21 May 2015
RS-485 Transceiver ESD Protection
Jeff Lies, Applications Engineer, Precision Products, Intersil Corporation looks how to design ESD protection into RS485 transceiver circuits.
Electrostatic discharge (ESD) protection is a key issue for serial data transceivers as they are likely to be subjected to transients that will appear on the data lines.
In designing ESD protection for serial data transceivers standards like Human Body Model and IEC61000-4-2 (IEC) are used. Techniques used to prevent damage include Over Voltage Protection (OVP) and a cable invert function, and it is also necessary to consider a snapshot of very high-speed transceivers (above 25Mbps) where data skew is critical.
Enhanced ESD Protection
Full featured transceivers also include some form of improved ESD protection on the bus pins. Bus pins typically connect to an exposed port on the exterior of a piece of equipment. This exposure makes the port especially susceptible to ESD events, and simply connecting a charged interface cable could destroy an unprotected transceiver.
Bus pin ESD protection is specified to one of two standards: the widely familiar Human Body Model (HBM) standard that is common in the U.S., and the IEC61000-4-2 (IEC) standard that is common in Europe, and is growing in worldwide acceptance. The HBM test emulates the types of ESD events seen during manufacturing and handling, while the IEC ESD test is an end-equipment test intended to harden equipment to ESD events seen in the field.
The IEC ESD standard contains two test methods: a contact method – like HBM - and an air gap method. In the air gap method, a charged electrode is brought toward the pin-under-test, until the voltage discharges across the air gap to the pin. The contact method has the electrode contact the pin-under-test before charging the electrode.
The biggest differences between the HBM and IEC61000 standards are the component values in the discharge network (see Figure 4). The charge storage cap is 50% larger on IEC61000, and the series current limiting resistor is 330Ω vs. 1.5kΩ. This lower value resistor increases the peak ESD current by nearly a factor of five, resulting in a much more severe ESD test.
Figure 4: Human Body Model and IEC61000-4-2 discharge networks
Table 1 highlights the differences between the IEC ESD model, and the human body model, and as you can see, every parameter is more severe for IEC. In addition to the nearly 5X higher peak current, the pulse rise time is much faster forcing the on-chip protection circuit to respond more quickly, all the pulse energy is delivered to the IC in less than half the time, and IEC requires ten ESD pulses rather than three.
|Table 1 Comparison of IEC61000-4-2 and Human Body Model Parameters, and Classification Levels|
|Number of applied pulses||1 - 3||10|
|Maximum current (at 15kV)||10A||45A|
|Maximum rise time||10ns||1ns|
|Classification Levels||HBM||IEC 61000 Air Gap Method||IEC61000 Contact Method|
|Level 3||≥ ±8kV||±8kV||±6kV|
|Level 4||N/A||≥ ±15kV||≥ ±8kV|
Table 1: Comparison of IEC61000-4-2 and Human Body Model Parameters, and Classification Levels
As shown in Table 1, there are three classification levels for HBM, and four levels for IEC ESD. The special ESD structures utilized in full featured transceivers allow them to meet the highest level for each of the standards. These high ESD structures protect the IC whether or not it is powered up, and without interfering with the RS-485 standard’s -7V to +12V CMR. Adding IEC61000 ESD protection to interface ICs saves designers time and money by eliminating the need for board level protection, and minimizes field returns due to ESD damage.
Full Featured Transceivers with Large Differential Output Voltage (VOD)
Some standards - like Profibus DP – use RS-485 for the physical layer, but require a much larger 2.1V Tx VOD for better drive and noise immunity.
The 5V powered ISL3150E family from Intersil, for example, features a 2.4V minimum VOD, which provides 900mV more noise immunity than standard RS-485 ICs, while the 40Mbps ISL3159E delivers a 2.1V minimum.
Additionally, the large VOD allows these transceivers to drive more than the two terminations required by the RS-485 standard. ISL315XE transmitters can drive the RS-485 required 1.5V VOD into six to eight terminations (3-4x the RS-485 requirement), making them ideal for “Star” and other nonstandard networks that require more than two terminations.
Over-Voltage Protected (OVP) Devices
Another application issue arises when power is routed in the same conduit as the data cable. Wiring errors, loose connections, or even solder debris may cause the power line to contact the data connection on the PCB or in the connector.
With industrial power supplies commonly exceeding 20V, contact with a data line ensures the destruction of a standard, unprotected RS-485 transceiver. Enter the over-voltage protected, or fault protected, transceiver, which is designed such that the RS-485 bus pins can survive voltages much higher than those required by the RS-485 standard.
OVP devices like the ISL3243XE and ISL3249XE offer over-voltage levels of ±40 to ±60V and a wide common mode voltage range (CMR) that is up to two times the range required by the RS-485 standard. Wider CMR allows for the common mode voltage pick-up that frequently occurs in long networks, or in noisy environments. Many OVP devices are specified with CMRs from ±15V to ±25V, meaning that a transmitter and receiver continue communicating even when faced with large common mode voltages.
