LTM2881 查看數據表（PDF） - Linear Technology

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LTM2881

Complete Isolated RS485/RS422 µModule Transceiver + Power

Linear Technology 

LTM2881

Applications Information

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 necessary and standard RS485 termination can

be used with control from TE.

VCC2, 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 VCC2 in order to

maintain the specified driver output swing.

VCC

VL

RO

3.3V (LTM2881-3)

5V (LTM2881-5)

PWR

DE

DI

TE

VCC2 390Ω

A

B

PROFIBUS CABLE

TYPE A

220Ω

Y

SHIELD

Z

• 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.

GND

GND2

LTM2881

390Ω

2881 F15

Figure 15. PROFIBUS-DP Connections with Termination

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 VCC 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 VCC2 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.

• 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.

2881fh

16

For more information www.linear.com/LTM2881

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