Datasheet

ADM2485
Rev. A | Page 16 of 20
APPLICATIONS INFORMATION
PCB LAYOUT
The ADM2485 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 33).
Bypass capacitors are most conveniently connected between
Pin 3 and Pin 4 for V
DD1
and between Pin 15 and Pin 16 for
V
DD2
. 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.
Bypassing between Pin 9 and Pin 16 is also recommended
unless the ground wires on the V
DD2
side are connected close to
the package.
D1
D2
GND
1
V
DD1
RxD
RE
RTS
TxD
V
DD2
GND
2
GND
2
B
A
GND
2
DE OUT
GND
2
ADM2485
0
6021-029
Figure 33. Recommended Printed Circuit Board Layout
In applications involving high common-mode transients, care
must be taken 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 exceeding the device absolute maximum ratings, thereby
leading to latch-up or permanent damage.
TRANSFORMER SUPPLIERS
The transformer primarily used with the ADM2485 must be a
center-tapped transformer winding. The turns ratio of the
transformer must be set to provide the minimum required
output voltage at the maximum anticipated load with the
minimum input voltage.
Table 1 2 shows ADM2485 transformer
suppliers.
Table 12. Transformer Suppliers
Manufacturer Primary Voltage 3.3 V Primary Voltage 5 V
Coilcraft DA2304-AL DA2303-AL
C&D Technologies 782485/35C 782485/55C
APPLICATIONS DIAGRAM
The ADM2485 integrates a transformer driver that, when used
with an external transformer and LDO, generates an isolated
5 V power supply, to be supplied between V
DD2
and GND
2
.
D1 and D2 of the ADM2485 drive the center-tapped
Transformer T1. A pair of Schottky diodes and a smoothing
capacitor is used to create a rectified signal from the secondary
winding. The ADP3330 linear voltage regulator provides a
regulated 5 V power supply to the ADM2485 bus-side circuitry
(V
DD2
), as shown in Figure 34.
When the ADM2485 is powered by 3.3 V on the logic side, a
1CT:2.2CT Transformer T1 is required to step up the 3.3 V to
6 V, ensuring enough headroom for the ADP3330 LDO to
output a regulated 5 V output.
If ADM2485 is powered by 5 V on the logic side, a 1CT:1.5CT
Transformer T1 is required, ensuring enough headroom for the
ADP3330 LDO to output a regulated 5 V output.
06021-030
V
CC
V
CC
ISO 5V
ADP3330
IN
10µF
100nF
OUT
5V
SD
1N5817
1N5817
V
DD1
D1 D2 V
DD2
GND
2
ADM2485
GND
ERR NR
22µF
10µF
MLC
T1
ISOLATION BARRIER
100nF
GND
1
Figure 34. Applications Diagram