feature

Isolated, RS-485/RS-422 transceiver, configurable for half or full duplex; ±8 kV ESD protection on RS-485 input/output pins; 16 Mbps data rate; ANSI TIA/EIA compliant RS-485-A-1998 and; ISO 8482:1987(e); suitable for 5V or 3.3V operation (VDD1); high common mode transient immunity: >25kv/μs; receiver with open circuit, fault protected design ; 32 nodes on bus; thermal shutdown protection; safety and regulatory approvals; UL recognition: 5000 V rms isolation voltage for 1 minute according to UL 1577; VDE certificate of conformity; -10): 2006-12; Reinforced insulation, viorm=849V peak; Operating temperature range: -40°C to +85°C; Wide body, 16 lead SOIC package.

application

Isolated RS-485/RS-422 interface; industrial field network; Interbus; multipoint data transmission system.

General Instructions

The ADM2491E is an isolated data transceiver with ±8KV ESD protection for high-speed, half-duplex or full-duplex communication on multipoint transmission lines. For half-duplex operation, the transmitter output and receiver input share the same transmission line. The transmitter output pin Y is externally connected to the receiver input pin A, and the transmitter output pin Z is connected to the receiver input pin B.

The ADM2491E is designed for balanced transmission lines and complies with ANSI TIA/EIA RS-485-A-1998 and ISO 8482:1987(E). Using Analog Devices, Inc., ICoupler® technology, this device combines a three-channel isolator, three-state differential line driver, and differential input receiver into a single package.

The differential transmitter output and receiver input employ electrostatic discharge circuits that provide ±8 kV protection using the Human Body Model (HBM). The logic side of the device can be powered with a 5V or 3.3V supply, while the bus side requires an isolated 5V supply.

The device features current limiting and thermal shutdown to prevent output shorts and bus contention situations that could lead to excessive power dissipation. The ADM2491E is available in a wide-body, 16-lead SOIC package and operates over the temperature range of -40°C to +85°C.

Specification

All voltages are relative to their respective grounds; 3V±V is less than 5.5V, and 4.5V is less than 5.5V. Unless otherwise stated, all min/max specifications apply over the entire recommended operating range. All typical specifications are T=25°C, V=V=5.0V unless otherwise noted.

Timing Specifications

1. T=-40°C to +85°C.

2. Devices considered as two-terminal devices: Pin 1, Pin 2, Pin 3, Pin 4, Pin 5, Pin 6, Pin 7 and Pin 8 shorted together, Pin 9, Pin 10, Pin 11, Pin 12 , Pin 13, Pin 14, Pin 15 and Pin 16 are shorted together.

3. The input capacitor is from any input data pin to ground.

Regulatory Information

1. According to UL 1577, each ADM2491E is verified by applying an insulation test voltage greater than or equal to 6000 V rms for one second (current leakage detection limit = 10 μA).

2. According to din v vde v 0884-10, each adm2491e is verified by applying insulation test voltage ≥1590v peak value for 1 second (partial discharge detection limit = 5pc).

VDE 0884 Insulation Characteristics

This isolator is only suitable for reinforced electrical isolation within the safety limit data. Maintenance of safety data must be ensured by protective circuits. An asterisk (*) on the package indicates VDE 0884 approved 849 V peak working voltage.

Absolute Maximum Ratings

Ta = 25°C unless otherwise noted. Each voltage relative to its stress is greater than the voltages listed under Absolute Maximum Ratings.

test circuit

Switching Characteristics

Typical performance characteristics

Circuit Description

Electrical isolation

In the ADM2491E, electrical isolation is implemented on the logic side of the interface. Therefore, this section has two main sections: the digital isolation section and the transceiver section (see Figure 25). The driver input signal applied to the TXD pin and referenced to logic ground (gnd) is coupled through the isolation barrier to appear on the transceiver section referenced to isolated ground (gnd). Similarly, the receiver input (referenced to isolated ground in the transceiver section) is coupled across the isolation barrier to appear at the rxd pin referenced to logic ground.

Digital signals are transmitted through the isolation barrier through ICouper technology. This technique uses chip-scale transformer windings to magnetically couple digital signals from one side of the isolation barrier to the other. The digital input is encoded into a waveform capable of exciting the main transformer windings. At the secondary winding, the induced waveform is decoded into the originally transmitted binary value.

