How To Design Electronic Circuits with Robust Protection for Mission-Critical Applications

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Introduction

Analog systems are omnipresent today, and system designers embed robust protection devices to ensure that these systems operate correctly in diverse and extreme conditions to fulfill their designated functional life-cycle. Robust protection devices maintain and monitor systems' environments, confirming safety and performance for mission-critical applications. In this Tech Spotlight article, we will discuss robustness from an analog design and protection control IC standpoint. We will examine some features of robust protection systems, designs, and a few examples of how robust protection ICs lead to increased system performance and life span.

Robust Protection Design

Robust protection, at its simplest, is the intentional addition of a 'link' in a circuit to protect and safeguard the system in the event of a fault – whether due to temperature variations, excessive current, overvoltage flow, or transients. The protection circuit feature introduced in the system does not intrude in the circuitry's normal function, but responds quickly to act on any hazardous events that may occur. Integrated protection circuits provide robust, high performance, and easy-to-implement solutions. These circuits are compact, energy-efficient, and ensure steady operation over many years, making them useful in industrial, automotive, medical, communications, and portable applications.

Let us discuss some of their significant characteristics, and understand the different types of integrated solutions available for robust protection.

Overcurrent Protection (OCP):  Overcurrent is an event in which a circuit's current exceeds its rated current capacity or the rating of the connected equipment or load. Overcurrent may occur due to overloading, short circuits, a current flowing back due to a surge of load, or an inrush current, thus adversely affecting system performance and efficiency.

Traditional discrete current-limiting solutions like fuses or circuit breakers commonly used for OCP are error-prone and need replacement. Integrated protection ICs and current sense amplifiers, current limiters, e-fuses, and ideal diode solutions help to precisely limit overload currents, consuming minimal power with reduced size.

Overvoltage/Under-voltage Protection (OVP/UVP): The presence of any voltage higher or lower than the rated maximum voltage of a component or device may cause component damage or degrade its performance. Voltage faults may happen due to power supply surges, the ringing of the load, system miswiring, or voltage reversal. Most voltage-protection methods as a discrete component utilize diodes to shunt voltage fault, which exhibits capacitance and leakage current that contribute to distortion at an early stage. Integrated robust protection solutions with voltage supervisor ICs (also known as voltage monitors or voltage detectors) continuously monitor system supplies or power rails, alarming or gating off downstream systems in case of OV or UV. Active voltage limiters may limit or block both positive and negative voltage, providing an overvoltage and under-voltage threshold.

Over-temperature protection (OTP): Thermal faults happen due to excessive heating of some component, or when the environment temperature exceeds rated operating temperature. High temperatures may develop within the supply due to various factors like faulty components, overloading, over-voltage at the supply or junctions, or other factors that stress the components.

The OTP circuit is widely used to alert the system of high temperatures and shut down or take corrective measures for system protection. Temperature sensors such as thermistors, RTDs, thermocouples, and silicon ICs are available for temperature measurement. When compared to discrete sensors, integrated solutions offer clinical-grade accuracy with the lowest power consumption and smallest size. Digital thermometers accurately and quickly measure temperature. Precision temperature switches with ultra-low threshold accuracy support high-speed temperature monitoring.

High Transient Protection (HTP): a severe voltage surge for a short duration occurs primarily due to high common-mode and faults in-ground loops. Isolators (aka surge protectors) prevent DC and uncontrolled AC currents between two parts of a system, while allowing signal and power transfer between these two parts. A typical solution employed in the past was optocouplers, but the use of LEDs creates design and reliability issues. Digital isolators offer identical isolation capability with reduced power requirements and board space.

EMI/ESD Protection: Electrostatic Discharge (ESD) due to charge imbalance of dissimilar materials, and electromagnetic interference (EMI) due to electrical-noise pollution generated by an external source in the circuit, raise severe concerns for long term reliability of an analog design. Circuit miniaturization, computing equipment, and high-speed interfaces adversely affect the situation. A designer, to protect against ESD, can either add external protection or choose ICs with high levels of built-in protection. Protection circuitry includes metal-oxide varistors and silicon-based diode arrays with low input capacitance, or transient-voltage suppressors (TVSs). The use of common-mode filters for EMI protection, in combination with modern ESD protection technology, improves system-level robustness. Other techniques to minimize EMI include proper power-supply design, layout methods, and enclosure shields.

Hardware and Software Malfunction Protection: a proper monitoring system is necessary to ensure system operation during power-up, power-down, and brownout situations in processor-based systems. Protection features like manual reset, watchdog timer, battery backup, and multi-voltage monitoring provide control over malfunctions in an embedded environment. Supervisor circuits include a wide range of low power integrated devices, including watchdog timers, multi-voltage monitors, voltage trackers, reset ICs, and pushbutton controllers/debouncers. Reset ICs ensure microcontrollers or microprocessors (and their peripherals) continue in their known safe state in case of a power supply or software malfunction. Pushbutton reset ICs generate clean reset outputs recovering systems from an error condition. Watchdog timer (WDT) protection modules can be both software and hardware-based on size and practical applications.

Robust Protection Solutions and ICs

Engineers understand the environment and the application, and decide on the kind of protection needed. The following solutions explain the importance of robust protection circuits and ICs that leads to increased system performance and life span.

