June 13, 2019
| By: Michael Jackson
Principal Writer, Maxim Integrated
Q. Dear Maxim:
My new piece of industrial equipment needs a high-speed USB interface that is also safe to use with all those high AC and DC voltages around. What are my options?
- Worried Engineer
A. Dear Worried Engineer:
Well, let’s start right back at the beginning so we’re fully understanding your problem.
USB is a four-wire interface used to connect an upstream host/charger to a downstream device (Figure 1). VBUS (+5 VDC) and GND (0V) are the power pins, while D+ and D- are used for either differential or single-ended data transfer. The data is organized into data frames or packets. Each frame can contain bits for clock synchronization, data type identifier, device address, data payload, and an end-of-packet sequence. Data transfer takes place at speeds of either 1.5Mbps (low speed), 12Mbps (full speed), or 480Mbps (high speed) using a range of connectors with different form factors. Types A, B, mini-B, and micro-B are the most common connector types.
Figure 1. Block diagram of a USB interface.
How Effective Are Traditional USB Protection Options?
Control of the USB data structure is handled at each end of the cable by a serial interface engine (SIE). This specialized controller, or subset of a larger controller, usually includes the USB transceiver hardware that handles the USB protocol. When a peripheral is first connected to the cable, the SIE provides the host with the configuration information and power requirements. During operation, the SIE formats all data according to the required transfer speed and provides error checking and fault handling. The SIE handles bus flow control as well as enables or disables the line drivers and receivers as necessary. The host initiates all transactions, which then follow a sequence of data exchanges between host and peripheral. This includes provisions for when data is corrupted or other faults that may occur.
Common IC solutions for USB data channel protection use process isolation technology to ensure no current path can exist between the high-voltage equipment and the low-voltage device. This involves using an isolation IC at a location between the controllers (Figure 2).
Figure 2. Isolation placed between USB controllers.
While this ensures that the USB port can continue to operate normally, even when exposed to voltages up to 5kV, this approach only allows data transfer rates up to 12Mbps.
Some IC solutions are limited in that they only provide upstream protection (i.e., the host side is protected but not the downstream device). Also, some isolation ICs protect the data transfer pins (D+/D) but not the power pins (VBUS/GND). It is possible to construct a smaller, custom USB protection circuit using discrete components. However, because many components are required, this solution is very complex while still only allowing data transfer at 12Mbps.
Clearly, none of these is the ideal solution to your problem.
Achieve the Protection You Need Without Isolation
Fortunately, we can suggest an alternative integrated solution to USB port protection for commercial and industrial equipment as shown below in Figure 3.
Figure 3. MAX22505 USB protection solution.
The MAX22505 USB protection solution protects a USB port from faults while allowing data transfer rates up to 480Mbps. The device ensures that the USB port is fully protected if exposed to the voltage levels typically encountered in industrial and commercial environments (±24V AC or ±40V DC). In the case of a wiring fault which brings the USB port into contact with a high voltage, this IC responds by effectively “open-circuiting” the connection to the port, thus protecting both the upstream host and any connected downstream device. No data transfer can take place until the fault has been resolved. Another benefit is board space savings. This solution requires an area of only 120mm2, representing a savings between 20% and 50% (compared to other solutions). In addition, all USB port pins are protected in the event of a fault (±50V DC for power and ±40.7V for data), with no configuration required. You can evaluate the IC via the evaluation kit. MAX22505EVKIT features a high-power peripheral section, a low-power peripheral section, and a high-power host section, with each section demonstrating USB bus protection from overvoltage conditions on the USB lines.
So, Worried Engineer, we think this might just be the answer to your problem…Happy designing!