ADM2481 Datasheet, PDF (16/20 Pages) Analog Devices – Half-Duplex, iCoupler Isolated RS-485 Transceiver

ADM2481
APPLICATIONS INFORMATION
PRINTED CIRCUIT BOARD (PCB) LAYOUT
The ADM2481 signal isolated RS-485 transceiver requires no
external interface circuitry for the logic interfaces. Power supply
bypassing is required at the input and output supply pins (see
Figure 29).
Bypass capacitors are most conveniently connected between
Pin 1 and Pin 2 for VDD1 and between Pin 15 and Pin 16 for
VDD2. The capacitor value must be between 0.01 μF and 0.1 μF.
The total lead length between both ends of the capacitor and
the input power supply pin must not exceed 20 mm.
V_DD1
GND₁
RxD
RE
DE
TxD
GND₁
GND₁
ADM2481
NC = NO CONNECT
V_DD2
GND₂
NC
B
A
NC
GND₂
GND₂
Figure 29. Recommended Printed Circuit Board Layout
In applications involving high common-mode transients, take
care to ensure that board coupling across the isolation barrier
is minimized. Furthermore, the board layout must be designed
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 that exceed the absolute maximum ratings of the device,
thereby leading to latch-up or permanent damage.
ISOLATED POWER SUPPLY CIRCUIT
The ADM2481 requires isolated power capable of 5 V at
100 mA to be supplied between the VDD2 and GND2 pins. If
no suitable integrated power supply is available, a discrete
circuit, such as the one in Figure 30, can be used. A center-
tapped transformer provides electrical isolation. The primary
winding is excited with a pair of square waveforms that are 180°
out of phase with each other. A pair of Schottky diodes and a
smoothing capacitor are used to create a rectified signal from
the secondary winding. The ADP3330 linear voltage regulator
provides a regulated power supply to the bus-side circuitry of
the ADM2481.
ISOLATION
V_CC
BARRIER
SD103C
TRANSFORMER
DRIVER
V_CC
+
22µF
78253
SD103C
V_CC
5V
IN
OUT
SD _ADP3330
+
10µF
ERR
NR _GND
V_DD1
V_DD2
ADM2481
GND₁
GND₂
Figure 30. Isolated Power Supply Circuit
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