feature

5kV rms isolated RS-485 /RS-422 transceiver, configurable as; half or full duplex; equal power integrated isolated dc-dc converter; ESD protection for RS-485 input/output pins; ANSI/ TIA/EIA-485-A-98 and ISO 8482-1987(E); data rates: 16 Mbps (ADM2682E), 500 kbps (ADM2687E); 5 V or 3.3 V operation; up to 256 nodes on one bus; Open and short circuit, fault protected receiver input; high common mode transient immunity: >25kv/m; thermal shutdown protection; safety and regulatory approvals; UL recognized; 5000 V rms per UL 1577 one minute; -35; 5A (pending); IEC 60601-1 : 400 V rms (basic), 250 V rms (enhanced); IEC 60950-1: 600 V rms (basic), 380 V rms (enhanced); VDE certificate of conformity; German Industrial Standard V VDE V 0884-10 (VDE V 0884-10): 2006-12; VIORM=846V peak; operating temperature range: 40oC to 85oC; 16-lead wide body SOIC, creepage and clearance greater than 8 mm.

application

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

General Instructions

The ADM2682E/ADM2687E are fully integrated 5 kV rms signal and power isolated data transceivers with ±15 kV electrostatic discharge protection for high-speed communication on multipoint transmission lines. The ADM2682E/ADM2687E include an integrated 5 kV rms isolated DC-DC power supply, eliminating the need for an external DC-DC isolation block. They are designed for balanced transmission lines and meet the requirements of ANSI/TIA/EIA-485-A-98 and ISO 8482:1987(E).

These devices integrate Analog devices, Inc., iCoupler® technology, combining a 3-channel isolator, a three-state differential line driver, a differential input receiver, and the analog device isoPower® dc-dc converter into a single package. The unit is powered by a single 5v or 3.3v supply, enabling a fully integrated signal and power isolated RS-485 solution.

The ADM2682E/ADM2687E driver has an active high enable. An active low receiver enable is also provided, which puts the receiver output into a high impedance state when disabled. These devices feature current limiting and thermal shutdown to prevent output shorts and bus contention situations that could lead to excessive power dissipation. Fully specified over the industrial temperature range, these parts are available in a highly integrated 16-lead wide-body SOIC package with creepage and clearance greater than 8 mm.

The ADM2682E/ADM2687E contain isopower technology that uses high frequency switching elements to transfer power through a transformer. Special care must be taken during printed circuit board (PCB) layout to meet emission standards. For more information on board layout considerations, see the AN-0971 application note, Controlling Radiated Emissions with Isopower Devices.

Typical performance characteristics

test circuit

Switching Characteristics

Circuit Description

Signal isolation

The ADM2682E/ADM2687E signal isolation is 5kV rms, implemented on the logic side of the interface. This section achieves signal isolation by having a digital isolation section and a transceiver section (see Figure 1). Data applied to the TxD and DE pins and referenced to logic ground (GND) is coupled through the isolation barrier to appear on the part of the transceiver referenced to isolated ground (GND). Similarly, the isolated ground referenced single ended receiver output signal in the transceiver section is coupled across the isolation barrier to appear at the logic ground referenced RxD pin.

Power isolation

The ADM2682E/ADM2687E achieve 5kV rms power isolation using an equal power integrated isolated dc-dc converter. The DC-DC converter section of the ADM2682E/ADM2687E operates on the same principle as most modern power supplies. It is a secondary-side controller architecture with isolated Pulse Width Modulation (PWM) feedback. The V power supply is provided to the oscillator circuit, which converts the current into a chip-scale air-core transformer. The power delivered to the secondary side is rectified and regulated to 3.3 V. The secondary side (VISO) controller regulates the output by creating a PWM control signal sent to the primary side (VCC) by a dedicated ICouper (5 kV rms signal isolation) data channel. PWM modulates the oscillator circuit to control the power sent to the secondary side. Feedback can significantly improve power and efficiency.

truth table

The truth tables in this section use the abbreviations in Table 11.

Thermal shutdown

The ADM2682E/ADM2687E contain 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 drive will re-enable at 140°C as the device cools.

