Isolating USB 2.0 and power for harsh environments

October 01, 2014 // By Chris Gobok, Linear Technology
Connect your desktop PC to industrial or medical equipment using a standard USB (Universal Serial Bus) cable and you may quickly learn an expensive lesson.

Your PC and the piece of machinery can easily be connected to different outlets at different ground potentials. So after connection, the USB cable can provide a lower impedance ground path between both devices, enabling everything along the USB power path to fry. Replace the PC with a device that depends on USB power, such as a portable scanner, and your dilemma might be that you are faced with a USB port that provides insufficient power. Perhaps more frustrating is when you know all of your devices are safely connected and powered properly, yet the electrically noisy environment makes communications go haywire.

The USB standard was developed in the mid-1990s and was never designed to operate in a noisy environment. It was meant to connect low power peripherals to PCs over short distances in a relatively quiet home or office environment. Fast forward to today, where it has grown wildly popular for characteristics such as its speed and ease of implementation, designers rely on USB to interface computers with a wide variety of custom peripherals, where it has become clear that isolation is required in certain applications, especially in the medical and industrial arenas. Isolated USB transceivers are available in the market, but these solutions exclude isolated power and passive devices, forcing larger, more complex designs. A more elegant solution is an isolated USB transceiver and power, which drastically simplifies any isolated USB 2.0 hub or peripheral design and prepares them for harsh environments.

Chipscale isolation and power

Galvanic isolation is used in a variety of industries, most commonly to provide safety against potentially lethal voltages. Isolation is also used to eliminate the effects of noise and common-mode voltage differences created by ground loops, or as a level shifter between dissimilar operating voltages. Typically, building an isolated system requires a number of passive and active components on either side of the isolation barrier in addition to the barrier components themselves. Barrier components are notoriously difficult to use, adding significant design time and cost to isolated systems. With this in mind, Linear Technology developed a line of μModule isolators that reduce the design of isolated systems to simply plugging in a module, with no complex barrier components required; these isolators require no external components at all. The LTM2884 USB μModule isolator, shown in Figure 1, provides 2500 VRMS of galvanic isolation and integrates a USB 2.0 transceiver, a no-opto flyback converter, ultra-low quiescent current LDOs and all required passives in a 15 x 15 x 5 mm surface mount BGA package.