Active ESD Protection for Microcontrollers (EE Tip #129)

Microcontrollers need to be protected from of electrostatic discharge (ESD). You can use the circuit described in this post when you have an application requires a greater degree of ESD protection than what you get from an IC on its I/O pins. Although there are many ESD clamping devices out there, they don’t typically enable you to precisely limit voltage overshoots and undershoots.

Normally, when dealing with a microcontroller or other digital circuit the connections on the device are protected against electrostatic dis­charge. Nevertheless engineers are 4ever taking special precautions when handling such devices to avoid the risks of ESD: the lab will have an anti-static covering on the floor, and nylon clothes and shoes with soles made of insulating material are avoided. And, in case that is not enough, it is normal to wear an anti-static wrist band when moving devices from their anti-static bags to the anti-static bench surface. But what exactly do we mean when we talk about ESD?

HUMAN BODY MODEL

The first model for static discharge, mentioned as early as the 19thcentury, was the “human body model” (HBM). This takes as its starting point a voltage of up to 40 kV, a body capaci­tance of a few hundred picofarads, and a (skin) resistance of 1.5 kΩ. We find that even with a static voltage of only 10 kV, as might easily be acquired by walking across an artificial fiber car­pet in shoes with synthetic soles, it is possible to discharge through a fingertip at peak cur­rents of up to 20 A! The discharge also hap­pens in a very short period, perhaps measured in nanoseconds.

The HBM was adopted in the electronics industry in the 1970s with the introduction of sensitive JFET devices in space applications. The compo­nents were tested using a simple RC circuit like the one shown in Figure 1. The discharge cur­rent depends only on the resistance in the cir­cuit, and the damped discharge curve is largely free of oscillation and is accurately reproducible.

Figure 1—Standard test circuit and current waveform for the human body model

Figure 1—Standard test circuit and current waveform for the human body model

There are also other models that deal with dis­charge through a sensitive component, for exam­ple when a low-resistance electrical connection is made between two devices (the “machine model,” or MM), or when a static charge present on the device itself is discharged (the “charged device model,” or CDM)…

ESD CLAMP CIRCUITS

Figure 2 shows the typical protection circuitry provided on a microcontroller’s I/O port. This example is from an Atmel ATmega. Other microcontrol­lers and logic devices use similar arrangements. Two bipolar protection diodes conduct discharge currents that could cause undershoots or overshoots to one of the supply rails, either VCC or ground. However, the diodes take about 6 ns before they conduct fully.

Figure 2—Typical ESD protection circuit, as found in an Atmel microcontroller

Figure 2—Typical ESD protection circuit, as found in an Atmel microcontroller

Since ESD transients can sometimes be considerably shorter than this, it is possible that the CMOS circuit structures will be damaged long before the diodes spring into action. The parasitic capacitance of the pin is around 6 pF, and this is quickly charged up by the energy in the electrostatic discharge. Unfortunately, we cannot increase this capacitance with­out increasing the impedance of the pin, which is not desirable.

Standard ESD protection circuits like this one are designed to meet the particular requirements set by the ESD Association. However, it is becoming apparent that the traditional models are not appropriate for modern applications. Recent efforts have been directed toward developing a new “system level model” (SLM), which takes into account the different aspects of the older models. This model employs two stored charges that are discharged in different ways, creating a high-amplitude current pulse that decays very quickly plus a low-amplitude pulse that dies away more slowly. The energy transferred in a dis­charge under the SLM can be very much higher than that in the traditional models (Figure 3).

Figure 3—Current waveform under the system-level model

Figure 3—Current waveform under the system-level model

It is readily apparent that the conventional I/O pin circuitry on the IC is not sufficient to provide ESD protection under this model. Also, the con­tinuing industry pressure to make smaller and more complex structures makes it very difficult for design engineers even to maintain current levels of ESD protection, let alone improve on them. In other words: the silicon area needed to provide ESD protection in accordance with the SLM is simply not available! For this reason, external ESD clamp circuits are becoming more rele­vant. If a component provides only a low level of ESD protection (or even none at all) it is possible to add such a circuit at the points most at risk. The clamp circuits usually use so-called transient suppression diodes (transils or tranzorbs) which, like Zener diodes, start to conduct at a specified threshold voltage. However, unlike Zener diodes, they react quickly and can withstand much higher current transients. There are many variations on the circuit design, but none has exceptional performance and none offers precise clamping of voltage undershoots and overshoots.