Thermal shutdown

The ADM2491E includes thermal shutdown circuitry to prevent excessive power dissipation of the part during fault conditions. Shorting the driver output to a low impedance source can result in high driver current. In this case, the thermal sensing circuit detects the increase in mold temperature and disables the driver output. This circuit is designed to disable the driver output when the die temperature reaches 150°C. The drives were re-enabled at 140°C as the device cooled.

Fail-safe receiver input

The receiver input includes a failsafe function that ensures the RXD pin is logic high when the A and B inputs are floating or open circuit.

Magnetic field immunity

Since non-magnetic devices use coreless technology, no magnetic components are present, and the problem of magnetic saturation of the magnetic core material does not exist. Therefore, ICoupler devices have essentially infinite DC field immunity. The following analysis defines the conditions under which this may occur. Check the 3V operating condition of the ADM2491E as it represents the most susceptible operating mode.

The limit on the AC magnetic field immunity of the i-coupler is set by the condition that an error voltage is induced in the receiver coil (in this case the bottom coil), which is large enough to incorrectly set or reset the decoder. The voltage induced through the bottom coil is given by:

where (if the pulse amplitude at the transformer output is greater than 1.0 V):

β is the magnetic flux density (Gaussian). n is the number of turns of the receiving coil. r is the n-turn radius (cm) of the receiving coil. nth

The sense threshold of the decoder is about 0.5v; therefore, there is a 0.5v margin to allow the sensed voltage.

Considering the geometry of the receiver coil and the applied requirements, in the decoder, the induced voltage is at most 50% of the 0.5 V margin, and the maximum allowable magnetic field is calculated, as shown in Figure 26.

For example, at a magnetic field frequency of 1 MHz, a maximum allowable magnetic field of 0.2 kGauss induces a voltage of 0.25 volts on the receiving coil. This is about 50% of the sensing threshold and will not cause false output transitions. Similarly, if such an event occurs during a transmit pulse and is a worst-case polarity, it reduces the receive pulse from >1.0v to 0.75v - still well above the decoder's 0.5v sensing threshold.

Figure 27 shows flux density values for more familiar quantities, such as the maximum allowable current, at a given distance from the ADM2491E transformer.

Under the combined effect of strong magnetic fields and high frequencies, any loops formed by printed circuit board traces can generate an error voltage large enough to trigger the thresholds of subsequent circuits. Care should be taken to avoid this possibility when laying out such traces.

application information

Isolated power circuit

The ADM2491E requires a power supply with isolation capability up to about 5 millivolts (this current depends on the data rate used and termination resistors) supplied between the V and GND pins. A transformer driver circuit with a center-tapped transformer and LDO can be used to generate an isolated 5 V supply, as shown in Figure 28. A center-tapped transformer provides electrical isolation from the 5 V supply. The primary winding of the transformer is excited by a pair of square waves that are 180° out of phase with each other. A rectified signal is generated from the secondary winding using a pair of Schottky diodes and a smoothing capacitor. The ADP3330 linear voltage regulator is the ADM2491E.

printed circuit board layout

The ADM2491E isolated RS-485 transceiver does not require external interface circuitry for logic interfaces. Power supply bypassing is required at the input and output power pins (see Figure 29). Bypass capacitors are conveniently connected between V pins 1 and 2 and between V pins 15 and 16. The capacitor value should be between 0.01µf and 0.1µf. The total lead length between the ends of the capacitor and the input power pins should not exceed 20 mm. Also consider bypassing between pin 1 and pin 8 and between pin 9 and pin 16, unless the ground pair on each package side is connected near the package.

In applications involving high common mode transients, care should be taken to ensure that plate coupling on the isolation barrier is minimized. Additionally, the board layout should be designed so that any coupling that occurs will have the same effect on all pins on a given component side. Failure to ensure this could cause voltage differences between pins to exceed the absolute maximum ratings of the device, resulting in latch-up or permanent damage.

typical application

Figure 30 and Figure 31 show typical applications of the ADM2491E in half-duplex and full-duplex RS-485 network configurations. Up to 32 transceivers can be connected to the RS-485 bus. To minimize reflections, the line must be terminated at the receiving end of its characteristic impedance and the stub length from the mains must be kept as short as possible. For half-duplex operation, this means that both ends of the line must be terminated, as either end can be the receiver.