Datasheet

LTM2881
16
2881fg
For more information www.linear.com/LTM2881
PROFIBUS Applications
The LTM2881 can be used in PROFIBUS-DP networks
where isolation is required. The standard PROFIBUS
termination differs from RS485 termination and is shown
in Figure15. If used in this way, the internal termination
should remain disabled (TE low). The 390Ω resistors in
Figure15 pre-bias the bus so that when the line is not
driven, the receiver delivers a high output. Since the
LTM2881 uses a fail-safe receiver, the pre-biasing resistors
are not necessar
y and standard RS485 termination can
be used with control from TE.
V
CC2
, provides an isolated source for the external termina-
tion resistor as shown in the Figure 15. When using the
LTM2881 in PROFIBUS applications, it is recommended
that
no additional loads are connected to V
CC2
in order to
maintain the specified driver output swing.
• Input and Output decoupling is not required, since these
components are integrated within the package. An ad
-
ditional bulk capacitor with a value of 6.8µF to 22µF is
recommended. The high ESR of this capacitor reduces
board resonances and minimizes voltage spikes caused
by hot plugging of the supply voltage. For EMI sensitive
applications, an additional low ESL ceramic capacitor of
1µF to 4.7µF, placed as close to the power and ground
terminals as possible, is recommended. Alternatively, a
number of smaller value parallel capacitors may be used
to reduce ESL and achieve the same net capacitance.
• Do not place copper on the PCB between the inner col
-
umns of pads. This area must remain open to withstand
the rated isolation voltage.
•
The use of solid ground planes for GND and GND2
is recommended for non-EMI critical applications to
optimize signal fidelity, thermal performance, and to
minimize RF emissions due to uncoupled PCB trace
conduction. The drawback of using ground planes,
where EMI is of concern, is the creation of a dipole
antenna structure which can radiate differential voltages
formed between GND and GND2. If ground planes are
used it is recommended to minimize their area, and
use contiguous planes as any openings or splits can
exacerbate RF emissions.
• For large ground planes a small capacitance (≤ 330pF)
from GND to GND2, either discrete or embedded within
the substrate, provides a low impedance current return
path for the module parasitic capacitance, minimizing
any high frequency differential voltages and substantially
reducing radiated emissions. Discrete capacitance will
not be as effective due to parasitic ESL. In addition, volt
-
age rating, leakage, and clearance must be considered
for component selection. Embedding the capacitance
within
the PCB
substrate provides a near ideal capacitor
and eliminates component selection issues; however,
the PCB must be 4 layers. Care must be exercised in
applying either technique to insure the voltage rating
of the barrier is not compromised.
APPLICATIONS INFORMATION
PCB Layout Considerations
The high integration of the LTM2881 makes PCB layout
very simple. However, to optimize its electrical isolation
characteristics, EMI, and thermal performance, some
layout considerations are necessary.
• Under heavily loaded conditions V
CC
and GND current
can exceed 300mA. Sufficient copper must be used
on the PCB to insure resistive losses do not cause the
supply voltage to drop below the minimum allowed
level. Similarly, the V
CC2
and GND2 conductors must
be sized to support any external load current. These
heavy copper traces will also help to reduce thermal
stress and improve the thermal conductivity.
Figure 15. PROFIBUS-DP Connections with Termination
A
390Ω
3.3V (LTM2881-3)
5V (LTM2881-5)
220Ω
390Ω
SHIELD
PROFIBUS CABLE
TYPE A
V
L
DE
RO
DI
GND
V
CC
V
CC2
2881 F15
B
Y
Z
TE
PWR
GND2
LTM2881
ISOLATION BARRIER