IL260 IL261
6
Application Notes
Dynamic Power Consumption
Isoloop® devices achieve their low power consumption from
the manner by which they transmit data across the isolation
barrier. By detecting the edge transitions of the input logic
signal and converting these to narrow current pulses, a
magnetic field is created around the GMR Wheatstone
bridge. Depending on the direction of the magnetic field, the
bridge causes the output comparator to switch following the
input logic signal. Since the current pulses are narrow, about
2.5ns wide, the power consumption is independent of mark-
to-space ratio and solely dependent on frequency. This has
obvious advantages over optocouplers whose power
consumption is heavily dependent on its on-state and
frequency.
The approximate power supply current per channel for
Power Supply Decoupling
Both power supplies to these devices must be decoupled
with low ESR 100 nF ceramic capacitors. For data rates in
excess of 10MBd, use of ground planes for both GND1 and
GND2 is highly recommended. Capacitors should be
located as close as possible to the device.
Signal Status on Start-up and Shut Down
To minimize power dissipation, the input signals are
differentiated and then latched on the output side of the
isolation barrier to reconstruct the signal. This could result
in an ambiguous output state depending on power up,
shutdown and power loss sequencing. Therefore, the
designer should consider the inclusion of an initialization
signal in his start-up circuit. Initialization consists of
toggling each channel either high then low or low then high,
depending on the desired state.
Data Transmission Rates
The reliability of a transmission system is directly related to
the accuracy and quality of the transmitted digital
information. For a digital system, those parameters which
determine the limits of the data transmission are pulse width
distortion and propagation delay skew.
Propagation delay is the time taken for the signal to travel
through the device. This is usually different when sending a
low-to-high than when sending a high-to-low signal. This
difference, or error, is called pulse width distortion (PWD)
and is usually in ns. It may also be expressed as a
percentage:
PWD% = Maximum Pulse Width Distortion (ns) x 100%
Signal Pulse Width (ns)
For example: For data rates of 12.5 Mb
PWD% = 3 ns x 100% = 3.75%
80 ns
This figure is almost three times better than for any available
optocoupler with the same temperature range, and two times
better than any optocoupler regardless of published
temperature range. The IsoLoop® range of isolators will run
at almost 35 Mb before reaching the 10% limit.
Propagation delay skew is the difference in time taken for
two or more channels to propagate their signals. This
becomes significant when clocking is involved since it is
undesirable for the clock pulse to arrive before the data has
settled. A short propagation delay skew is therefore critical,
especially in high data rate parallel systems, to establish and
maintain accuracy and repeatability. The IsoLoop® range of
isolators all have a maximum propagation delay skew of 6
ns, which is five times better than any optocoupler. The
maximum channel-to-channel skew in the IsoLoop® coupler
is only 3 ns which is ten times better than any optocoupler.