The Universal Serial Bus (USB) is quickly becoming the standard interface for most PC peripherals. It is replacing RS-232 and parallel printer ports because of its speed, flexibility, and support for hot-plugging of devices. Industrial and medical device manufacturers are also eager to use this bus, but adoption has been slow because there is no good way to provide the necessary isolation for machine connections that control dangerous voltages or for low-leak, defibrillation-proof connections in medical applications. .

The ADuM4160 is primarily designed to be used as an isolation component for USB peripherals. But in some cases, it can also be used to isolate the host. To do this, several issues must be resolved first. The buffers upstream and downstream of the ADuM4160 are identical and capable of driving the USB cable, but the downstream buffer must also be able to adjust its speed depending on the full-speed or low-speed peripheral connected to it.

In applications building dedicated peripheral interfaces, the speed is known and will not change, unlike host applications, which must improvise. The ADuM4160 is hardwired through pins to determine a single speed. So when the speed of the peripheral plugged into the downstream side is correct, it works fine; when the speed of the connected peripheral is incorrect, it doesn't work. 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, which can be easily isolated using the ADuM4160, while the speed of the downstream port is handled by the hub controller. The hub controller converts peripherals of different speeds to match the upstream port speed. The circuit in Figure 1 shows how a dual-port hub chip can be used in a design to isolate two downstream host ports, making the design fully compliant with the USB specification.

The ADuM4160 provides an economical and simple way to implement isolation buffers for industrial and medical peripherals. The challenge in utilizing this device is that it must be paired with a hub chip to build a fully compatible host port. As with any peripheral isolation, the ADuM4160 and hub provide the following features: 

1. Isolating the USB D+ and D− lines directly upstream of the hub chip allows the hub to manage downstream host port activity.
2. Implement automated control schemes for control data flow that do not require external control lines.
3. Provide medical grade isolation.
4. Allows the creation of one or more USB-IF certified host ports.
5. Supports full signaling rates.
6. Supports flexible power configuration.

The goal of this application circuit is to isolate the hub like a full-speed peripheral. The hub or host functionality requires 2.5 W of power per downstream port. Power for the downstream side of the isolator as well as power for the hubs and ports is provided as part of the solution. This application circuit is typical of many medical and industrial applications.

The power used by the upstream USB connector is derived from the 5 V VBUS voltage provided by the USB cable. The hub chip must provide all signals and pull-up/pull-down resistors required when the ADuM4160 is not used. The selected hub chip is the SMSC USB2512 dual-port USB hub controller, which has the characteristics of low cost and small size. Additionally, a 4-channel version of the same size is also available. This design is fully functional, supporting the current limiting function of each channel through the MIC2026 power distribution switch, and has sufficient offline regulated power supply to provide 2.5 W power to each downstream channel. Power on the downstream side is provided by a wall power adapter and the ADP3339 LDO regulator (5 V option). This device provides very low dropout voltage, reducing wall adapter regulation requirements.

Its small size and 1.5 A current capability are ideal for such general-purpose circuits, where downstream peripherals may require the full power of the cable to operate.

The ADuM4160 has multiple power, speed, and protection options that must be determined. Peripherals operate 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 blocks the handshake used to negotiate that speed. The selected hub chip supports high-speed operation, but normal operation of the ADuM4160 does not allow this mode. The ADuM4160 must be set up to operate at full speed through the status of the SPU and SPD pins. In the current schematic, the SPU and SPD pins are connected to the 3.3 V regulated power supplies VDD1 and VDD2, which sets the device to run at full speed.

5 V power can be provided through the VBUSx pin. The 3.3 V signal voltage is generated at the VDDx pin by an internal 3.3 V regulator. Another option is to supply 3.3 V directly to VBUSx and VDDx. The device is able to detect this configuration and use the 3.3 V supply directly, disabling the internal regulator. For illustration purposes, the ADuM4160 is configured to accept an external 3.3 V supply and bypass the internal regulator. The VBUS2 and VDD2 pins are shorted together and powered by an external 3.3 V voltage generated by the ADP3330 LDO regulator.

The ADuM4160 also offers an option to delay the application of an upstream pull-up resistor under peripheral control. This feature is controlled by PIN entry. In this application, the PIN input is shorted to high, so the upstream pull-up resistor is used whenever power is applied to the hub chip. In some applications, it can be connected to a controller's GPIO pin, a fixed delay circuit can be used, or it can be connected like this circuit. How to use this feature is up to the designer.

This circuit also shows EOS/ESD protection devices. These devices are selected from manufacturers that offer a variety of different devices, and the specific device chosen allows them to be replaced with a 0 Ω short-circuit resistor to remove it from the circuit. Designers should carefully consider protection device selection, ranging from situations where no external protection is required to a full suite of transient suppressor and filter components. The components included in this application circuit are typical of what could be used.

When the circuit is working, packet detection occurs and data is transferred from one side of the isolation to the other. The data shown in Figures 2 and 3 illustrate typical full-speed processing in the form of time domain data and eye diagrams respectively. In real-time data, characteristics to note are that the packet starts in the passive idle state, which transitions to the driven J state, and that the end of the packet at the end of processing shows a single-ended 0 state, followed by the idle J state. It is this automatic control flow and handling of these special logic states that enables the ADuM4160 chip and is unique on the market.

The hub is designed to be fully isolated from upstream data connections and can withstand transient voltages up to 5 kV. The downstream ports are powered from an offline power supply to support full power applications. Low-speed, full-speed, and high-speed peripherals can be connected to downstream ports in any combination, and the hub controller correctly negotiates the speed according to the USB standard. This design also includes current switching and limiting functions, and also reserves space for other output protection devices, which designers can choose to install as needed.

Applicable test reference documents are listed below: 

• Upstream full-speed signal quality test reference document—USB 2.0 specification Section 7.1.11, Section 7.1.2.1
• Upstream full-speed rise time test reference document—USB 2.0 specification Section 7.1.11, Section 7.1.2.2
• Upstream Full Speed ​​Fall Time Test Reference Document—USB 2.0 Specification Section 7.1.11, Section 7.1.2.2

Figure 3 is a full-speed eye diagram showing that the ADuM4160 is able to provide a fully open eye and stay away from the no-go zone. The exception is one transition, which invades a no-go zone. This is acceptable for this type of test certification. Similar data were obtained for the low-speed evaluation. A photo of the evaluation board is shown in Figure 4. For layout files, please visit: http://www.analog.com/CN0158-DesignSupport.