The momentum behind Universal Serial Bus (USB) has picked up steam to the point where buyers are beginning to differentiate PCs based on its presence or absence. When this happens in a market fast-paced growth cannot be far behind. As a result, many more designers are working with USB and some may not be familiar with the finer points of USB power distribution.

Effective power distribution is critical to a finely tuned USB-based I/O subsystem. The 12-Mbit/s USB architecture is essentially a tiered-star topology, supporting as many as 127 devices on one PC. Central to each star is a hub with one or more ports for downstream peripheral devices.

Since USB is a cable power architecture, devices connected to hubs rely on the hub for power distribution. USB hubs can be either bus-powered, and derive their power from another hub, one tier upstream, or be self-powered, in which case the hub would derive its power from a source external to the USB architecture.

Only a matter of time

Given the nature of personal computers and how they are used, catastrophes happen. Coffee is spilled on a keyboard, a desk chair rolls over a misplaced mouse wire: some occurrence will eventually take place and the ability of the USB subsystem to effectively manage power will be challenged. How a hub handles an over-current situation provides a real test of the power management devices such as the power switches or polyfuses used.

Historically, many USB hubs have used polyfuses because of their simplicity and low cost. During an over-current condition, a polyfuse simply heats up to a predefined point where the material that makes up the fuse changes resistance and the current going through the circuit is reduced to a minimum. The polyfuse will remain in this state until the problem is removed. Once the shorted condition is no longer present the polyfuse will begin to cool, and the resistance will be reduced to a normal level, restoring circuit operation.

The response time of polyfuses can be a problem. For example, the fuse might take several seconds to heat up and reduce current flow. In that period of time, all the downstream devices may be reset due to low input power if the short decreases the line voltage. These shortcomings have caused many USB system designers to implement the more sophisticated power distribution switches.

The switch is on

USB power distribution switches offer several advantages over polyfuses. In the event of an over-current condition, a switch will immediately respond and limit current to prevent any damage to the upstream devices.

The switch's response time is usually negligible. In addition, a switch can provide over-current feedback through an open drain response pin to the upstream USB controller or host. By itself, a polyfuse cannot provide any information to the USB controller and unlike a polyfuse, a switch can be turned off by the controller to prevent additional power loss.

The use of a power distribution switch certainly gives a USB system greater protection, but designing in the correct device for the application requires some consideration and planning.

Resistance tolerances

USB power is a 5-V bus with 3.3-V data. The power output of a host controller, or self-powered hub, is specified as 4.75 V. To assure that this voltage is distributed to downstream devices on a USB subsystem, designers must be cognizant of the resistance stack-up of components between the power supply and the USB connector.

For example, let's say the host controller has a 5-V power supply with five percent tolerance, so the voltage coming out of the supply could be as low as 4.75 V. But it will drop further because of the resistance in the power distribution switch in addition to the resistance in the traces and vias on the printed circuit board of the USB host controller. By the time it reaches the USB line, the power may very well have dropped below 4.75 V.

The designer may think that implementing a power supply with, say, three percent tolerance would solve the problem. Unfortunately, this may not provide the entire solution unless the resistance of the power distribution switch is relatively low. Even though a three percent 5-V power supply will output no less than 4.85 V, a power distribution switch with 200-mW of resistance will drop the output voltage of the USB controller to the level of 4 .75 V when distributing 0.5 A. And this does not leave any headroom for power lost from the resistance in the controller's traces, vias and other components. USB designers must carefully consider the voltage loss across the entire control circuit to assure power is adequately distributed downstream. Elements such as lower-resistance switches, higher-precision power supplies or larger power supply voltages help to improve design flexibility.

Managing power

Most integrated FETs typically include a back-gate diode (that is, a diode connected from the drain to the source in the circuit manufacturing process.) If such a diode is included in the FET that is used in a USB power switch, a USB peripheral device inadvertently left on could become a power source to upstream devices. For example, a PC user turns off his computer when he leaves the office at the end of the day, but he neglects to turn off the attached bus-powered printer. The printer includes a USB hub with a power switch. If this power switch has a back-gate diode, the host PC could draw power from the hub through the back-gate diode. Unless the PC is properly protected from undercurrent conditions, this could create a problem for the host computer. The simplest solution is to implement a USB power switch in the printer without a back-gate diode.

To prevent inadvertent switching, an under-voltage lockout capability forces the power supply to reach the minimum level and then remain there for a predefined period before power is distributed down the line.

Other issues

To avoid any unforeseen difficulties, several other characteristics of USB power switching devices should also be considered. For example, the level of compatibility between a USB controller and the hub's power switch will affect the implementation of a USB subsystem. Some controllers on the market use an active high signal to turn on the power switch while others have adopted an active low enable. Compatibility between the controller and power switch will eliminate the need for intervening logic.

Power switch packaging has also become more important as USB is implemented in applications where portability is an issue. If the USB hub in a laptop computer is very small, for example, the designer is able to use the freed-up board space to design-in additional functionality that could provide a competitive edge in the marketplace.

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