New generations of portable products are increasingly sensitive to damage from Electrostatic Discharge (ESD) voltage as newer integrated circuit (IC) technologies employ smaller geometries and lower working voltages. Because of this, designers of portable products such as cell phones, MP3 players and digital cameras must evaluate ESD protection options to ensure that they select a solution that responds to the ever-changing needs of today's ICs. This article will explain the key steps involved in selecting effective ESD protection.

ESD waveform
The most commonly used waveform for defining a typical ESD event at the system level is the IEC61000-4-2 waveform which is distinguished by its sub nanosecond rise time and high current levels (see Figure 1). The spec for this waveform calls out 4 levels of ESD magnitudes. Most designers are required to qualify their products to the highest level which is 8kV contact discharge or 15kV air discharge. When testing at the component level, the contact discharge test is the most appropriate test because the air discharge test is not repeatable on such small components.

Figure 1. IEC61000-4-2 spec; IEC 61000-4.2 Current Waveform

ESD considerations--recent design trends
The purposes of an ESD protection device is to reduce an ESD input of thousands of volts down to a safe voltage for the IC being protected and shunt current away from the IC. Although the input voltages and currents of the required ESD waveforms have not changed over the last few years, the safe voltage level required to protect ICs has decreased. In the past, IC designs were more robust to ESD and could handle higher voltages so it was sufficient to choose any protection diode that was capable of surviving the IEC61000-4-2 level 4 requirements. With newer, more sensitive ICs a designer today must not only ensure that the protection device is capable of surviving the IEC61000-4-2 level 4 standard, but also make sure that the device will clamp the ESD pulse at a low enough voltage to ensure that the IC is not damaged. When selecting the best protection device for a given application the designer must consider how low the ESD protection device clamps an incoming ESD event.

How to choose the most effective protection solution
The key DC specifications on protection diode datasheets are the breakdown voltage, leakage, and capacitance. Most datasheets will also state a max rating for IEC61000-4-2 which indicates that the diode will not be damaged by an ESD pulse at the specified level. The problem with most datasheets is that they do not have any information about the clamping voltage for high frequency, high current transients such as ESD. The clamping voltage of protection diodes is considerably higher during these types of transient events compared to the DC voltage specified on the datasheet. It is difficult to specify a clamping voltage for the IEC61000-4-2 spec, however, because it was intended to be a pass/fail spec at the system level and the frequency is so high. To apply this spec to a protection device it is crucial to examine not only if the protection diode passes or fails, but also how low it clamps the ESD voltage.

The best way to compare the clamping voltages of protection diodes is to take an oscilloscope screenshot of the actual voltage waveform across the diode during an ESD event. This is accomplished using a test setup (see Figure 2).

When looking at a voltage waveform of an ESD protection device exposed to the IEC61000-4-2 there will typically be an initial voltage spike followed by a secondary peak and eventually the voltage will level off. The initial spike is caused by a combination of the initial current spike of the IEC61000-4-2 waveform and an overshoot resulting from the inductance in the testing structure. The initial spike duration is short, however, which limits the energy transferred to the IC. The protection device's clamping performance is best displayed in the curve following the initial overshoot. The secondary peak is the primary concern because the voltage waveform holds for a longer duration, increasing the total energy that the IC will be exposed to. In the study below the clamping voltage is defined as the maximum voltage of the secondary peak.

Benchmarking Study Example
To do a fair comparison, the parts chosen should have similar package sizes and datasheet specifications. The parts chosen for this comparison are three ESD protection diodes that are considered drop-in replacements for each other when comparing electrical characteristics from the datasheets. The devices are all bi-directional ESD protection diodes that have identical breakdown voltages (6.8V), capacitance (15pf) and packages outlines (1.0 mm x 0.6 mm x 0.4 mm). The products chosen for this study were the RSB6.8CS from Rohm, the PG05DBTFC from KEC, and the ESD9B5.0ST5G from ON Semiconductor.

When comparing the DC performance of the parts above they seemingly look identical (see curve trace in Figure 2 below). In addition, they all claim to be compliant with IEC61000-4-2 level 4 standard meaning that they will all survive an ESD strike up to 8 kV contact. The crucial performance characteristic of an ESD protection device to ensure protection of a sensitive IC is not the DC performance; however, and although it is important that the device survives the IEC61000-4-2 level 4 standard it is more important that the IC being protected survives. To ensure IC survival during an ESD event, the protection diode must clamp the ESD voltage to a low enough value that the IC does not get damaged.