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Isolator Reference Designs

Isolated RS-485 Reference Design

Isolated RS-485 Reference Design

Faster and longer networks should be be terminated. Reflections are minimized by terminating both ends of the bus but not every node. Two 120 ohm termination resistors (on each end of the bus) are used to match a 60 ohm cable impedance.

With terminated networks the differential voltage across the conductor pair will be close to zero with no active drivers. In this case, the idle bus is indeterminate and susceptible to noise. External fail-safe biasing resistors (RFS-EXT) at one end of the bus ensure fail-safe operation with a terminated bus. Biasing should provide at least 200 mV across the conductor pair to meet the RS-485 input sensitivity specification.

The more loads on the bus, the lower the required values of the biasing resistors. The reference design has a 120 ohm termination resistor and 560 ohm biasing resistors for fail-safe operation for up to four Unit Loads.

  RS-485 Tips (04:03)

AB-5: Isolated RS-485 and RS-422 (.pdf)
AB-6: RS-485 Systems (.pdf)

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Isolated PROFIBUS Reference Design

PROFIBUS Reference Design

The 220 ohm termination resistor, along with another 220 ohm resistor and the other end of the bus, matches the bus impedance to minimize reflections. The 390 ohm “fail-safe” resistors guarantee a known state with no active transceivers. Bypass capacitors should be placed as close as possible to the appropriate IC.

IL3685 product details

AB-5: Isolated RS-485 and RS-422 (.pdf)
AB-6: RS-485 Systems (.pdf) 
AB-19: PROFIBUS Compliance: A Hardware Design Guide (.pdf) 

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Isolated USB Reference Design

Isolating the input to a USB controller (rather than the bus lines) requires fewer isolation channels, and is therefore usually the simplest solution as shown in this reference design:

Isolated USB UART Using an IL712

The Microchip MCP2200 is a USB 2.0 to UART Protocol Converter. The IL712/IL721 Series are best-in-class bidirectional isolators with a remarkable 100 ps of jitter, and parts are available in unique MSOPs to reduce board space.

IL712-Series data sheet (.pdf)

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Isolated CAN Transceiver With DC-DC Convertor Reference Design

IL41050 Isolated CAN Transceiver Reference Design

The IL41050 single-chip isolated CAN transceiver provides an isolated CAN interface in a single chip. The isolator operates on the 3.3 V microcontroller power supply, and a low-cost dc-dc convertor provides 5 V to power the bus side. The IL41050 can withstand 200 V transients on the bus; the PSM712 provides additional ESD protection if necessary.

Designed for harsh CAN and DeviceNet environments, IL41050 transceivers have a -55 to +125°C operating range, transmit data dominant time-out, bus pin transient protection, thermal shutdown protection, short-circuit protection, 2,500 V isolation, and typical transient immunity of 30 kV/µs. Unique edge-triggered inputs improve noise performance.

An innovative nonvolatile programmable power-up feature prevents unstable nodes. A state that needs to be present at node power up can be programmed at the last power down. For example if a CAN node is required to “pulse” dominant at power up, TxD can be sent low by the controller immediately prior to power down. When power is resumed, the node will immediately go dominant allowing self-check code in the microcontroller to verify node operation. If desired, the node can also power up silently by presetting the TxD line high at power down. At the next power on, the IL41050 will remain silent, awaiting a dominant state from the bus.

IL41050TA Isolated CAN Transceiver data sheet (.pdf)

  CAN Isolation (02:02)

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Isolated I²C Reference Design

Isolated I2C Reference Design

This circuit provides bidirectional isolation of I²C bus signals with no restrictions on data rate and none of the I²C bus latch-up problems common with other isolation circuits. The SCL section is similar as shown in the schematic using the other half of the P82B96.

IL600-Series Data Sheet (.pdf)

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