Preliminary Technical Data

ADM2682E/ADM2687E

Rev. PrE | Page 17 of 20

APPLICATIONS INFORMATION

PCB LAYOUT

The ADM2682E/ADM2687E isolated RS-422/RS-485 transceiver

contains an isoPower integrated dc-to-dc converter, requiring

no external interface circuitry for the logic interfaces. Power

supply bypassing is required at the input and output supply pins

(see Figure 35). The power supply section of the ADM2682E/

ADM2687E uses an 180 MHz oscillator frequency to pass power

efficiently through its chip-scale transformers. In addition, the

normal operation of the data section of the iCoupler introduces

switching transients on the power supply pins.

Bypass capacitors are required for several operating frequencies.

Noise suppression requires a low inductance, high frequency

capacitor, whereas ripple suppression and proper regulation

require a large value capacitor. These capacitors are connected

ripple, a parallel combination of at least two capacitors is required

with the smaller of the two capacitors located closest to the device.

The recommended capacitor values are 0.1 µF and 10 µF for

use a very low inductance ceramic capacitor, or its equivalent,

for the smaller value capacitors. The total lead length between

both ends of the capacitor and the input power supply pin

should not exceed 10 mm.

Figure 35. Recommended PCB Layout

In applications involving high common-mode transients, ensure

that board coupling across the isolation barrier is minimized.

Furthermore, design the board layout such that any coupling

that does occur equally affects all pins on a given component

side. Failure to ensure this can cause voltage differentials between

pins exceeding the absolute maximum ratings for the device,

thereby leading to latch-up and/or permanent damage.

The ADM2682E/ADM2687E dissipate approximately 650 mW

of power when fully loaded. Because it is not possible to apply

a heat sink to an isolation device, the devices primarily depend

on heat dissipation into the PCB through the GND pins. If the

devices are used at high ambient temperatures, provide a thermal

path from the GND pins to the PCB ground plane. The board

layout in Figure 35 shows enlarged pads for Pin 1, Pin 8, Pin 9,

and Pin 16. Implement multiple vias from the pad to the ground

plane to reduce the temperature inside the chip significantly. The

dimensions of the expanded pads are at the discretion of the

designer and dependent on the available board space.

EMI CONSIDERATIONS

The dc-to-dc converter section of the ADM2682E/ADM2687E

components must, of necessity, operate at very high frequency

to allow efficient power transfer through the small transformers.

This creates high frequency currents that can propagate in circuit

board ground and power planes, causing edge and dipole radiation.

Grounded enclosures are recommended for applications that use

these devices. If grounded enclosures are not possible, good RF

design practices should be followed in the layout of the PCB.

See Application Note AN-0971, Control of Radiated Emissions

with isoPower Devices, for more information.

INSULATION LIFETIME

All insulation structures eventually break down when subjected to

voltage stress over a sufficiently long period. The rate of insulation

degradation is dependent on the characteristics of the voltage

waveform applied across the insulation. Analog Devices conducts

an extensive set of evaluations to determine the lifetime of the

insulation structure within the ADM2682E/ADM2687E.

Accelerated life testing is performed using voltage levels higher

than the rated continuous working voltage. Acceleration factors for

several operating conditions are determined, allowing calculation

of the time to failure at the working voltage of interest. The values

shown in Table 9 summarize the peak voltages for 50 years of

service life in several operating conditions. In many cases, the

working voltage approved by agency testing is higher than the

50-year service life voltage. Operation at working voltages higher

than the service life voltage listed leads to premature insulation

failure.

The insulation lifetime of the ADM2682E/ADM2687E depends

on the voltage waveform type imposed across the isolation barrier.

The iCoupler insulation structure degrades at different rates,

depending on whether the waveform is bipolar ac, unipolar ac,

or dc. Figure 36, Figure 37, and Figure 38 illustrate these different

isolation voltage waveforms.

Bipolar ac voltage is the most stringent environment. A 50-year

operating lifetime under the bipolar ac condition determines

the Analog Devices recommended maximum working voltage.

In the case of unipolar ac or dc voltage, the stress on the insulation

is significantly lower. This allows operation at higher working

voltages while still achieving a 50-year service life. The working

1

2

RxD

3

16

A

15

B

14

RE

4

13

DE

ADM2682E

ADM2687E

5

Z

12

TxD

6

11

7

Y

10

8

9

100nF

100nF

100nF

10µF

10µF

100nF

10nF

10nF