LTM2884 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM2884' PDF : 24 Pages View PDF
LTM2884
Applications Information
USB Connectivity
The LTM2884 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 LTM2884. 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 LTM2884 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 LTM2884 isolator with a delay of approximately 80ns.
Idle State Communication and Automatic Speed
Selection
The LTM2884 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, the LTM2884 monitors the speed of the downstream
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.
UPSTREAM CONNECTION
VLO
DOWNSTREAM CONNECTION
LTM2884
3.3V 3.3V
SW1 SW2
RPU RPU
1.5k 1.5k
D1+
D1–
15k 15k
REFRESH
FULL LOW
SPEED SPEED
1.5k 1.5k
D2+
D2– OR
RPD
15k
RPD
15k
OR DISCONNECTED
2884 F04
Figure 4. Idle State Automatic Resistor Setting
2884f
14
For more information www.linear.com/LTM2884
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