By Eva Murphy and Padraig Fitzgerald

With the increasing prevalence of USB functions in both portable and handheld applications, high-quality switches using ultralow power are playing a key role in implementations of this connectivity solution. Driven by the consumer demand for continual innovation, accelerated design and reduced manufacturing cycles, reliable low-cost USB switches are critical components for designers.

USB has become the most popular standard for communicating between PCs and peripherals. Keyboards, printers, Webcams, data storage devices, mobile phones, MP3 players, digital cameras and game consoles are among the many devices that can be attached to a PC using the USB port.

USB data is transmitted using a two-wire half-duplex system. The cable also includes a 5V power line and ground, eliminating the need for batteries in low-power devices such as USB pen drives, Webcams and keyboards.

The designer must take several factors into account when choosing a switch for USB:

• On-resistance
  The on-resistance varies with bias voltage, temperature and supply voltage, so the amplitude seen by the receiver will also vary with these parameters. On-resistance variation with bias voltage also causes rising and falling edges to see different impedances, resulting in poor crossover in the eye diagram. As a rule, the lower the on-resistance, the lower the variation with parameters. It is important to keep on-resistance below 5 ohms.
• Capacitance and bandwidth
  Capacitance should be kept below 15pF and bandwidth should be kept above 400MHz. The switch layout must be tailored to maintain capacitive symmetry.
• Propagation delay
  CMOS switches, which can be modeled as series resistors, add minimal delay to digital signals passing through them. The delay, measured in picoseconds, is limited only by the time it takes the signal to get to and from the die.
• Power supplies
  For low- and full-speed USB, the specified signal amplitude is 3.3V ±10 percent, so 3.6V is the minimum allowable supply voltage.
• Switch protection
  The USB spec states that the data lines of a USB device must be able to withstand being shorted to the 5V supply line for a period of 24 hours. The easiest way of doing this is using a resistor to limit the current flow. However, the most common solution is to use a switch powered from 5V supplies.