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

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.