LTM2882
11
2882fd
Figure 5. V
CC
and V
L
Are Independent
2882 F05
ON
DIN
T1IN
R1OUT
T2IN
R2OUT
DE
DOUT
T1OUT
R1IN
T2OUT
R2IN
LTM2882
ANY VOLTAGE FROM
1.62V TO 5.5V
3.0V TO 3.6V LTM2882-3
4.5V TO 5.5V LTM2882-5
EXTERNAL
DEVICE
V
L
V
CC
V
CC2
GND
ISOLATION BARRIER
GND2
applicaTions inForMaTion
Overview
The LTM2882 µModule transceiver provides a galvanically-
isolated robust RS232 interface, powered by an integrated,
regulated DC/DC converter, complete with decoupling
capacitors. The LTM2882 is ideal for use in networks
where grounds can take on different voltages. Isolation in
the LTM2882 blocks high voltage differences, eliminates
ground loops and is extremely tolerant of common mode
transients between grounds. Error-free operation is main-
tained through common mode events greater than 30kV/
μs providing excellent noise isolation.
µModule Technology
The LTM2882 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 core-
less 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 advanced dual
RS232 transceiver and powerful isolated DC/DC converter
in one small package.
DC/DC Converter
The LTM2882 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 at 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 regulator
(LDO) to provide a regulated low noise 5V output, V
CC2
.
An integrated boost converter generates a 7V V
DD
supply
and a charge pumped –6.3V V
EE
supply. V
DD
and V
EE
power
the output stage of the RS232 drivers and are regulated
to levels that guarantee greater than ±5V output swing.
The internal power solution is sufficient to support the
transceiver interface at its maximum specified load and
data rate, and has the capacity to provide additional 5V
power on the isolated side V
CC2
and GND2 pins. V
CC
and
V
CC2
are each bypassed internally with 2.2µF ceramic
capacitors.
V
L
Logic Supply
A separate logic supply pin V
L
allows the LTM2882 to in-
terface with any logic signal from 1.62V to 5.5V as shown
in
Figure 5. Simply connect the desired logic supply to V
L
.
There is no interdependency between V
CC
and V
L
; they
may simultaneously operate at any voltage within their
specified operating ranges and sequence in any order. V
L
is bypassed internally by a 2.2µF capacitor.
Hot Plugging Safely
Caution must be exercised in applications where power is
plugged into the LTM2882’s power supplies, V
CC
or V
L
,
due to the integrated ceramic decoupling capacitors. The
parasitic cable inductance along with the high Q char-
acteristics of ceramic capacitors can cause substantial
ringing which could exceed the maximum voltage ratings
and damage the LTM2882. Refer to Linear Technology Ap-
plication Note 88, entitled “Ceramic Input Capacitors Can
Cause Overvoltage Transients” for a detailed discussion
and mitigation of this phenomenon.
