ADM2485
Preliminary Technical Data
APPLICATIONS INFORMATION
PC BOARD LAYOUT
output a regulated 5V output.
The ADM2485 isolated RS-485 transceiver requires no external
interface circuitry for the logic interfaces. Power supply
bypassing is strongly recommended at the input and output
supply pins (see Figure ).
If ADM2485 is powered by 5V on the logic side a 1CT:1.5CT
transformer T1 is required so that therefore is enough
headroom for the ADP667 LDO to output a regulated 5V
output.
Bypass capacitors are most conveniently connected between Pin
3 and Pin 4 for VDD1 and between Pin 15 and Pin 16 for VDD2
.
ISOLATION BARRIER
1N5817
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.
+5V
IN
OUT
ADP667
VCC
22µF
10µF
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.
SET GND SHDN
T1
1N5817
D1
D2
V
DD2
GND
GND
B
VCC
2
2
ISO 5V
VDD2
GND
1
ADM2485
100nF
V
100nF
DD1
D1D2
RxD
RE
A
VDD1
GND
2
RTS
TxD
DE OUT
GND
ADM2485
GND1
2
Figure 19. Recommended Printed Circuit Board Layout
GND2
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.
Figure 20. Applications Diagram
Failure to ensure this could cause voltage differentials between
pins exceeding the device’s absolute maximum ratings, thereby
leading to latch-up or permanent damage.
APPLICATIONS DIAGRAM
The ADM2485 integrates a transformer driver which when
used with an external transformer and LDO generates an
isolated 5V power supply, to be supplied between the VDD2 and
the GND2 pins.
Pins D1 and D2 of the ADM2485 drive a center-tapped
transformer T1, A pair of Schottky diodes and a smoothing
capacitor are used to create a rectified signal from the
secondary winding. The ADP667 linear voltage regulator
provides a regulated 5V power supply to the ADM2485’s bus-
side circuitry (VDD2), as shown in Figure 20.
When the ADM2485 is powered by 3V on the logic side a
1CT:2.2CT transformer T1 is required to step up the 3V to 6V,
so that therefore is enough headroom for the ADP667 LDO to
Rev. PrK | Page 14 of 15
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