A key advantage of high voltage tolerant bus pins is that they ease the design of bus pin protection networks. If DC or transient bus voltages can exceed a transceiver’s bus pin voltage rating, then external protection devices like Transient Voltage Suppressor (TVS) ICs must be added to the transceiver design. The asymmetrical nature of the -7V to +12V standard CMR makes it difficult to use basic bidirectional TVS ICs. Choosing a ±12V TVS allows negative voltages to exceed the transceiver’s -7V limit, while utilizing a ±7V TVS cuts off 40% of the standard’s +CMR. Conversely, the symmetrical bus pin voltages of the OVP transceivers easily accommodate bidirectional TVS protection, and the protection is more robust because of the extra headroom between the TVS hard clamping voltage and the bus pin damage voltage.
For example, when trying to protect an OVP device with a ±25V CMR, you simply pick a bidirectional TVS with a standoff voltage above ±25V, and below the OVP level. Keep in mind that TVS devices typically hard clamp at voltages 50% higher than their standoff voltage, so select the lowest TVS voltage that allows the needed CMR. TVS voltages in the range of ±25V to ±40V have been shown to give good protection for ±60V OVP ICs.
Coupled with the ±16.5kV HBM ESD, the OVP and wide CMR features make these devices some of the most robust RS-485 transceivers on the market. These are also full featured devices, so they are FFS, and present only a 1/4th UL to the bus.
OVP Devices with a Cable Invert Function
High node count RS-485 networks often end up with nodes mis-wired (e.g., data lines swapped) but testing and rewiring connectors is a manual, time consuming task. A better solution is to utilize RS-485 transceivers with a cable invert – also known as polarity reversal – function. Simply moving a jumper, or changing the state of a GPIO line, inverts the transceiver’s polarity, allowing the mis-wired node to communicate properly on the bus.
With a traditional RS-485 transceiver, the receiver/transmitter A/Y pins are the non-inverting input, and the B/Z pins are the inverting connections. Reversing the connections from these pins to the bus inverts the received and transmitted data, resulting in unintelligible communications.
A transceiver with a polarity reversal function operates as a normal transceiver with the polarity reversal input in the inactive state, but the transceiver flips the polarity of the bus pins when the polarity select input is switched to the active state. Thus, the B/Z pins become the non-inverting pins, while A/Y become the inverting pins, so the transceiver now communicates properly, even though its bus connections are reversed. The ISL3248XE 5V family, and the 3-5V ISL32437E and ISL32457E all include the cable invert function.
One problem with prior cable invert functions is that the previously described inversion also inverts the full failsafe output state. Thus, activating the cable invert function causes the receiver to output a logic low when its inputs are floating or shorted together, which is the opposite of what the µC expects. Intersil has solved this problem by implementing a patented function that maintains FFS whether the receiver is in normal or inverted polarity.
Very High-Speed Transceivers (> 25Mbps)
Near real-time applications, such as robotics, motor control (e.g., EnDat2.2) and data acquisition, require the highest data rates (> 25Mbps) to minimize latency and to increase throughput. Very high data rates require low Tx and Rx skews to minimize duty cycle distortion, and low part-to-part skews enable high speed parallel applications (e.g., SCSI Fast-20 and Fast-40) where data skew is critical.
As an example of the importance of low skews, consider an Rx and Tx where each have skews of 5ns. Inputting a 100ns (10Mbps) pulse to either device results in an output pulse between 95ns and 105ns. If the skews are in the same direction (additive), then a bit sent between two µC may be as small as 90ns at the receive end. This is only a 10% distortion, but if the same Rx and Tx transmit a 40Mbps signal (25ns bit width) the same skews result in an unacceptable 40% pulse width distortion.
High-speed devices offer maximum Rx and Tx skews at 1.5ns, and maximum part-to-part skew at 4ns. The ISL3179E (3V) and ISL3159E (5V) are specified at 40Mbps, while the ISL3259E (5V) operates at data rates up to 100Mbps. All parts have a 125°C option (extended industrial range) to accommodate the high temperatures encountered in motor control applications, are available in MSOP and DFN packages for space constrained applications, and feature ±15kV IEC ESD levels. Additionally, the ISL3159E and ISL3259E have a Tx VOD > 2.1V, making them ideal for high speed Profibus DP networks.
Despite the large number of RS-485/RS-422 devices on the market, understanding common design problems - and the transceiver features developed to solve those problems - simplifies the designer’s task of choosing the best RS-485 device for a particular application.
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About the author
Jeff Lies is an Applications Engineer for Precision Products at Intersil Corporation. He specialises in interface products, including RS-485 and RS-232 transceivers. Jeff holds a BSEE from U.C. Berkeley.
Intersil Corporation is a leading provider of innovative power management and precision analogue solutions. The company's products form the building blocks of increasingly intelligent, mobile and power hungry electronics, enabling advances in power management to improve efficiency and extend battery life. With a deep portfolio of intellectual property and a rich history of design and process innovation, Intersil is the trusted partner to leading companies in some of the world's largest markets, including industrial and infrastructure, mobile computing, automotive and aerospace.
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