Circuit Note CN-0419 Devices Connected/Referenced Circuits from the Lab reference designs are engineered and Full/Low Speed 5 kV USB Digital ADuM4160 tested for quick and easy system integration to help solve todays Isolator analog, mixed-signal, and RF design challenges. For more Low Emission, Isolated DC-to-DC information and/or support, visit www.analog.com/CN0419. ADuM5020 Converters Universal Serial Bus (USB) Peripheral Isolator Circuit EVAL-CN0419-EBZ galvanically isolates a peripheral device EVALUATION AND DESIGN SUPPORT that implements a USB interface and is bus powered with a Circuit Evaluation Boards small isolated dc-to-dc converter based on Analog Devices, CN-0419 Circuit Evaluation Board (EVAL-CN0419-EBZ) Inc., iCoupler technology. The application circuit shown in Design and Integration Files Figure 1 is typically necessary for many medical and industrial Schematics, Layout Files, Bill of Materials applications. This circuit provides the following features: CIRCUIT FUNCTION AND BENEFITS 1. Directly isolates, in the upstream, the USB D+ and D Universal serial bus (USB) has been established as the standard lines of a cable. interface for most computer peripherals. It has mostly displaced 2. Implements an automatic scheme for data flow that RS-232 and the parallel printer port because of superior speed, does not require external control lines. flexibility, and support of device hot swapping. There has been 3. Provides medical grade isolation. a strong desire on the part of industrial and medical equipment 4. Allow a complete peripheral to meet the USB-IF manufacturers to use the USB standard as well. However, adoption certification standards. has been slow because there has not been a good way to provide 5. Support full speed (12 Mbps) and low speed the required isolation for safe and robust connections to machines (1.5 Mbps) signaling rates. that control dangerous voltages or low leakage, defibrillation 6. Provides up to 92 mA isolated power for the attached proof connections in medical applications. peripheral. ISOLATION BARRIER 5V FS OPTOCOUPLER LS SPU ADuM4160 DATA DATA USB TYPE A PROTECTION PROTECTION USB TYPE A PLUG DEVICES DEVICES RECEPTACLE POWER POWER ADuM5020 Figure 1. USB Cable Isolator Circuit Rev. 0 Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages Tel: 781.329.4700 www.analog.com due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2019 Analog Devices, Inc. All rights reserved. 17298-001CN-0419 Circuit Note USB Speed Selection CIRCUIT DESCRIPTION The ADuM4160 has several options for power, bus speed, and USB Isolation electrostatic discharge/electrical overstress (ESD/EOS) protection USB is the serial interface of choice for most computer peripherals. that must be determined. The ADuM4160 does not support Supported by all common commercial operating systems, it enables high speed operation and blocks handshaking signals that are on-the-fly connection of hardware and loading of drivers. Data used to negotiate that speed. High speed mode starts as a full transfer modes handle everything from large bulk data transfers for speed configuration. The peripheral requests high speed support memory devices, to isochronous transfers for streaming media, to through a process called a high speed chirp. The ADuM4160 interrupt-driven transfers for time critical data such as mouse ignores the high speed chirp therefore, the request for high movements. USB operates at three data transfer rates: low speed speed operation is never passed to the host, and the peripheral (1.5 Mbps), full speed (12 Mbps), and high speed (480 Mbps). continues to work at full speed. 3.3V PERIPHERAL HOST Because the peripheral operates at a single speed, the isolator is hardwired for the desired speed setting, either full speed or low D+ speed. For the ADuM4160 to operate at full or low speed, the D SPU and SPD signals must be configured properly. In this application, a switch at the upstream side allows the user to select between low and full speed. An optocoupler was used to transfer the same pulse signal from the SPU pin on the upstream side to the SPD pin on the downstream side. TRANSCIEVER TRANSCIEVER Figure 2. Standard Elements of USB To switch between the two modes, flip the actuator of the S1 switch to full speed (FS) mode for full speed operation and to The USB physical layer consists of only four wires: two provide 5 V low speed (LS) mode for low speed operation. Figure 3 and power and ground to the peripheral device the other two, D+ and Figure 4 show the switch position for each mode of operation. D, form a twisted pair that carries differential data with a signaling voltage of 3.3 V. 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. Control of this complex data structure is handled at each end of the cable by a serial interface engine (SIE). This specialized controller Figure 3.S1 Switch Position for Full Speed Operation or portion of a larger controller, which usually includes the USB transceiver hardware, takes care of the USB protocol. It handles all flow of control on the bus, enabling and disabling the line drivers and receivers, as required. The primary challenges in developing a USB isolator are properly determining the direction of data transmission and when to disable Figure 4. S1 Switch Position for Low Speed Operation drivers to allow an idle bus state. The packet oriented nature of USB data allows a simple method of determining data direction without When the circuit is functioning, packets are detected and data is the overhead of a complete SIE. When the bus is idle, pull-up and transferred from one side of the isolation barrier to the other. pull-down resistors hold the USB in an idle state with no buffers Data shown in Figure 5 show a typical full speed transaction as driving the bus. time domain data. Features to note in the real-time data are the passive idle state at the start of the packet, which transitions to a These challenges have been met with the ADuM4160 USB port driven J state, and then the end of packet at the end of the trans- isolator, based on Analog Devices iCoupler technology. The action that shows as a single-ended zero state followed by an idle J ADuM4160 provides mechanisms for detecting the direction of state. It is the automatic flow of control and the handling of data flow and control over the state of the output buffers. Data these special logic states that make the ADuM4160 chip direction is determined on a packet by packet basis. By combining operate. high speed CMOS and monolithic air core transformer technology, ADuM4160 provides outstanding performance characteristics and is easily integrated with low and full speed USB-compatible peripheral devices. Rev. 0 Page 2 of 4 LOCAL CONTROLLER AND SIE LOCAL CONTROLLER AND SIE 17298-002 17298-004 17298-003