Circuit Designs Using Analog Devices Products
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CN-0031
Devices Connected/Referenced
ADM2485 High Speed Isolated RS-485 Transceiver
ADP3330
High Accuracy, Low Dropout Linear
Regulator
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 ADM2485 is a half-duplex RS-485 transceiver that can easily achieve a fully isolated RS-485 PROFIBUS compliant 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.
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.
T1 RECTIFIER
V
DD1
1N5817
22µF
IN
LDO
OUT
+5V ISO
ADP3330
10µF
SD GND ERR
5V/3.3V
POWER
SUPPLY
100nF
10µF
MLC
1N5817
100nF
GND
2
D1 D2
V
DD1 ADM2485 V
DD2
OSC
V
DD1
TxD
ENCODE DECODE D
A
B
R
T
RS-485 CABLE
RTS
ENCODE DECODE DE OUT
UART
RxD
DECODE ENCODE R
RE DIGITAL ISOLATION i Coupler RS-485 TRANSCEIVER
GND
1
GND
1 ISOLATION
BARRIER
GND
2
Figure 1. Half-Duplex Isolated RS-485 Interface Using ADM2485 and ADP3330 (Simplified Schematic)
Rev. A
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CN-0031
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.
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.
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.
Table 1. Transformer Suppliers
Manufacturer
Primary
Voltage 3.3V
Coilcraft DA2304-AL
C&D Technologies 782482/35C
Primary
Voltage 5V
DA2303-AL
782485/55C
Chen, Baoxing. 2006. iCoupler® Products with isoPower®
Technology: Signal and Power Transfer Across Isolation
Barrier Using Microtransformers . Analog Devices.
Wayne, Scott. 2005. “ i Coupler® Digital Isolators Protect RS-232,
RS-485, and CAN Buses in Industrial, Instrumentation, and
Computer Applications.” Analog Dialogue , Volume 39.
Analog Devices (October).
Data Sheets
ADM2485 Data Sheet.
ADP3330 Data Sheet.
6/09—Rev. 0 to Rev. A
Updated Format .................................................................. Universal
10/08—Revision 0: Initial Version
(Continued from first page) "Circuits from the Lab" are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the "Circuits from the Lab" in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the "Circuits from the Lab". Information furnished by Analog Devices is believed to be accurate and reliable. However, "Circuits from the Lab" are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any "Circuits from the Lab" at any time without notice, but is under no obligation to do so. Trademarks and registered trademarks are the property of their respective owners.
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