitors the rise time. Due to the huge difference in capacitance of a OTG device to a standard USB device, the B-device can detect if it is connected to a OTG device or standard USB device.

The TPS65030 is a power supply chip for USB implementation

The complete implementation of a high speed USB-OTG implementation not only requires a USB transceiver with a power management device to provide 5V for Vbus but also a host processor that is capable of handling the data rate of 480MBit/s. Furthermore, also the USB transceiver requires different supply voltage rails, which need to be generated from the power source available. Therefore it makes sense to define a power management unit that not only generates 5V for Vbus, but also all voltages for the USB transceiver. With this approach, a complete high-speed USB-OTG solution can be defined based on two chips: a power management chip and a USB transceiver. This solution can be connected to a host processor via a host interface. Such a system can be implemented using TUSB6010B high speed USB-OTG controller with the power management chip TPS65030.

The TPS65030 is a power supply chip for such an USB implementation. It contains 4 different converters to generate the voltages listed below:

  • 5V at 100mA for Vbus
  • 3.3V at 22mA for the analog blocks in TUSB6010B
  • 1.5V at 200mA core voltage of TUSB6010B
  • 1.8V at 60mA I/O voltage of TUSB6010B

To achieve smallest solution size, a device based on charge pumps was developed. The TPS65030 comes in a small 25-ball chip scale package. A fractional charge pump generates 5V for Vbus. As the core voltage of 1.5V is less than half of the typical voltage of a LiIon cell, the core voltage can be generated using a step-down charge pump. For the 3.3V, a LDO provides high efficiency as long as the input voltage is above 3.4V. For lower input voltages, it is switched to charge pump mode to boost the input voltage to 3.3V. The I/O voltage, provided by Vout4, is supplied by an LDO only. With this approach, a total efficiency of up to 90% is achieved. For an input voltage of greater than 3.6V, the typical efficiency is above 70%.

Efficiency vs input voltage

In USB-suspend mode, the TPS65030 can be switched to a low power mode by pulling the SLEEP pin to Vin. In this case, the quiescent current for TPS65030 goes down to some uA, allowing a total supply current for the power supply and the USB transceiver TUSB6010B of only 100uA.

The TPS65030 allows to be switched to bus powered mode

In addition to this feature, the TPS65030 allows to be switched to bus powered mode for Vout2 and Vout3. In this case, the 5V bus voltage from a host, connected to TPS65030, is used to power the device, helping to extend battery life in the application. The switchover to bus powered mode is done using SW_EN1 and SW_EN2 pins. When these inputs are pulled to Vin, the voltage at Vbus is supervised by a comparator that switches to inputs of Vout2 and Vout3 to the voltage applied at Vbus.

 

 

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