Open and short circuit, failsafe receiver input

The receiver input is open-circuit and short-circuit, fault-protected to ensure that the receiver output is high when the input is open or short-circuited. During line idle conditions, when no drivers are enabled on the bus, the voltage across the termination resistors at the receiver input decays to 0V. With traditional transceivers, the receiver input threshold specified between -200 mV and +200 mV means that external bias resistors are required on the A and B pins to ensure that the receiver output is in a known state. The short-circuit, fault-protected receiver input feature eliminates the need for biasing resistors by specifying receiver input thresholds between -30 mV and -200 mV. Guaranteed negative threshold means that the receiver output is guaranteed to be high when the voltage between A and B decays to 0v.

DC Correctness and Magnetic Field Immunity

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.

Positive and negative logic transitions at the input of the isolator cause narrow pulses (~1ns) to be sent through the transformer to the decoder. The decoder is bistable, so it can be set or reset with a pulse, indicating input logic transitions. In the absence of more than 1 μs logic transitions at the input, a periodic set of refresh pulses indicating the correct input state is sent to ensure dc correctness at the output. If the decoder does not receive internal pulses greater than about 5µs, the input is assumed to be unpowered or nonfunctional, in which case the isolator output is forced to the default state by the watchdog timer circuit.

This condition should only occur during power up and power down operations of the ADM2682E/ADM2687E device. The limit of the ADM2682E/ADM2687E magnetic field immunity is set by the condition that the induced voltage in the receiver coil of the transformer is large enough to incorrectly set or reset the decoder. The following analysis defines the conditions under which this occurs.

Check the 3.3V operating condition for the ADM2682E/ADM2687E as it represents the most susceptible operating mode. The pulse amplitude at the transformer output is greater than 1.0v. The induction threshold of the decoder is about 0.5v, creating a 0.5v margin in which the induced voltage can be tolerated. The voltage induced by the receiver coil is given by:

where: β is the magnetic flux density (Gaussian). N is the number of turns of the receiving coil. rn is the radius (cm) of the nth turn of the receive coil.

Considering the geometry of the receive coils in the ADM2682E/ADM268E, and the induced voltage applied in the decoder is at most 50% of the 0.5 V margin, the maximum allowable magnetic field is calculated as shown in Figure 39.

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 the transmit pulse (and has the worst polarity), it reduces the receive pulse from >1.0v to 0.75v, which is still well above the decoder's 0.5v sensing threshold .

The flux density values above correspond to specific current amplitudes at a given distance from the ADM2682E/ADM2687E transformers. Figure 40 presents these allowable current amplitudes as a function of frequency for selected distances. As shown in Figure 40, the ADM2682E/ADM2687E are extremely immune and can only be affected by extremely large currents operating at high frequencies very close to the components. For the 1 MHz example, a 0.5 kA current must be placed 5 mm from the ADM2682E/ADM2687E to affect component operation.

The ADM2682E/ADM2687E state that under a combination of strong magnetic fields and high frequencies, any loops formed by the PCB traces can generate enough error voltages to trigger the thresholds of subsequent circuits. Pay attention to the layout of these traces to avoid this possibility.

application information

printed circuit board layout

The ADM2682E/ADM2687E isolated RS-422/RS-485 transceivers contain an equal-power integrated dc-dc converter that eliminates the need for external interface circuitry for the logic interface. Power supply bypassing is required at the input and output power pins (see Figure 41). The power section of the ADM2682E/ADM2687E uses an oscillator frequency of 180 MHz to efficiently pass power through its chip-scale transformer. Additionally, the normal operation of the iCoupler data section can introduce switching transients on the power supply pins.

Several operating frequencies require bypass capacitors. Noise suppression requires a low inductance, high frequency capacitor, while ripple suppression and proper regulation require a large value capacitor. These capacitors are connected between pins 1 (GND) and 2 (VCC) of VCC and between pins 7 (VCC) and 8 (GND1). The VISOIN and VISOOUT capacitors are connected between pins 9 (GND2) and 10 (VISOOUT) and between pins 15 (VISOIN) and 16 (GND2). To suppress noise and reduce ripple, a parallel combination of at least two capacitors is required, with the smaller of the two capacitors located closest to the device. For VISOOUT at pin 9 and pin 10, VCC at pin 7 and pin 8. Capacitor values of 0.01µF and 0.1µF are recommended for VISOIN at pins 15 and 16 and VCC at pins 1 and 2. The recommended best practice is to use a very low inductance ceramic capacitor, or its equivalent, for a smaller value capacitor. The total wire length between the capacitor ends and the input power pins should not exceed 10 mm.

