CN0550

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 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 V_BUS1 and V_BUS2 pins. V_BUS1 is tied directly to the USB host connector, and V_BUS2 is tied to the isolated 5 V supply. Internal low dropout (LDO) regulators generate the required +3.3 V V_DD1 and V_DD2 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.

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.

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 winding 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 of 30 V. A 150 μF capacitor is used to reduce the ripple of the output voltage. This combination of components allows the design to meet the standard 100 mA maximum load current drawn by low-power USB 2.0 devices using the available 5 V bus power on the host side.

Additionally, this circuit is capable of supporting the 500 mA maximum load current drawn by high-power USB 2.0 devices when connected to an external power supply of at least 8 V. Higher load currents can be achieved with more input voltage, up to 1.3 A at the maximum input voltage of 32 V. For the general design procedure in selecting these components, refer to the LT8301 data sheet.

Note that while the ADuM3165 has an insulation voltage rating of 3.75 kV rms and the LT8301 has a maximum input voltage of 42 V, these parameters will be limited by the transformer specifications in this application. The flyback transformer used in the CN0550 is tested at 1.5 kV rms insulation, but is only designed to comply with IEC61558-2-16 basic insulation for a primary circuit at a working voltage of 50 V rms. This transformer is suitable for isolation in most laboratory test and measurement applications.

The output voltage of the LT8301 is set by an external feedback resistor connected between the SW and RFB pins; the value of which is calculated using Equation 1:

where:
R_FB is the required feedback resistance in Ω.
N_PS is the transformer effective primary-to-secondary turns ratio.
V_OUT is the output voltage.
V_F is the output diode forward voltage.

For the required output voltage of 5 V, a 3:1 transformer turns ratio, and an output diode forward voltage of 0.3 V, Equation 1 results in an RFB value of 159 kΩ; the closest standard 1% value to this is 158 kΩ is considered. In practice, however, the unique sampling scheme used by the LT8301 can introduce errors in the output voltage. Equation 2 can be used to adjust the feedback resistance based on the actual output voltage.

where:
R_FB(FINAL) is the adjusted feedback resistance in Ω.
V_OUT(MEAS) is the actual output voltage measured in V.

With a target V_OUT of 5.0 V, a measured V_OUT of 5.26 V and an initial R_FB of 158 kΩ, Equation 2 results in a resistance value of 150.19 kΩ. In CN0550, a 150 kΩ ±1% feedback resistor is used. The resulting load regulation using these values are shown in Figure 6.