LTM2894
9
2894f
For more information www.linear.com/LTM2894
operaTion
The LTM2894 µModule transceiver provides a galvanically
isolated robust USB interface, complete with decoupling
capacitors. An automatically configured pull up resistor is
included to represent the condition of the isolated down
-
stream USB bus to the upstream USB bus. The LTM2894
is
ideal for use in USB connections where grounds between
upstream hub/host and downstream devices can take on
different voltages. Isolation in the LTM2894 blocks high
voltage differences and eliminates ground loops and is
extremely tolerant of common mode transients between
ground potentials. Error free operation is maintained
though common mode events exceeding 50kV/µs provid
-
ing excellent noise isolation.
The
integrated USB transceiver on both sides of the isola-
tion barrier
supports full and low speed modes defined in
the
USB 2.0 Specification. The communication through the
isolation barrier for USB is bidirectional and as such the
LTM2894 determines data flow direction based on which
side a start of packet (SOP) begins first. The direction of
data is maintained until an end of packet (EOP) pattern is
observed or a timeout occurs due to a lack of activity. The
USB interface maintains a small consistent propagation
delay representative of a single connection or hub delay
and transfers all bus state and data information.
Pull-up
resistors integrated in the upstream interface
automatically indicate device connections and discon
-
nections. A downstream device connection automatically
selects
the proper pull-up resistor at the upstream facing
port after sensing the idle state of the downstream de
-
vice at connection time. Disconnection of a downstream
device automatically releases the pull-up resistor on the
upstream facing port allowing the upstream 15k pull-down
resistors to pull the bus signals to a disconnect condition.
This function makes the LTM2894 ideal for host, hub, bus
splitter, or peripheral device integration.
µModule Technology
The LTM2894 utilizes isolator µModule technology to
translate signals across an isolation barrier. Signals on
either side of the barrier are encoded into pulses and
translated across the isolation boundary using coreless
transformers formed in the µModule substrate. This
system, complete with data refresh, error checking, safe
shutdown on fail, and extremely high common mode im
-
munity, provides
a robust solution for bidirectional signal
isolation. The µModule technology provides the means to
combine the isolated signaling with our USB transceiver
in one small package.
USB Transceiver Pin Protection
The LTM2894 USB transceiver pins D1
+
, D1
–
, D2
+
, and
D2
–
have protection from ESD and short-circuit faults.
The transceiver pins withstand ±20KV HBM ESD events.
Overcurrent circuitry on the transceiver pins monitor fault
conditions from D1
+
and D1
–
to GND, V
LO
, or V
BUS
(5.3V
MAX) and from D2
+
and D2
–
to GND2, V
LO2
, or V
BUS2
(5.3V
MAX). A current detection circuit disables the transceiver
pin if the pin sinks about 40mA for greater than 600ns.
The V
LO
and V
LO2
output supplies protect the USB trans-
ceiver pins from shorts to GND or GND2 respectively with
a 40mA current limit.
applicaTions inForMaTion
USB Connectivity
The LTM2894 µModule transceiver connects directly to
USB ports on the upstream side and the downstream side
without the addition of external components. The trans
-
ceiver passes
through all data and does not act as a hub
or intelligent device. The bus lines are monitored for idle
conditions, start of packet, and end of packet conditions
to properly maintain bus speed and data direction. The
series resistance, pull-up, and pull-down resistors are
built into the LTM2894. The upstream facing USB port
contains automatically configured 1.5k pull-up resistors
which are switched in or out based on the downstream
side peripheral device configuration. This implementation
allows upstream reporting of the downstream bus speed
and connection/disconnection conditions. Built-in 15k pull-
down resistors are included from the D2
+
and D2
–
signals
to GND2 supporting the downstream bus configuration.
