LTM2883 datasheet(11/18 Pages) AD

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LTM2894

Rev A

For more information www.analog.com

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

loadbetweentheK-stateandJ-stateistransferredthrough

the LTM2894 isolator with a delay of approximately 80ns.

Idle State Communication

The LTM2894 μModule transceiver maintains the condi-

tions of the USB bus idle state by monitoring the down-

stream side bus idle condition and refreshing the state

across the isolation barrier at a consistent rate. Further-

more,theLTM2894monitorsthespeedofthedownstream

peripheral once connected and sets its own operation to

match. Figure 4 shows the abbreviated circuitry of the

automatic monitoring and reporting of the bus speeds.

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 pull-

up resistor will maintain the condition prior to detecting

the suspend command.

APPLICATIONS INFORMATION

Suspend Mode

When the upstream USB bus is idle for greater than 3ms

theLTM2894willentersuspendmode.Oncethepartenters

suspend the VBUS supply current reduces below 2mA.

TheLTM2894wakesupfromsuspendduetodisconnects,

reconnects, and downstream initiated resume signaling

when the LTM2894 is in a hub, host, or peripheral device.

The upstream transceiver maintains observation and re-

portsdownstreamtransceiverdisconnectsandreconnects.

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

externalcircuitswillbesuppliedfromVBUSandmayexceed

the limits set in the USB specification.

VBUS2 current does not reduce in suspend mode.

VLO and VLO2 Supplies

The VLO and VLO2 output supply pins are available for use

as a low current 3.3V supply on both sides of the isolation

barrier. They also serve as supplies for the USB interface

circuitry.Aninternallinearregulatormaintains3.3voltson

VLO from the VBUS input supply. A separate linear regula-

tor maintains 3.3V on VLO2 from VBUS2. The current is

limited to 10mA for external applications. Exceeding this

limit may cause degradation in the VLO or VLO2 supplies

and undesirable operation from the USB isolator. Connec-

tion of signals ON to VLO or ON2 to VLO2 will not cause a

significant change in the available current. These supplies

are available to support interface logic to the isolated USB

port. In order to meet the suspend mode current limit,

minimize the DC current of external applications on the

VLO output supply. VLO and VLO2 are protected from over

current and over temperature conditions.

Supply Current

LoadingonthemultipleoutputsupplypinsoftheLTM2894

affect the supply current consumption on VBUS. The

VBUS input supplies current to the upstream side of the

transceiver and to the VLO pin. The VBUS2 input supplies

power to the downstream side of the transceiver and to

the VLO2 pin.

Figure 4. Idle State Automatic Resistor Setting

2894 F04

LTM2894

REFRESH

VLO

D1+

RPU

1.5k

RPU

1.5k

SW1

SW2

D1–

D2+

D2–

RPD

15k

RPD

15k

1.5k

FULL

SPEED

UPSTREAM CONNECTION

DOWNSTREAM CONNECTION

OR DISCONNECTED

LOW

SPEED

1.5k

3.3V

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