USB On-The-Go presents benefits, challenges to power designers

Users need to connect these devices directly to each other, a use not supported by USB. In response to this limitation, a supplement to the USB 2.0 specification, called USB On-The-Go (OTG) addresses mobile interconnectivity by defining how two devices connect without the need for a computer host. The USB OTC spec has backing from heavy hitters such as Nokia, Ericsson, Motorola, HP, Intel, Microsoft, Qualcomm, TI, Palm, Cypress, and Philips. As vendors introduce controller and software platforms to OEMs, it becomes increasingly important for OEM designers to understand the power management complexities the spec presents.

Under USB OTG the user plugs two devices together to establish a link. The devices take care of all the host/peripheral negotiation without any input from the user to make the experience as simple as possible. USB OTG defines two types of configurations: A-devices (devices that have a Standard-A or Mini-A plug inserted), are hosts by default when connected, and B-devices (devices that have a Standard-B or Mini-B plug inserted), are peripherals by default when connected. OTG-devices (formerly known as dual-role-devices) can be either an A-device or B-device, giving it the potential to be either host or peripheral. The status is negotiated between the devices.

In addition to providing new device definitions, USB OTG presents many challenges for portable electronic system engineers. USB OTG requires that battery-powered portable devices create a 5V supply typically from a 3.0V to 4.2V single-cell lithium ion source. Additional requirements define VBUS monitoring and protocols for handling power-saving modes.

According to the OTG Supplement Rev 1.0, an OTG A-device must provide at least 8mA between 4.4V and 5.25V to power VBUS. Devices can negotiate for more current, depending on the host's ability to supply more power. Voltage rise time must be less than 100ms in case the peripheral draws more current than can be supplied by the A-device. In this instance the A-device must turn off VBUS and terminate the session.

When VBUS is not powered up, the A-device must have only 40kohm-100kohm resistance on VBUS, and voltage due to leakage on VBUS must be less than 0.2V. Finally, the OTG device's VBUS decoupling capacitance must be between 1µF and 6.5µF. A standard USB host has a minimum 96µF.

USB OTG defines low power consumption for portable devices. When there is no active session, VBUS is turned off to save battery power. If the A-device turns off the VBUS, but the B-device wants to use the bus, the B-device can request that the A-device turn on VBUS. This request is called Session Request Protocol (SRP) by the USB OTG Supplement and it is performed by data-line pulsing and VBUS pulsing.

Before starting VBUS pulsing, the VBUS voltage must be below 0.8V to make sure that the A-device recognizes there is no session in progress, and also so that it can detect low to high voltage when VBUS pulsing begins.

During VBUS pulsing the B-device drives VBUS high to signal the A-device to turn on VBUS. While driving VBUS, the B-device must monitor VBUS to make sure it is connected to an OTG device instead of a standard USB host. This is detected via the difference in capacitance described above.

The OTG spec defines four voltage thresholds to monitor:

• VA_VBUS_VLD monitors VBUS, reporting when it is above 4.4V, inside a valid range. This detector also functions as a simple over-current detector in case the load requires more current than the host can provide.

• VA_SESS_VLD monitors VBUS. The range of 0.8V to 2.0V sig nals an A-device that the session is valid. This voltage range is used to detect a B-device VBUS-pulsing to initiate the SRP.

• VB_SESS_VLD monitors VBUS. The range 0.8V to 4.0V signals a B-device that the session is valid.

• VB_SESS_END monitors VBUS, the range is 0.2V to 0.8V to signal a B-device that the session has ended. This state must be recognized by the B-device before initiating SRP.

The USB 2.0 spec's power management issues were mostly confined to keeping track of voltage drops across current-limiting ICs, connectors, and PCB traces. USB OTG introduces many more challenges for engineers to consider.

Creating the VBUS supply from a battery is a primary concern. A power management system for an OTG-device needs to include a 4.4V to 5.25V output, supplying at least 8mA. In common practice, the majority of portable devices use a single-cell lithium ion or lithium polymer battery. This means a voltage converter is required to step up from 3.0-4.2V to 4.4-5.25V .

High-efficiency conversion is required to provide the longest possible battery run time. The OTG spec calls for at least 8mA and allows negotiation for higher currents if the peripheral needs more power. OTG devices can provide up to 500mA, but in realistic terms, handheld portable electronics don't have 500mA to spare for external loads. 100mA is a commonly accepted realistic maximum.

The spec also states that the supply must have a rise time less than 100ms. Typically this is not an issue for voltage converters because they have turn-on times of less than a few milliseconds.

Up to capacitance standards

A high-frequency fractional (1.5x) charge pump is the preferred conversion method for several reasons. The first reason is meeting the required output capacitance. A high-frequency switched capacitor regulator can use small capacitors on the output to easily meet the OTG spec's maximum 6.5µF requirement. Other solutions, such as inductor-based D C/DC boost converters, can have stability problems with small output capacitors.

Another reason for using a fractional charge pump as opposed to a standard regulated voltage doubler is to increase efficiency, which improves battery run time. For example, if the lithium ion battery voltage is 3.6V, a 5V-output regulated voltage-doubler's theoretical efficiency would be 69 percent. With the same input and output conditions, a 1.5x fractional charge pump achieves 93 percent. These numbers are theoretical, but in actual testing charge pumps achieve efficiencies within 1% of theoretical calculations.

Other minor considerations include the leakage requirement, which states that when the VBUS is turned off, the A-device must not float the VBUS any higher than 0.2V. Additionally, a pull-down resistor is required in the range of 40kohm and 100kohm.

For OTG-device compatibility, four voltage detectors are required to watch VBUS for (greater than) 4.4V, (0.8V to) 4.0V, (0.8V to) 2.0V, and (0.2V to) 0.8V. These levels correspond to the different states of VBUS as seen by an A-device or B-device. From a practical standpoint, the voltage detectors should be extremely low quiescent-current devices in order to maximize power savings. This is especially true for the 2.0V voltage detector because it must remain enabled even when the A-device turns off the bus, so it can recognize an SRP event.

Furthermore, when the OTG-device is acting as a B-device, the maximum allowed unconfigured current is 150µA. Clearly, the power budgets must be managed carefully.

Another function that sets OTG apart from its USB predecessor is the ability of a B-device to wake up VBUS when it is turned off to save power. OTG-devices must be able to perform data-line pulsing and VBUS pulsing to initiate an SRP. The power management system must be able to drive VBUS and monitor voltage and timing simultaneously.

The purpos e of this is to see the difference between the capacitance of an OTG device (maximum 6.5µF) and a standard USB host (minimum 96µF). The monitor must take into account the timing which would charge 2 x 6.5µF above 2.1V, yet 96+1µF (1µF is the minimum allowed OTG capacitance) would remain below 2.0V. This is important so that a standard USB host that is not SRP capable won't be damaged due to reverse current flow on VBUS.

A designer of an OTG-device could use the voltage converter for VBUS pulsing, but this approach would needlessly burn power. The preferred method for VBUS pulsing is a current source because it simplifies timing. A current source charging a capacitor provides a linear voltage ramp, in contrast to a voltage source charging an RC, which gives an exponential curve.

The OTG spec states that one of the initial conditions required before a B-device can start SRP is that the VBUS must be less than 0.8V. It can be helpful to speed up SRP by switching in a pull -down resistor to discharge VBUS. The only requirement of this discharge resistor is that it can't draw more than 8mA.