DC1746A-A Linear Technology, DC1746A-A Datasheet - Page 12

• Manufacturer Part Number: DC1746A-A
• Description: EVALUATION KIT LOW EMI LTM2881-3
• Manufacturer: Linear Technology
• Series: µModuler

Specifications of DC1746A-A

• Main Purpose: Interface, Transceiver RS-422/RS-485
• Embedded: No
• Utilized Ic / Part: LTM2881
• Primary Attributes: 3V Supply, Isolated, Transceiver + Power
• Kit Contents: Board, Datasheet
• Tool / Board Applications: Interface-Backplane, PCI, PCI Express, RS-XXX, Digital & Opto Isolator
• Mcu Supported Families: LTM2881
• Development Tool Type: Hardware - Eval/Demo Board
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Secondary Attributes: -

LTM2881
APPLICATIONS INFORMATION
Overview
The LTM2881 μModule transceiver provides a galvani-
cally-isolated robust RS485/RS422 interface, powered by
an integrated, regulated DC/DC converter, complete with
decoupling capacitors. A switchable termination resistor is
integrated at the receiver input to provide proper termina-
tion to the RS485 bus. The LTM2881 is ideal for use in
networks where grounds can take on different voltages.
Isolation in the LTM2881 blocks high voltage differences
and eliminates ground loops and is extremely tolerant of
common mode transients between ground potentials. Error
free operation is maintained through common mode events
greater than 30kV/μs providing excellent noise isolation.
μModule Technology
The LTM2881 utilizes isolator μModule technology to
translate signals and power 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
immunity, provides a robust solution for bidirectional
signal isolation. The μModule technology provides the
means to combine the isolated signaling with our RS485
transceiver and powerful isolated DC/DC converter in one
small package.
DC/DC Converter
The LTM2881 contains a fully integrated isolated DC/DC
converter, including the transformer, so that no external
components are necessary. The logic side contains a full-
bridge driver, running about 2MHz, and is AC-coupled
to a single transformer primary. A series DC blocking
capacitor prevents transformer saturation due to driver
duty cycle imbalance. The transformer scales the primary
voltage, and is rectified by a full-wave voltage doubler.
This topology eliminates transformer saturation caused
by secondary imbalances.
The DC/DC converter is connected to a low dropout reg-
ulator (LDO) to provide a regulated low noise 5V output.
The internal power solution is sufficient to support the
transceiver interface at its maximum specified load and data
12
rate, and external pins are supplied for extra decoupling
(optional) and heat dissipation. The logic supplies, V
V
isolated supply V
to GND2 within the μModule package.
V
The on-board DC/DC converter provides isolated 5V power
to output V
power at 5V in the LTM2881-5 option and up to 600mW
of power in the LTM2881-3 option. This surplus current is
available to external applications. The amount of surplus
current is dependent upon the implementation and current
delivered to the RS485 driver and line load. An example
of available surplus current is shown in the Typical Per-
formance Characteristics graph, V
Temperature. Figure 19 demonstrates a method of using
the V
that is controlled with the isolated RS485 data channel.
Driver
The driver provides full RS485 and RS422 compatibility.
When enabled, if DI is high, Y–Z is positive. When the
driver is disabled, both outputs are high impedance with
less than 10μA of leakage current over the entire common
mode range of –7V to 12V, with respect to GND2.
Driver Overvoltage and Overcurrent Protection
The driver outputs are protected from short circuits to
any voltage within the absolute maximum range of (V
–15V) to (GND2 +15V) levels. The maximum V
rent in this condition is 250mA. If the pin voltage exceeds
about ±10V, current limit folds back to about half of the
peak value to reduce overall power dissipation and avoid
damaging the part.
The device also features thermal shutdown protection
that disables the driver and receiver output in case of
excessive power dissipation (See Note 4 in the Electrical
Characteristics section).
SLO Mode
The LTM2881 features a logic-selectable reduced slew rate
mode (SLO mode) that softens the driver output edges to
L
CC2
have a 2.2μF decoupling capacitance to GND and the
CC2
Output
output directly and with a switched power path
CC2
. V
CC2
CC2
has a 2.2μF decoupling capacitance
is capable of suppling up to 1W of
CC2
Surplus Current vs
CC2
CC
2881fb
cur-
and
CC2