• Switch Debouncer using MAX16150 NanoPower Pushbutton Controller

Maxim's MAX16150 is an extremely low-power, pushbutton, on/off controller with a switch debouncer and built-in latch. It serves as an electronic switch that separates the power supply voltage from the rest of the circuit, as shown in Figure 1, providing a sanitized, glitch-free output for microcontrollers. It delivers debounce logic, input overvoltage protection to ±60V, and ESD protection to ±15kV for harsh industrial environments. The MAX16150 operates within a +1.3V to +5.5V supply range and consumes a meager ( less than 20nA) supply current to ensure minimal battery drain. All these features result in CPU time and overhead reduction, and the replacement of excessive passive components makes the entire system robust and reliable. It can operate over the -40°C to +125°C temperature range and is available in a 1mm x 1.5mm, 6-bump wafer-level package (WLP) and a 6-pin thin SOT-23 package.

Figure 1. Electronic Switch Providing Power Directly To Low-Current Load

• Battery Backup Circuit Using the MAX40200 Ideal Diode

Battery-powered portable devices need a diode ORing to power the device from the battery or charging cable, as shown in Figure 2. Diodes inherently trade off between forward voltage, leakage, size, and cost, making them unreliable and inefficient. The Maxim MAX40200 ideal diode features an internal pMOSFET to pass the current from the VDD input to the OUT output and operates from a supply voltage of 1.5V to 5.5V. It conducts with as little as 85mV of voltage drop, while carrying currents as high as 1A, and it has a typical low leakage of 70nA when reverse-biased. The overvoltage protection and ultra-low voltage drop feature helps to protect the adjacent circuit and increase comparative battery operation time. It comes in an ultra-tiny 0.73mm square 4-bump WLP or a SOT23-5 package fit and thermally self-protects to work between -40° to 125°C.

Figure 2: MAX40200: Functional Diagram and Package

Figure 3: Standard power selection circuit for portable devices.

• Smart Meter Application Using MAX22445 Isolator IC

The figure below illustrates a reference design using Maxim's MAX22445 isolator IC for a smart meter application. Smart meters use isolation to protect internal low-voltage integrated circuits from high-voltage mains and maintain signal integrity under common-mode interference. The MAX22445 is a 4-channel, 5kVRMS low power digital galvanic isolator that offers high electromagnetic interference (EMI) immunity and stable temperature performance. The MAX22445 IC consumes 0.41mA per channel current and 0.74mW power at 1Mbps, with VDD at 1.8V. Dual barriers of isolation provide 5kVRMS and 10kV surge for reliable performance over longer product lifetimes, and they satisfy relevant safety standards required for operator protection. The MAX22445 eliminates tradeoffs between isolation reliability, dynamic performance, and power consumption.

Figure 4: Smart meter application featuring MAX22445 Isolator IC

• Monitoring Microcontroller Temperature Using the MAX31875 Temperature Sensor

Maxim's MAX31875 is a high accuracy, low-power temperature sensor in a tiny 0.84 mm square WLP package with accuracy as minute as ±0.5°C. The average current drawn is less than 10 µA while operating from a 1.6 V to 3.6 V supply. A tiny package, low power consumption, and high digital accuracy help the MAX31875 to monitor microcontroller temperature inside any portable or handheld device to avoid overheating conditions. The I2C/SMBus interface reads and writes control bits with useful SMBus functions, including selectable Packet Error Checking (PEC) for reliable communications. The selectable resolution capability (up to 12 bits) provides flexibility for a tradeoff between temperature measurement resolution and power, based on design requirements.

Figure 5: MAX31875 block diagram        

Figure 6: Temperature monitoring system using MAX31875

• Portable Blood Glucose Monitoring with the MAX31341B Real-Time Clock (RTC)

Healthcare portable devices provide remote patient monitoring, and must keep track of real-time data, even during sleep mode or in the event of a main battery failure. Real-time clock (RTC) ICs embedded in electronic circuits keep track of time relative to the "real" world while rendering reliable protection, as shown in Figure 7. The external discrete RTC in the design extends battery life, increases power efficiency, and reduces overall solution size.

Figure 7: Blood Glucose Monitoring

Figure 8: MAX31341B RTC typical operating circuit.

The MAX31341B low-current real-time clock with I2C interface achieves the twin goal of minimal power consumption (180nA timekeeping) and small size (2mm x 1.5mm, 12-bump WLP) for a portable device. Its integrated, factory-calibrated crystal operates at 32.768 kHz for minute time deviation. The click board also has an onboard external (and more precise) crystal oscillator. For integrated protection, in the event of a power failure, it provides the option to automatically switch to a backup battery (or a supercapacitor) until power is restored.

• Modern System Protection Using the MAX17608 Current Limiter

Maxim's MAX17608/09/10 adjustable overvoltage and overcurrent protection devices are the industry's smallest robust integrated system protection solutions, ideal for securing systems against positive and negative input voltage faults up to +60V and -65V. The adjustable input overvoltage protection range is 5.5V to 60V, and the adjustable input under-voltage protection range is 4.5V to 59V. They integrate a pFET and nFET for forward/reverse voltage/current protection, programmable UV/OV, current-limit threshold (up to 1A) and fault response modes, and thermal protection with warning flags. The ICs come in a tiny 3mm x 3mm, 12-pin TDFN-EP package. The device enables monitoring of system current consumption via SETI pin and features reverse current blocking protection.

Figure 9. The MAX17608/09 are highly integrated, space-efficient protection ICs.

Examples of Devices for Robust Protection