Datasheet

Data Sheet ADuM3160
Rev. C | Page 9 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 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 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 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 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 input can be tied to a 3.3 V control logic signal or to the
V
DD2
rail depending on whether enumeration must be controlled
or will occur when power is first applied.
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
MICRO-
CONTROLLER
PERIPHERAL
POWER
SUPPLY
V
BUS
GND
BUS
D+
D–
D+
3.3V
D–
09125-004
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 maintain compatibility with low speed devices.
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