ADuM3160

Rev. A | Page 10 of 16

APPLICATIONS INFORMATION

FUNCTIONAL DESCRIPTION

USB isolation in the D+/D− lines is challenging for several

reasons. First, access to the output enable signals is normally

required to control the transceiver. Some level of intelligence

must be built into the isolator to interpret the data stream and

determine when to enable and disable its upstream and down-

stream output buffers. Second, the signal must be faithfully

reconstructed on the output side of the coupler while retaining

precise timing and not passing transient states such as invalid

SE0 and SE1 states. In addition, the part must meet the low

power requirements of the suspend mode.

The iCoupler technology is based on edge detection and,

therefore, lends itself well to the USB application. The flow of

data through the device is accomplished by monitoring the

inputs for activity and setting the direction for data transfer

based on a transition from the idle state. After data directionality

is established, data is transferred until either an end of packet

(EOP) or a sufficiently long idle state is encountered. At this

point, the coupler disables its output buffers and monitors its

inputs for the next activity.

During the data transfers, the input side of the coupler holds its

output buffers disabled. The output side enables its output buffers

and disables edge detection from the input buffers. This allows

the data to flow in one direction without wrapping back through

the coupler causing the iCoupler to latch. Timing is based on

the differential input signal transition. Logic is included to

eliminate any artifacts due to different input thresholds of the

differential and single-ended buffers. The input state is trans-

ferred across the isolation barrier as one of three valid states,

J, K, or SE0. The signal is reconstructed at the output side with a

fixed time delay from the input side differential input.

The iCoupler does not have a special suspend mode, nor does it

need one because its power supply current is below the suspend

current limit of 2.5 mA when the USB bus is idle.

The ADuM3160 is designed to interface with an upstream-

facing low/full speed USB port by isolating the D+/D− lines.

An upstream-facing port supports only one speed of operation;

therefore, the speed-related parameters, J/K logic level, and

D+/D− slew rate are set to match the speed of the upstream-

facing peripheral port (see Table 10).

A control line on the downstream side of the ADuM3160

activates the idle state pull-up resistor. This allows the

downstream port to control when the upstream port attaches

to the USB bus. The pin can be tied to the peripheral pull-up,

a control line, or the peripheral power supply depending on

when the initial bus connect is performed.

PRODUCT USAGE

The ADuM3160 is designed to be integrated into a USB

peripheral with an upstream-facing USB port as shown in

Figure 4. The key design points are as follows:

•

The USB host provides power for the upstream side of the

ADuM3160 through the cable.

•

The peripheral supply provides power to the downstream

side of the ADuM3160.

•

The DD+/DD− lines of the isolator interface with the

peripheral controller, and the UD+/UD− lines of the

isolator connect to the cable or host.

•

Peripheral devices have a fixed data rate that is set at design

time. The ADuM3160 has configuration pins, SPU and

SPD, that are set by the user to match this speed on the

upstream and downstream sides of the coupler.

•

USB enumeration begins when either the D+ or D−line is

pulled high at the peripheral end of the USB cable. Control

of the timing of this event is provided by the PIN input on

the downstream side of the coupler.

•

Pull-up and pull-down resistors are implemented inside

the coupler. Only external series resistors and bypass

capacitors are required for operation.

USB

HOST

ADuM3160

µCPU

PERIPHERAL

POWER

SUPPLY

D+

D–

D+

3.3V

D–

Figure 4. Typical ADuM3160 Application

Other than the delayed application of pull-up resistors, the

ADuM3160 is transparent to USB traffic, and no modifications

to the peripheral design are required to provide isolation. The

isolator adds propagation delay to the signals comparable to a

hub and cable. Isolated peripherals must be treated as if there

were a built-in hub when determining the maximum number of

hubs in a data chain.

Hubs can be isolated like any other peripheral. Isolated hubs

can be created by placing an ADuM3160 on the upstream port

of a hub chip. This configuration can be made compliant if

counted as two hub delays. The hub chip allows the ADuM3160

to operate at full speed yet maintains compatibility with low

speed devices.