ADM2687E datasheet(18/24 Pages) AD

18 / 24 page

ADM2682E/ADM2687E

Rev. 0 | Page 18 of 24

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 41). 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

between Pin 1 (GND1) and Pin 2 (VCC) and Pin 7 (VCC) and

Pin 8 (GND1) for VCC. The VISOIN and VISOOUT capacitors are

connected between Pin 9 (GND2) and Pin 10 (VISOOUT) and

Pin 15 (VISOIN) and Pin 16 (GND2). To suppress noise and reduce

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

VISOOUT at Pin 9 and Pin 10 and VCC at Pin 7 and Pin 8. Capacitor

values of 0.01 μF and 0.1 μF are recommended for VISOIN at Pin 15

and Pin 16 and VCC at Pin 1 and Pin 2. The recommended best

practice is to 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.

GND1

VCC

RxD

GND2

VISOIN

TxD

VCC

VISOOUT

GND1

GND2

ADM2682E/

ADM2687E

10nF

10µF

100nF

Figure 41. 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 675 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 41 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 the AN-0971 Application Note, Recommendations for

Control of Radiated Emissions with isoPower Devices, for more

information.

ADM2687E Datasheet(PDF) 18 Page - Analog Devices

ADM2687E Datasheet(HTML) 18 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL