Universal Serial Bus (USB) Cable Isolator Circuit

Crystal patch 2520 2.5*2.0MM 12MHZ 12.000MHZ 1.6V~3.6V universal OSC

Photocoupler

Circuit function and advantage

The Universal Serial Bus (USB) is quickly becoming the standard interface for most PC peripherals. It is replacing RS232 and parallel printer ports because of its speed, flexibility, and support for hot swapping of devices. Industrial and medical device manufacturers are also eager to use this bus, but suffer from a lack of good ways to provide the necessary isolation for machine connections that control dangerous voltages or low-leakage anti-defibrillation connections in medical applications, leading to slow application adoption. .

The ADuM4160 is primarily designed as an isolation component for USB peripherals. However, in some cases it can also be used to implement isolated cable functions. To this end, several issues must be resolved first. The upstream and downstream buffers of the ADuM4160 are identical and can drive the USB Cable, but the downstream buffer must also be able to adjust the speed based on the full-speed or low-speed peripherals connected to it. The upstream connection must work like a peripheral, and the downstream connection must work like a host.

In applications where a dedicated peripheral interface is built, the speed is known and does not change, while the host application is different and must be adapted to detect whether the connected low or full speed device is connected. The ADuM4160 hard-wires the pin to determine a single speed; therefore, it works fine when the peripherals plugged into the downstream side are at the correct speed; it does not work when the connected peripherals are not at the correct speed. The best way to solve this problem is to combine the ADuM4160 with a hub controller.

The upstream side of the hub controller can be thought of as a standard fixed-speed peripheral port that can be easily isolated using the ADuM4160, while the downstream ports are all handled by the hub controller. However, in many cases, although it is not considered to be fully compliant with the USB standard, a single speed cable is acceptable from a practical point of view, especially if a custom connector is used, so that it is not confused with compatible devices. The hub chip can be removed, so the design will be very small and simple.

The ADuM4160 provides an economical and simple way to implement isolation buffers for industrial and medical peripherals. The challenge with this device is that it must be paired with a small isolated DC-DC converter such as the ADuM5000 to build a bus-powered cable isolator with this isolation buffer. As with any device isolation, the ADuM4160 offers the following features:

• Directly isolate the USB D+ and D- lines of the cable upstream.

• Implement an automatic control scheme for control data streams that do not require an external control line.

· Provide medical grade isolation.

· Support full speed or low speed signal rate.

· Support for isolated power supply via cable.

The goal of the application circuit shown in Figure 1 is to isolate a peripheral that already implements a USB interface. Since a 100% inefficient power converter can be used to transmit bus voltage across isolation, a fully compatible bus-powered cable is not available. In addition, the converter's quiescent current does not meet the USB standard standby current requirements, and the ADuM4160 has speed detection limits. What can be achieved is a fixed speed or switch-controlled speed cable that provides moderate power to downstream peripherals. However, this is a custom application that does not fully comply with the USB standard.

Circuit description

The power used by the upstream USB Connector is obtained from the 5 V VBUS voltage provided by the USB cable. The bus voltage also drives the ADuM5000 to generate the VBUS2 voltage for use on the downstream side of the ADuM4160 and to provide up to 100 mA of power to the peripherals. The reason for choosing the ADuM5000 is its high isolation voltage and small size. It provides enough power for small bus-powered devices such as mice, keyboards, and memory sticks. Since the device uses a chip-scale micro-transformer, the internal switching frequency is very high, so ferrite beads must be used on the cable during design and follow the recommendations in the application note AN-0971 to minimize the effects of electromagnetic radiation. In order for the system to pass EMI/RFI testing, special layout, decoupling, and grounding techniques must be used. For guidance, please refer to the tutorial MT-031 and the tutorial MT-101.

The ADuM4160 isolated cable application has a variety of power, bus speed and ESD/EOS protection options that must be determined. The peripheral operates at one of three speeds: low speed (1.5 Mbps), full speed (12 Mbps), and high speed (480 Mbps). The ADuM4160 does not support high speed operation and will block the handshake signal used to negotiate this speed. The high-speed mode begins with a full-speed configuration, and the peripherals request high-speed support through a process called high-speed chirp. The ADuM4160 ignores this high-speed FM; therefore, high-speed running requests are never passed to the host, and the peripherals automatically continue to run at full speed. This application circuit includes a switch and a single-channel isolator that allows the user to select the cable speed by setting the SPU and SPD pins: full speed or low speed. This feature is optional if single-speed operation is fully satisfactory.

The VBUSx pin provides power. The 3.3 V signal voltage is generated by the internal 3.3 V regulator on the VDDx pin. The ADuM4160 supports other power supply configurations. See the other circuit notes for details. In the circuit shown in Figure 1, both the upstream and downstream sides of the ADuM4160 are configured to draw power from the VBUSx line and the internal regulator.

The ADuM4160 also provides an option to delay the application of upstream pull-up resistors under peripheral control. This feature is controlled by the PIN input. In this application, the PIN input is shorted to a high level, so an upstream pull-up resistor is used whenever a peripheral power supply is applied.