DESCRIPTION

The LTM2894 is a complete galvanically-isolated USB 2.0 compatible μModule® (micromodule) transceiver. The LTM2894 is ideal for isolation in host, hub, bus splitter or peripheral device applications. It is compatible with USB 2.0 full speed (12Mbps) and low speed (1.5Mbps) operation. Automatic speed selection configures integrated pull-up resistors on the upstream port to match those sensed on the downstream device. The isolator µModule technology uses coupled inductors to provide 7500VRMS of isolation and 17.4mm of creepage between the upstream and downstream USB interface. This device is ideal for systems requiring isolated ground returns or large common mode voltage variations. Uninterrupted communication is guaranteed for common mode transients greater than 50kV/μs. Enhanced ESD protection allows this part to withstand up to ±20kV (human body model) on the USB transceiver interface pins to local supplies and ±20kV through the isolation barrier to supplies without latchup or damage.

 

FEATURES

Isolated USB Transceiver: 7500VRMS for 1 Minute

CSA (IEC/UL) Approved, File #255632

Reinforced Insulation

USB 2.0 Full Speed and Low Speed Compatible

Auto-Configuration of Bus Speed

4.4V to 36V VBUS and VBUS2 Operating Range

3.3V LDO Output Supply Signal References VLO and VLO2

50kV/μs Common Mode Transient Immunity

±20kV HBM ESD on USB Interface Pins

1414VPEAK Maximum Continuous Working Voltage

17.4mm Creepage Distance

22mm × 6.25mm Surface Mount BGA

 

APPLICATIONS

Isolated USB Interfaces

Host, Hub, or Device Isolation

Industrial/Medical Data Acquisition

 

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 transceiver passes through all data and does not act as a hub or intelligent device. The bus lines are monitored for idleconditions, 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.Monitoring the USB data pins, the LTM2894 detects a K-state to begin a data packet and set the data direction. The data is monitored for an end of packet signature and a finishing J-state before the bus is released. The data pay load between the K-state and J-state is transferred through the LTM2894 isolator with a delay of approximately 80ns.

Idle State Communication

The LTM2894 μModule transceiver maintains the conditions of the USB bus idle state by monitoring the downstream side bus idle condition and refreshing the state across the isolation barrier at a consistent rate. Furthermore, the LTM2894 monitors the speed of the downstream peripheral once connected and sets its own operation to match. The D2+ or D2–signals are monitored for a connection to pull-ups on D2+ or D2– and the result is processed as full speed or low speed, otherwise disconnect. The idle state is communicated to the upstream side through a refresh transmission. The switches SW1 or SW2 are controlled based on the received information. SW1 is closed if D2+ is detected to have a pull-up and D2– was open. SW2 is closed if D2– is detected to have a pull-up and D2+ was open. Both SW1 and SW2 are opened if the downstream USB bus is disconnected. During a USB suspend, the pullup resistor will maintain the condition prior to detecting the suspend command.

Suspend Mode

When the upstream USB bus is idle for greater than 3ms the LTM2894 will enter suspend mode. Once the part enters suspend the VBUS supply current reduces below 2mA. The LTM2894 wakes up from suspend due to disconnects, reconnects, and downstream initiated resume signaling when the LTM2894 is in a hub, host, or peripheral device. The upstream transceiver maintains observation and reports downstream transceiver disconnects and reconnects. Recovery from suspend mode occurs when a data state change is detected and may take up to 10µs. During suspend mode, DC current drawn from VLO into external circuits will be supplied from VBUS and may exceed the limits set in the USB specification. VBUS2 current does not reduce in suspend mode.