STATE-OF-THE-ART ESD CLAMPING

If we are in the lucky position of not having to worry about the last cent of materials cost or the last square millimeter of board area, we can easily cre­ate a state-of-the-art active ESD protection circuit from discrete components (Figure 4).

Figure 4—This protection circuit clamps voltage transients outside defined upper and lower thresholds

Figure 4—This protection circuit clamps voltage transients outside defined upper and lower thresholds

The transistor circuit forms a kind of regulated voltage divider. The current through the two resistors R2 and R3 is such that the voltages across them are just enough that transistors T1 and T4 start to conduct and T2 and T3 are just short of saturation. So we have one base-emitter voltage (about 600 mV) across each of these two resistors, which means in turn that the emitters of T2 and T3 are 600 mV below VCC and above ground respectively. The circuit as shown is suitable for a 5 V supply; R1 can be changed to suit supplies of 3.3 V or 2.7 V if needed.

What is the point of this complexity? If the I/O pin is high (at +5 V) the upper 1N4148 switching diode will conduct fully as its cathode is at only 4.4 V. If a positive voltage transient should occur it will be conducted by the 1N4148, without switching delay, to the positive rail by 1N5817 Schottky diode D2, which acts quickly and has a low forward voltage. The same thing happens with polarities reversed when a negative voltage transient (below ground) occurs. Hence the digital inputs and outputs are protected against voltage excursions outside the range of the supply rails. In addition, voltage peaks are limited by the use of suppression inductors. The Murata BLM series inductor presents a relatively high impedance to signals in the 100 MHz range and so can significantly reduce the level of transients.

Although the approach we have described works well with digital levels, it is not suitable for use with signals destined for the analog-to-digital converter (ADC) on a microcontroller. In this case a reverse-biased diode between the signal and each supply rail is required to clamp overshoots and undershoots, with a pair of 10 kΩ series resistors to limit the transient current.

The series-connected capacitors C2 and C3 present a low-impedance path for transients between VCC and ground, and hence spikes on the supply rails will also be conducted away.—P. Kruger, “Active ESD Protection,” Elektor January/February 2014

Editor’s note: This article originally appeared in Elektor January/February 2014. It was shortened and updated for publication on CircuitCellar.com, which is an Elektor International Media Publication.

 

RESOURCES

www.teseq.de/de/de/service_support/technical_information/01_Transient_ immunity_testing_e.pdf

www.ti.com/lit/sg/sszb130b/sszb130b.pdf

www.semtech.com/circuit-protection/esd-protection/

www.murata.com/products/emc/basic/feature/bl_intro.html

Gigabit Ethernet Designs

WurthWurth Electronics Midcom and Lantiq recently announced The Evaluation Kit, a jointly developed demonstration kit. The kit enables users to easily add Ethernet hardware to an application or device and provides all necessary information to understand the demands of an Ethernet hardware design.

The Evaluation Kit includes an easy-to-use 1-Gbps demonstration board. The (54-mm × 92-mm) credit card-sized demonstration board is powered by USB. The board plugs into PCs and provides up to 1-Gbps bidirectional data rates.

The Evaluation Kit costs approximately $175.

Wurth Electronics Midcom, Inc.
www.we-online.com

Lantiq
www.lantiq.com

16-Bit Digitizer

SpectrumThe M2i.4960 and the M2i.4961 mid-speed 16-bit digitizers are available for PCI/PCI-X and PCIe. The devices offer two or four synchronous channels with a 60 megasamples-per-second (MSPS) speed and a 30-MHz bandwidth.

The channels can be individually switched between single-ended and true differential input mode, therefore single-ended and differential signals can be simultaneously acquired with one digitizer. Each input channel includes an on-board calibration. The channels can be software programmed for proper termination, user offset, and input range.

The devices’ acquisition modes include segmented acquisition, gated acquisition, or streaming mode. The devices also feature a versatile clock and trigger section, making them suitable for a variety of different applications. Multiple cards can be internally synchronized to obtain more synchronous channels or to directly synchronize to arbitrary and digital waveform generators or digital waveform capture cards.

A digital input option enables up to 32 synchronous digital input channels to be acquired by multiplexing them into the analog data in different ways. Each of the 16 digital inputs can completely replace one analog channel or each of the 2/4 digital inputs can be stored together with the A/D sample by reducing its resolution.

Contact Spectrum for pricing.

