LTM2886 datasheet(10/18 Pages) AD

10 / 18 page

LTM2894

Rev A

For more information www.analog.com

OPERATION

TheLTM2894µModuletransceiverprovidesagalvanically

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

isidealforuseinUSBconnectionswheregroundsbetween

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

theUSB2.0Specification.Thecommunicationthroughthe

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

upstreamfacingportallowingtheupstream15kpull-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, VLO, or VBUS (5.3V

MAX)andfromD2+andD2–toGND2,VLO2,orVBUS2(5.3V

MAX). A current detection circuit disables the transceiver

pin if the pin sinks about 40mA for greater than 600ns.

The VLO and VLO2 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

andconnection/disconnectionconditions.Built-in15kpull-

down resistors are included from the D2+ and D2– signals

to GND2 supporting the downstream bus configuration.

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