Considerations on design factors affecting circuit robustness

introduce

What is the robustness, operating temperature range, and how does it handle high electrical noise? What about ESD and fault protection? These questions are not necessarily the first things that design engineers think of when selecting an IC. Nevertheless, robustness is a key performance parameter for a long-term and reliable, reputable high-end product. This is especially true when designing systems in industrial environments where harsh operating environments are common. Industrial equipment can be exposed to a wide range of temperatures, high electrical noise on power or data lines, and fault events such as ESD or short circuits.

Today the de facto temperature standard for integrated circuits in industrial environments is -40°C to +85°C, an "extended" +15°C temperature range above the standard -40°C to +70°C of a few years ago. The industry trend is to operate at higher and higher temperatures, and ultimately the new expectations in the automotive industry will be -40°C to +125°C. Higher current and power densities make it obvious that IC companies must design circuits to withstand wider temperature ranges... being left out of the decision-making process.

Handling Voltage Transients

Voltage transients often occur on power lines because of improper wiring or accidental shorts. If the inputs are not protected, these transients can damage downstream circuitry. A simple and discreet circuit consisting of a series fuse with a transient voltage suppressor (TVS) diode has traditionally been used to protect against most transient voltages (Figure 1).

Figure 1. Transient voltage protection circuit using careful components.

Transient protection using prudent components, however, has its own limitations. TVS diode protection thresholds are often not well controlled and can change dramatically with temperature. Additionally, fuse replacement is required after an overvoltage condition occurs. Finally, large transients require large TVS diodes that consume board space and dissipate additional heat.

A more controlled way to manage overvoltage and transient events is to integrate protection thresholds and reaction circuitry into an integrated circuit. To ensure a reliable response, internal comparators and diodes are designed into the monitoring and protection IC. Some ICs integrate high-voltage fault protection for the data lines. To protect itself from damage, the fault protection device will latch off when the normal data line voltage level is exceeded. An example of this is the MAX4708 multiplexer family. The MAX4708/MAX4709 include two fault detectors: a high-side detector for the NO_ voltage above the positive rail (V+), and a low-side detector for the NO_ voltage below the negative rail (V) (Figure 2). A fault condition occurs when the NO_ voltage exceeds the supply rails, at which point both the N1 and P1 FETs are turned off. This approach quickly disconnects the input and output switches when a fault condition occurs.

Figure 2. Functional block diagram of the MAX4708/MAX4709.

With RS-485 transceivers, you can also manage voltage transients in data systems. The receiver inputs and output drivers of an RS-485 transceiver may be exposed to significantly higher voltages than the -7V to +12V common mode range specified in the EIA/TIA-485 standard for industrial systems. New transceivers have been designed to withstand these voltage events and can now even withstand up to ±80V (to ground) without damage. This state-of-the-art technology ensures reliable protection and business longevity.

Preventing ESD and Failure

Integrated ESD circuitry protects the IC from damage from ESD events and helps make the entire system more robust.

Electrostatic discharge (ESD), another overvoltage event, occurs when two materials with different electrical potentials come into contact, transferring the stored static charge and generating a spark. Static sparks are often generated by the interaction of people with their surroundings. These unintentional sparks can alter the performance of a semiconductor device, degrading or completely destroying it. ESD is a serious industrial problem, estimated to cause billions of dollars in damages each year. ESD events can cause single component failures and sometimes even catastrophic system failures.

External discrete component circuits such as ESD diodes and other types of protection for data lines can be used. Many IC devices integrate some level of ESD protection so that the IC itself does not require further external protection. Figure 3 shows a very simplified functional block diagram of a common integrated protection scheme. Spike voltage signals at input/output (I/O's) are clamped to VCC or GND and protect the internal circuitry. Many interface products and analog switches have integrated ESD protection designed to meet the IEC 1000-4-2 standard. Notably, Maxim recently made its PROFIBUS RS-485 transceiver, the MAX14770E, ESD Human Body Model (HBM) level ±35 kV.

Figure 3. Simplified integrated ESD protection circuit.

in conclusion

Although robustness, operating temperature range, ESD and short-circuit fault and line protection, cover a different set of design issues, it is not usually the first thing that design engineers address. This can be a serious mistake. Applications in the industrial market require robust components that can withstand harsh environments and territories. Robustness should be considered as early as the design stage of these applications.