Datasheet

ADM2490E
Rev. A | Page 14 of 16
APPLICATIONS INFORMATION
ISOLATED POWER SUPPLY CIRCUIT
The ADM2490E requires isolated power capable of 5 V at up
to approximately 65 mA (this current is dependent on the data
rate and termination resistors used) to be supplied between the
V
DD2
and the GND
2
pins. A transformer driver circuit with a
center-tapped transformer and LDO can be used to generate the
isolated 5 V supply, as shown in Figure 25. The center-tapped
transformer provides electrical isolation of the 5 V power supply.
The primary winding of the transformer 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 (V
DD2
) of the ADM2490E.
PCB LAYOUT
The ADM2490E isolated RS-485 transceiver requires no external
interface circuitry for the logic interfaces. Power supply bypass-
ing is required at the input and output supply pins (see Figure 24).
Bypass capacitors are conveniently connected between Pin 1
and Pin 2 for V
DD1
and between Pin 15 and Pin 16 for V
DD2
. The
capacitor value should 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 should not exceed 20 mm. Bypassing between
Pin 1 and Pin 8 and between Pin 9 and Pin 16 should also be
considered unless the ground pair on each package side is
connected close to the package.
V
DD1
GND
1
RxD
NC
GND
1
TxD
NC
GND
1
V
DD2
GND
2
A
B
NC
Z
Y
GND
2
NC = NO CONNECT
ADM2490E
0
5889-013
Figure 24. Recommended Printed Circuit Board Layout
In applications involving high common-mode transients, care
should be taken to ensure that board coupling across the isola-
tion barrier is minimized. Furthermore, the board layout should
be designed such that any coupling that does occur equally affects
all pins on a given component side. Failure to ensure this could
cause voltage differentials between pins exceeding the absolute
maximum ratings of the device, thereby leading to latch-up or
permanent damage.
ISO
L
A
TION
BARRIER
V
CC
78253
SD103C
SD103C
22µF
10µF
5V
OUT
IN
SD GND ERR
V
CC
V
DD1
V
DD2
GND
1
GND
2
TRANSFORMER
DRIVER
V
CC
ADP3330
ADM2490E
05889-012
Figure 25. Isolated Power-Supply Circuit