Ensure that plate coupling on the isolation barrier is minimized in applications involving high common mode transients. Additionally, design the board layout so that any coupling that occurs also affects 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 and/or permanent damage.

The ADM2682E/ADM268E consume approximately 675 MW at full load. Because it is not possible to apply heat sinks to isolated devices, these devices mainly rely on dissipating heat to the PCB through the GND pin. If the device is used at high ambient temperatures, provide a thermal path from the ground pins to the PCB ground plane. The board layout in Figure 41 shows enlarged pads for pin 1, pin 8, pin 9, and pin 16. Implementing multiple vias from the pad to the ground plane significantly reduces the temperature inside the chip. The size of the expansion pad is determined by the designer and depends on the available board space.

Electromagnetic Interference Considerations

The DC-DC converter portion of the ADM2682E/ADM2687E components must operate at very high frequencies for efficient power transfer through small transformers. This creates high frequency currents that can propagate in the board ground and power planes, causing fringe and dipole radiation. For applications using these devices, a grounded cabinet is recommended. If grounding the case is not possible, good RF design practices should be followed in the layout of the PCB. For more information, see AN-0971 Application Note, Controlling Radiated Emissions Using Isopower Devices.

Insulation life

Over enough time, all insulating structures will eventually collapse when subjected to voltage stress. The insulation degradation rate depends on the characteristics of the voltage waveform applied to the insulation. The simulation equipment performs an extensive series of evaluations to determine the lifetime of the insulating structures in the ADM2682E/ADM2687E.

Accelerated life testing is performed using a voltage level higher than the rated continuous operating voltage. Acceleration coefficients under several operating conditions are determined so that the time-to-failure at the relevant operating voltage can be calculated. The values shown in Table 9 summarize the peak voltage for a 50-year lifetime under several operating conditions. In many cases, agency testing approved operating voltages are higher than the 50-year service life voltage. Operation at operating voltages higher than the listed service life voltages can cause premature insulation failure.

The insulation lifetime of the ADM2682E/ADM2687E depends on the type of voltage waveform applied across the isolation barrier. The insulating structure of an I-coupler degrades at different rates depending on whether the waveform is bipolar AC, unipolar AC, or DC. Figure 42, Figure 43, and Figure 44 illustrate these different isolation voltage waveforms.

Bipolar AC voltage is the most demanding environment. The 50-year operating life under bipolar AC conditions determines the maximum recommended operating voltage for analog devices.

In the case of unipolar AC or DC voltages, the stress on the insulation is significantly reduced. This allows operation at higher operating voltages while still achieving a 50-year lifespan. The operating voltages listed in Table 9 may be applied while maintaining a minimum life of 50 years, as long as the voltage conforms to the unipolar AC or DC voltage conditions. Any cross-insulation voltage waveform not conforming to Figure 43 or Figure 44 shall be treated as a bipolar AC waveform and its peak voltage shall be limited to the 50-year lifetime voltage values listed in Table 9.

Precautions for Isolated Power Supply

The typical output voltage of the integrated isopower DC-to-DC isolated power supply is 3.3 V. The isolated power supplies in the ADM2682E/ADM2687E are typically capable of delivering 55 mA when the connected temperature of the device is kept below 130°C. This includes the current required by the internal RS-485 circuitry, typically, there is no additional current on VISOOUT for external applications.

typical application

An example application of the ADM2682E/ADM2687E for a full-duplex RS-485 node is shown in the circuit diagram in Figure 45. See the PCB Layout section for suggested locations of capacitors shown in this circuit diagram. The location of the R termination resistor depends on the node location and network configuration. See AN-960 Application Note for termination guidelines. TRS-485/RS-422 Circuit Implementation Guidelines Figure 46 and Figure 47 show the ADM2682E/ADM2687E in half-duplex and full-duplex RS-485 network configurations. Up to 256 transceivers can be connected to the RS-485 bus. To minimize reflections, terminate the line at its characteristic impedance at the receiving end and keep the stub 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.

Dimensions