ADuM4160

Rev. C | 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 a 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 (J) state. When data

direction 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 making the iCoupler latch. Logic is included to

eliminate any artifacts due to different input thresholds of the

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

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 ADuM4160 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,

thus, the speed related parameters, J/K logic levels, 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 ADuM4160 activates

a pull-up resistor integrated into the upstream side. 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

connect is to be performed.

PRODUCT USAGE

The ADuM4160 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:

1. The USB host provides power for the upstream side of the

ADuM4160 through the cable.

2. The peripheral supply provides power to the downstream

side of the ADuM4160

3. 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.

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

time. The ADuM4160 has configuration pins, SPU and

SPD, that determine the buffer speed and logic convention

for each side. These must be set identically and match the

desired peripheral speed.

5. USB enumeration begins when either the UD+ or UD−

line is pulled high at the peripheral end of the USB cable,

which is the upstream side of the ADuM4160. Control of

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

downstream side of the coupler.

6. Pull-up and pull-down resistors are implemented inside

the coupler. Only external series resistors and bypass

capacitors are required for operation.

USB

HOST

ADuM4160

DD+

DD–

DD+

3.3V

DD–

PIN

MICRO-

CONTROLLER

POWER

SUPPLY

PERIPHERAL

Figure 4. Typical Application

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

ADuM4160 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 ADuM4160 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 ADuM4160

to operate at full speed yet maintains compatibility with low

speed devices.

COMPATIBILITY OF UPSTREAM APPLICATIONS

The ADuM4160 is designed specifically for isolating a USB

peripheral. However, the chip does have two USB interfaces that

meet the electrical requirements for driving USB cables. This

opens the possibility of implementing isolation in downstream

USB ports such as isolated cables, which have generic connections

to both upstream and downstream devices, as well as isolating

host ports.