Isolating USB 2.0 and power for harsh environments
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.
Figure 1. Isolated USB transceiver with isolated power
The module employs inductively coupled coils, or coreless transformers, to pass data across the 2500 VRMS isolation barrier, while dedicated ICs perform the data transmission and receiving functions for both USB channels and both data directions. USB signals on either side of the barrier are encoded into pulses and translated across the barrier using differential signalling through the coreless transformers formed in the μModule substrate. This system, complete with data refresh, error checking, safe shutdown on fail, and extremely high common mode immunity, provides a robust solution for bidirectional signal isolation.
Isolated power in the module is generated by more conventional means. Power is derived from a boundary mode flyback converter with primary side voltage sense regulation. This overall power topology provides a simple, flexible, fault tolerant and relatively efficient design (~65%). The LTM2884 has two separate inputs for powering the onboard USB transceiver and DC/DC converter. The USB transceiver accepts power from a 4.4V to 16.5V bus or external supply. The DC/DC converter also accepts power from the same sources; if connected to a 5V bus, isolated peripherals can harness up to 1W (200 mA at 5V) of isolated power; otherwise, connecting a greater-than-8.6V external supply provides peripherals with 2.5W (500 mA at 5V).
next; simplifying rugged designs
Simplifying rugged designs
The USB path is compatible with both USB 2.0 full speed (12 Mbps) and low speed (1.5 Mbps) operation (figure 2). The device detects the speed of a connected USB device on the downstream port and then sets its own internal pull-up resistors on the upstream port to match the speed accordingly. Integrated pull-down resistors also support the downstream bus configuration. The LTM2884 maintains the conditions of the USB bus idle state by monitoring the downstream idle condition and refreshing the state across the isolation barrier at a consistent rate. If the upstream port is idle for greater than 3 msec, the module can enter suspend mode to reduce its own power consumption to less than 500 µA. This automatic speed selection feature eliminates the need to set jumpers or configure software, enabling truly plug-and-play designs.
Figure 2. Automatic selection of internal upstream pull-up resistors to match downstream speed
While isolation safeguards against ground differentials, other protection mechanisms are required to combat the noise, transients and surges that are commonly present in noisy environments. One of the major benefits of galvanic isolation is the ability of the isolation barrier to hold off large voltage potentials, eliminating the need for other protection devices such as bulky transient voltage suppressors (MOV, TVS, etc). The module withstands 2500 Vrms (3500 Vpeak) for up to 1 minute, and also withstands ±15 kV HBM (human body model) ESD on the USB pins to local supplies and ±15 kV HBM through the isolation barrier to supplies. The high ESD ratings derive from a carefully designed, high performance isolation barrier, which also supports a minimum common mode transient rate of 30 kV/µsec (50 kV/µsec typical) and allows the LTM2884 to operate through transient events error-free while transmitting USB data.
The module possesses strong immunity to RF and magnetic fields, and is also a friendly neighbour that does not generate high levels of EMI (electromagnetic interference). The µModule isolator technology has been independently evaluated and successfully passed the RF and magnetic field immunity testing requirements according to European Standard EN 55024, in accordance with the EN 61000-4-3 test standard. Design and layout techniques also allow the LTM2884 to minimise EMI to a point far below the Class B limit of CISPR 22, eliminating the need for complicated EMI shielding and mitigation techniques.
Next; applications…
Isolated USB applications
There are not too many possible circuit configurations when it comes to isolating USB ports. The isolating component can either be placed at the input of a peripheral, in line with the USB cable (repeater), or output of a hub/host. With that said, the device can be employed in practically any USB device, regardless of the end application or market segment. Minimal board space is required by the 225 mm² footprint, while almost all electrical specifications are tested and guaranteed over a -40°C to 105°C operating temperature range. UL 1577 component-level certification is in progress and the LTM2884 meets all of the requirements in the medical safety standard, IEC 60601-1, except for the few two-MOOP (means of operator protection) or two-MOPP (means of patient protection) tests that call for AC test voltage higher than 2500 kVrms. Otherwise, the package’s greater than 9 mm pin spacing (creepage and clearance), as well as 100 µm distance through insulation for data and 114 µm for power, are some safety related parameters that will help USB systems pass various equipment level standards.
The concept is so widely applicable that it found its way into multiple designs within Linear Technology, prior to its public release. Figure 3 shows the module at the heart of Linear Technology’s LTP2884 USB inline isolator, a complete standalone device that serves as a complete reference design and can also be purchased as-is so that USB isolation can be quickly deployed around the work environment. The LTP2884 provides all of the functions of the LTM2884 in a small rugged enclosure, complete with ±20kV ESD protection, LED status and warning indicators, and high retention USB connectors. Drivers and software are not required, just plug and play.
Figure 3. USB inline isolator based on the LTM2884
The LTM2884 has also found its way into Linear Technology’s DC2026 Linduino One Isolated Arduino-compatible demonstration board. Compatible with the popular Arduino Uno open-source electronics prototyping platform, the Linduino uses Arduino hardware that consists of an Atmel microcontroller with a bootloader, allowing quick in-circuit firmware updates. The software is a simple C programming environment based on the AVRGCC compiler. Linear Technology uses the Linduino to effectively demonstrate and distribute libraries for ICs that have digital interfaces, such as I²C or SPI. The LTM2884 breaks the ground connection to the PC, allowing projects connected to the Linduino to operate at a different ground potential than the computer that is controlling it.
The number of USB applications that require isolation is still growing, especially in the industrial and medical sectors, for safety and reliability reasons. The device described here has distinct advantages over older discrete solutions, including data-plus-power isolation features in a small package. 2500 VRMS galvanic isolation, automatic speed selection, excellent common mode rejection, high ESD, and low EMI all help expedite robust designs. Furthermore, no external components, including power decoupling capacitors, are required which designers working with space-constrained USB applications will appreciate. A highly integrated device is long overdue and should let USB flourish in the harshest environments!
Chris Gobok is a product marketing Engineer for mixed-signal products at Linear Technology.
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