Gateways featuring Semtech’s LoRa® devices often reside in electrically harsh industrial environments. As the centerpiece for a LoRaWAN® network, these gateways need to operate in the field for many years. The longevity of the network requires that gateways be immune to a range of transient threats the equipment may encounter over its lifetime. As such, gateway data ports need to be safeguarded from overvoltage transient threats, including electrostatic discharge (ESD), electrical fast transients (EFT) and lightning surge.
There are multiple data interface ports on a typical LoRa-based gateway. Each port or system exposure can present a vulnerability to overvoltage transients. It is helpful to classify the gateway data ports into basic categories: high-speed data ports, serial service ports, RF antennas, power/DC bus ports, and on some gateway devices, memory cards for loading software.
With fast response time and excellent clamping voltage characteristics, TVS diodes are proven and effective solutions for protecting system data interfaces. These protection elements are designed to clamp transient voltage and shunt transient current away from CMOS transceiver circuitry. Advanced TVS diodes offer very low clamping voltages and high-surge immunity with a minimal capacitance penalty. Unlike other transient suppression technologies, TVS diodes do not degrade in performance over time and maintain a consistent clamping performance.
Transient Voltage Suppressor (TVS) Operation © Semtech 2020
For gateway products, the stability of clamping performance over the product’s lifetime is critical, and TVS diodes offer a compelling advantage. TVS diodes are also well suited for protecting against both ESD (fast rise-time transient events) and surge (high power, slow rise time transient events). This is important as many gateways reside in outdoor environments and are thus subject to frequent lightning induced surges.
High-Speed Port Protection
Protecting a data line involves two basic challenges. The first is to safeguard the system such that the protection element will clamp the transient energy to a sufficiently low threshold, protecting the transceiver IC. The second task is to insure that the TVS does not adversely degrade the interface signal integrity. For high-speed ports (like Ethernet or USB), it is critical to minimize the capacitance the protection presents to the circuit.
With respect to the trace routing, high-speed ports are best protected with TVS components that offer a flow-through layout. A flow-through package simply means the input and output pins are arranged such that the package allows for differential pair signal traces to route cleanly underneath the package without excessive trace bends or vias. The following diagrams show examples of protecting Ethernet and USB 2.0 circuits with flow-through TVS devices.
Gigabit Ethernet Protection Scheme using the Semtech RClamp®3374N © Semtech 2020
USB 2.0 Protection Scheme using the Semtech RClamp0512TQ © Semtech 2020
Serial Port Protection
LoRa-based gateways may also offer several serial data ports, with the most commonly used being RS-232 and RS-485. While at first glance, the need for low capacitance may not seem as critical, the RS-485 link can have multiple nodes and thus it is important to consider the overall capacitance budget of the data link. Generally, for RS-232 and RS-485, a TVS element needs to standoff higher voltage (±15V typically for RS-232 and +12/-7V for standard RS-485 or +/-36V for extended common mode range applications). The image below shows a typical ESD and surge protection scheme using a low-capacitance TVS array to protect extended mode range RS-485 links.
RS-485 Protection Scheme for Extended Common Mode Range using TClamp®3602P © Semtech 2020
Additionally, LoRa-based gateways offer multiple antennas with different RF protocols: Wi-Fi, NFC, GPS, 4G, and of course, LoRaWAN. As the TVS is a nonlinear element that can and will generate unwanted harmonics, not only should the capacitance be minimized (generally less than 1pF), but the capacitance versus working voltage should be as stable (non-varying) as possible. Generally, antennas are best protected using a bidirectional TVS diode configuration with symmetrical clamping in the positive or negative polarity. The image below shows an example of ESD protection on a LoRa RF link with a Semtech RClamp2451ZA device.
LoRa Antenna Protection with RClamp2451ZA © Semtech 2020
Power Port Protection
System power lines and DC buses also represent vulnerabilities for overvoltage transients to enter the PCB. As with any aperture or system opening, it is important to arrest the aggressor transient as close to the system entry as possible. As power buses, these lines tolerate additional capacitance, and using a TVS with a higher Cj rating is generally acceptable. For Vbus/DC lines, unidirectional TVS devices are preferred. The following table shows appropriate TVS protection solutions at different power bus voltages.
DC Line/Vbus TVS Protection Diodes © Semtech 2020
DC Line/Vbus TVS Protection with Semtech uClamp®xx71P Family © Semtech 2020
SD Card Protection
For software updates to the gateway, the system may occasionally employ a memory card. Though the memory card slot is rarely touched, the exposed pins make the interface exceedingly susceptible to damage from ESD charge transfers when handled by a user. The diagram below illustrates a typical example of a protection solution for an SD card.
ESD Protection for SD Card Connection © Semtech 2020
As the backbone link for aggregating data collected by LoRa-based devices, LoRa-based gateways are frequently subjected to transient voltage threats. With careful planning on the front end of the design, the gateways can be effectively safeguarded with TVS protection diodes. It is important to take an inventory of all the key system data ports as well as to consider the potential transient threats in the intended environment. A forward thinking approach in planning overvoltage protection on the front end of the hardware design helps insure a well-hardened gateway in the field, and can pay welcome dividends in terms of time to market and a quality product.
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