Circuit Note

CN-0550

analog.com

Rev. 0 | 2 of 5

CIRCUIT DESCRIPTION

USB DIGITAL ISOLATION

The CN0550 employs an ADuM3165 digital isolator to provide

galvanic isolation of the USB data lines between the host and the

peripheral device. The ADuM3165 supports low-speed (1.5 Mbps),

full-speed (12 Mbps), and high-speed (480 Mbps) USB 2.0, while

maintaining an isolation voltage rating of 3.75 kV rms.

The ADuM3165 utilizes the Analog Devices' iCoupler® technology

to implement the isolation on the digital lines, and is capable

of automatically negotiating the appropriate data transfer speed

without user intervention or the need for external circuitry.

Figure 2. ADuM3165 Functional Block Diagram

Figure 2 shows the ADuM3165 circuitry, which greatly simplifies

the isolation design as the device is highly integrated. Excluding

the signals from the USB connectors, the only external circuitry

required to operate the device is a 24 MHz reference clock source

for the internal phase-locked loop (PLL). This can be implemented

either by connecting a crystal between its XI and XO pins, or by

connecting a 24 MHz clock source from the host controller to the

XI pin and leaving XO floating. In CN0550, a 24 MHz crystal with

its necessary load capacitance is connected across the XI and XO

pins.

Power to the ADuM3165 is supplied as +5 V on the VBUS1 and

VBUS2 pins. VBUS1 is tied directly to the USB host connector, and

VBUS2 is tied to the isolated 5 V supply. Internal low dropout (LDO)

regulators generate the required +3.3 V VDD1 and VDD2 supplies

for internal circuitry. The power dissipation of the LDOs reduces

the maximum ambient operating temperature slightly. Refer to the

ADuM3165 data sheet for details.

To verify high-speed USB electrical requirement compliance, two

signal quality tests were performed using the CN0550. Figure 3 and

Figure 4 show open eye diagrams at the far-end and near-end,

respectively.

Figure 3. High-Speed, Far-End Device Eye Diagram

Figure 4. High-Speed, Near-End Device Eye Diagram

ISOLATED USB POWER

An LT8301 micropower no-opto isolated flyback converter provides

the isolated 5 V bus power on the peripheral side. Depending on

the input voltage, this device is capable of delivering up to 6 W

of isolated output power, allowing it to support both low-power and

high-power USB 2.0 devices. Figure 5 shows the simplified isolated

flyback circuit configuration used for the CN0550.

Figure 5. Simplified LT8301 Flyback Converter Circuit

In CN0550, the LT8301 uses a flyback transformer with a turns ratio

of 3:1 and a primary inductance of 40 μH to produce the isolated

power. A diode-Zener snubber is included across the primary wind-

ing to protect the device from high voltage spikes generated by

leakage inductance. A diode provides rectification on the secondary

side, and is rated for a forward current of 5 A and a reverse voltage