CN0031: Half-Duplex, Isolated RS-485 Interface
CIRCUIT OVERVIEW
Benefits & Features
- Fully isolated signal and power traces
- RS-485 half duplex tranciever circuit
- Low power with reduced board space
- Communications
- Field Instruments/ Smart Transmitters
- Programmable Logic Controllers/ Distributed Control Systems
Applications:
Design Resources
Connectivity Options
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.
Half-Duplex, Isolated RS-485 Interface (CN0031)
Figure 1: Half-Duplex Isolated RS-485 Interface Using ADM2485 and ADP3330 (Simplified Schematic)
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Figure 1: Half-Duplex Isolated RS-485 Interface Using ADM2485 and ADP3330 (Simplified Schematic)
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CIRCUIT DESCRIPTION
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 bus-side circuitry (VDD2) as shown in Figure 1.
COMMON VARIATIONS
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.
| Manufacturer | Primary Voltage 3.3V | Primary Voltage 5V |
|---|---|---|
| Coilcraft | DA2304-AL | DA2303-AL |
| C&D Technologies | 782482/35C | 782485/55C |
Table 1: Transformer Suppliers
