CN-0031: Half-Duplex, Isolated RS-485 Interface PDF

Circuit Note
CN-0031
Devices Connected/Referenced
Circuit Designs Using Analog Devices Products
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ADM2485
High Speed Isolated RS-485 Transceiver
ADP3330
High Accuracy, Low Dropout Linear
Regulator
Half-Duplex, Isolated RS-485 Interface
node. 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 VDD2
and GND2.
CIRCUIT FUNCTION AND BENEFITS
This circuit provides a half-duplex, isolated RS-485 interface
using the ADM2485 high speed, isolated RS-485 transceiver
and the ADP3330 high accuracy linear regulator. This circuit
achieves signal and power isolation and at the same time
decreases board space and power consumption.
The oscillator outputs D1 and D2 of the ADM2485 drive the
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 ADP3330 linear voltage regulator
provides a regulated 5 V power supply to the ADM2485 busside circuitry (VDD2) as shown in Figure 1.
CIRCUIT DESCRIPTION
The ADM2485 is a half-duplex RS-485 transceiver that can
easily achieve a fully isolated RS-485 PROFIBUS compliant
T1
RECTIFIER
LDO
1N5817
IN
VDD1
+5V ISO
OUT
10µF
ADP3330
SD GND ERR
5V/3.3V
POWER
SUPPLY
22µF
10µF
MLC
1N5817
100nF
100nF
D1
GND2
D2
VDD1
VDD2
ADM2485
OSC
VDD1
A
TxD
ENCODE
DECODE
RS-485 CABLE
RT
D
B
RTS
ENCODE
DECODE
DECODE
ENCODE
DE OUT
UART
RE
GND1
DIGITAL ISOLATION iCoupler
GND1
ISOLATION
BARRIER
R
RS-485 TRANSCEIVER
GND2
08256-001
RxD
Figure 1. Half-Duplex Isolated RS-485 Interface Using ADM2485 and ADP3330 (Simplified Schematic)
Rev. A
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CN-0031
Circuit Note
COMMON VARIATIONS
LEARN MORE
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,
thereby ensuring enough headroom for the ADP3330 LDO to
output a regulated 5 V output.
Chen, Baoxing. 2006. iCoupler® Products with isoPower®
Technology: Signal and Power Transfer Across Isolation
Barrier Using Microtransformers. Analog Devices.
If the ADM2485 is powered by 5 V on the logic side, a
1CT:1.5CT transformer T1 is required to ensure enough
headroom for the ADP3330 LDO to output a regulated 5 V
output.
Wayne, Scott. 2005. “iCoupler® Digital Isolators Protect RS-232,
RS-485, and CAN Buses in Industrial, Instrumentation, and
Computer Applications.” Analog Dialogue, Volume 39.
Analog Devices (October).
The external transformer used with the ADM2485 must have a
center-tapped winding. The turns ratio of the transformer must
be set to provide the minimum required output voltage at the
maximum anticipated load current with the minimum input
voltage. Table 1 shows suitable transformers for the ADM2485
circuit of Figure 1.
Data Sheets
Table 1. Transformer Suppliers
6/09—Rev. 0 to Rev. A
Manufacturer
Primary
Voltage 3.3V
Primary
Voltage 5V
Coilcraft
DA2304-AL
DA2303-AL
C&D Technologies
782482/35C
782485/55C
ADM2485 Data Sheet.
ADP3330 Data Sheet.
REVISION HISTORY
Updated Format .................................................................. Universal
10/08—Revision 0: Initial Version
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CN08256-0-6/09(A)
Rev. A | Page 2 of 2
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