Spectrum GmbH
www.spectrum-instrumentation.com

The Future of Monolithically Integrated LED Arrays

LEDs are ubiquitous in our electronic lives. They are widely used in notification lighting, flash photography, and light bulbs, to name a few. For displays, LEDs have been commercialized as backlights in televisions and projectors. However, their use in image formation has been limited.

A prototype emissive LED display chip is shown. The chip includes an emissive compass pattern ready to embed into new applications.

A prototype emissive LED display chip is shown. The chip includes an emissive compass pattern ready to embed into new applications.

The developing arena of monolithically integrated LED arrays, which involves fabricating millions of LEDs with corresponding transistors on a single chip, provides many new applications not possible with current technologies, as the LEDs can simultaneously act as the backlight and the image source.

The common method of creating images is to first generate light (using LEDs) and then filter that light using a spatial light modulator. The filter could be an LCD, liquid crystal on silicon (LCoS), or a digital micromirror device (DMD) such as a Digital Light Processing (DLP) projector. The filtering processes cause significant loss of light in these systems, despite the brightness available from LEDs. For example, a typical LCD uses only 1% to 5% of the light generated.

Two pieces are essential to a display: a light source and a light controller. In most display technologies, the light source and light control functionalities are served by two separate components (e.g., an LED backlight and an LCD). However, in emissive displays, both functionalities are combined into a single component, enabling light to be directly controlled without the inherent inefficiencies and losses associated with filtering. Because each light-emitting pixel is individually controlled, light can be generated and emitted exactly where and when needed.

Emissive displays have been developed in all sizes. Very-large-format “Times Square” and stadium displays are powered by large arrays of individual conventional LEDs, while new organic LED (OLED) materials are found in televisions, mobile phones, and other micro-size applications. However, there is still a void. Emissive “Times Square” displays cannot be scaled to small sizes and emissive OLEDs do not have the brightness available for outdoor environments and newer envisioned applications. An emissive display with high brightness but in a micro format is required for applications such as embedded cell phone projectors or displays on see-through glasses.

We know that optimization by the entire LED industry has made LEDs the brightest controllable light source available. We also know that a display requires a light source and a method of controlling the light. So, why not make an array of LEDs and control individual LEDs with a matching array of transistors?

The marrying of LED materials (light source) to transistors (light control) has long been researched. There are three approaches to this problem: fabricate the LEDs and transistors separately, then bond them together; fabricate transistors first, then integrate LEDs on top; and fabricate LEDs first, then integrate transistors on top. The first method is not monolithic. Two fabricated chips are electrically and mechanically bonded, limiting integration density and thus final display resolutions. The second method, starting with transistors and then growing LEDs, offers some advantages in monolithic (single-wafer) processing, but growth of high-quality, high-efficiency LEDs on transistors has proven difficult.

My start-up company, Lumiode (www.lumiode.com), is developing the third method, starting with optimized LEDs and then fabricating silicon transistors on top. This leverages existing LED materials for efficient light output. It also requires careful fabrication of the integrated transistor layer as to not damage the underlying LED structures. The core technology uses a laser method to provide extremely local high temperatures to the silicon while preventing thermal damage to the LED. This overcomes typical process incompatibilities, which have previously held back development of monolithically integrated LED arrays. In the end, there is an array of LEDs (light source) and corresponding transistors to control each individual LED (light control), which can reach the brightness and density requirements of future microdisplays.

Regardless of the specific integration method employed, a monolithically integrated LED and transistor structure creates a new range of applications requiring higher efficiency and brightness. The brightness available from integrated LED arrays can enable projection on truly see-through glass, even in outdoor daylight environments. The efficiency of an emissive display enables extended battery lifetimes and device portability. Perhaps we can soon achieve the types of displays dreamed up in movies.

Ultra-Compact Ultrasonic Sensor Series

MaxbotixThe UCXL-MaxSonar-WR series of sensors are flexible, OEM-customizable products that can be integrated into a system with MaxBotix’s horns or flush-mounted into an existing housing. Mounting design recommendations are provided through MaxBotix’s 3-D CAD models (available in multiple formats) to facilitate your design process. The sensor layout offers four conveniently placed mounting holes for design flexibility.

The rugged, high performance sensors are individually calibrated and feature a 1-cm resolution, an operational temperature range from –40˚C to 70˚C, real-time automatic calibration (voltage, humidity, and ambient noise), 200,000+ h mean time between failures (MTBF), and an operational 3-to-5.5-V voltage range with a low 3.4-mA average current requirement.

Contact MaxBotix for pricing.

MaxBotix, Inc.
www.maxbotix.com