Universal Serial Bus (USB) Hub Isolator Circuit
The ADuM4160 is designed primarily as an isolation element for a peripheral USB device. However, there are occasions when it is useful to isolate a host device. Several issues must be addressed to use the ADuM4160 for this application. Whereas the buffers on the upstream and downstream sides of the ADuM4160 are the same and capable of driving a USB cable, the downstream buffers must be capable of adjusting speed to a full or low speed peripheral that is connected to it.
Unlike the case of building a dedicated peripheral interface where the speed is known and not changed, host applications must adapt. The ADuM4160 is intended to be hardwired to a single speed via pins; therefore, it works when the peripheral plugged into its downstream side is the correct speed, but it fails when the wrong speed peripheral is attached. The best way to address this is to combine the ADuM4160 with a hub controller.
The upstream side of a hub controller can be thought of as a standard fixed speed peripheral port that can be easily isolated with the ADuM4160, whereas the speed of the downstream ports is handled by the hub controller. The hub controller converts peripherals of different speeds to match the upstream port speed. The circuit shown in Figure 1 shows how a two-port hub chip can be used to isolate two downstream host ports in a design that can be made fully compliant with the USB specification.
Figure 1. USB Cable Isolator Circuit
The ADuM4160 provides an inexpensive and easy to implement isolation buffer for medical and industrial peripherals. The challenge that must be met is to use this to create a fully com-pliant host port by pairing the ADuM4160 with a hub chip. As with isolating any peripheral device, the services that the ADuM4160 and hub provide are as follows:
- Directly isolates, in the upstream, the USB D+ and D− lines of a hub chip, allowing the hub to manage the downstream host port activity.
- Implements an automatic scheme for data flow of control that does not require external control lines.
- Provides medical grade isolation.
- Allows creation of one or more host ports that meet the USB-IF certification standards.
- Supports full speed signaling rates.
- Supports flexible power configurations.
The goal of the application circuit is to isolate a hub as if it were a full speed peripheral device. The hub or host function requires that 2.5 W of power be available to each downstream port. Power to run the downstream side of the isolator and power the hub and ports is provided as part of the solution. The application circuit is typical of many medical and industrial applications.
Figure 2. Full Speed Test Packet Traffic Driven by the Upstream ADuM4160 Port
This hub design is fully isolated from the upstream data con-nection for transients up to 5 kV. The downstream ports are powered through an offline power source to support full power applications. Low, full, and high speed peripherals can be connected to the downstream ports in any combination; the hub controller properly negotiates speed based on the USB standard. The design also includes current switching and limiting as well as positions for other output protection devices that can be installed as the designer chooses.
The following are applicable test references:
- Upstream full speed signal quality test reference—USB 2.0 Specification, Section 7.1.11., Section 188.8.131.52.
- Upstream full speed rise time test reference—USB 2.0 Specification, Section 7.1.11., Section 184.108.40.206.
- Upstream full speed fall time test reference—USB 2.0 Specification, Section 7.1.11., Section 220.127.116.11.
Figure 3 is a full speed eye diagram showing that the ADuM4160 provides an adequate open eye, staying well out of the keep out region. There is an exception for a single transition that violates the keep out zone. This type of artifact is acceptable to qualification in this test. Similar data is taken for low speed evaluation as well. A photograph of the board is shown in Figure 4. A complete design support package for this circuit note can be found at http://www.analog.com/CN0158-DesignSupport.
Figure 4. ADuM4160 Isolated USB Hub Evaluation Board
The following equipment is needed: a USB port hub connection, with an upstream-downstream data path, the ADuM4160 isolated USB hub, the ADuM4160 evaluation board, three USB cables, and a high speed digital oscilloscope.
In addition to the circuit, circuit schematic, and block diagram of the test setup described in this Circuit Note, details regarding the ADuM4160 isolated USB hub evaluation board, Gerber’s, bill of materials, layout, and assembly information are contained in the CN-0158 design support package. Information regarding the components used in the isolator circuit is available in the ADuM4160, ADP3330, and ADP3339 data sheets and the ADuM4160 evaluation board user guide, UG-043.
Functional Block Diagram
See Figure 1 for the schematic of the described test setup.
|ADP3339||High-Accuracy Ultralow Iq, 1.5 A, anyCAP® Low Dropout Regulator||
|ADP3330||High Accuracy Ultralow I(Q), 200 mA, SOT-23, anyCAP® Low Dropout Regulator|
|ADUM4160||Full/Low Speed USB Digital Isolator||