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Two different materials after fiction, one bringing positive charge, and the other bringing the negative charge produce a certain voltage between the two. Voltage difference depends on the material nature of air dryness and other factors. If the object with a static near a grounded conductor, the moment it will produce a stronger discharge, which is static electricity shocks (Electrostatic Discharge). In general, an object with static, in theory, can simply be looked as small charged high voltage capacitors.

When the integrated circuit (IC) attacked by electrostatic, the discharge circuit resistance is usually very small, can not restrict discharge current. For example, the cable plug into the circuit with a static interface, the discharge circuit resistance is almost zero; this would result in up to tens of amps of instantaneous peak discharge current flows to the corresponding IC pins. Instantaneous large current will seriously damage IC, localized heating of heat even melt silicon. ESD damage to the IC also generally include that an internal metal connections have been blown, passivation layer is damaged, the transistor cell has been burned and so on.

ESD can also cause IC latch-up. This affect similar with CMOS silicon devices with activated cell. High-voltage can activate these structures to form a high-current channels, generally from VCC to ground. Locked current of serial interface device normally is 1 amp. Current will remain locked until the device is power off. But then, IC is usually burned down long ago due to overheating.

For the serial interface device, ESD will make IC not work properly, communication error occurs, more seriously even complete damage. In order to analyze causes of fault phenomena, MAXIM made ESD test of RS-232 interface device manufactured by different plants. It was found that there are two common symptoms: one is crosstalk; when the signal receiver receives signal it interfere the transmitter, resulting in error. Another failure was the IC form a reverse current within the channel, so that the receiver port receives the RS-232 signal level (± 10V) back to the power supply terminal (+5 V). If you do not have the power to absorb current regulator features, high feedback voltage may damage the parts of a single power supply (+5 V).

The simplest protective measures for the serial interface device is add extra capacitor components to each signal line. Serial resistance can limit the peak current; parallel to ground capacitance also can limit transient voltage spikes. This has the advantage of low cost, but the protection is limited. Although the destructive power of ESD can to some extent be contained, but still exists. Because the RC components does not reduce the spike voltage peak, but the slope rate of the voltage rise. And the RC components can also cause signal distortion, which limits the length of communication cable and communication speed. External resistance / capacitance also increase the circuit board area. Another widely used technique is to apply an voltage transient suppressor or TransZorbTM diode. Such protection is very effective. However, the device will increase occupation of circuit board area, and the protective effect of the device capacitance will increase the equivalent capacitance of signal lines, the cost is also higher because of higher prices TransZorbTM diode (about 25 cents / each), a typical three-fat / 5 received COM terminal needs 8 TransZorbTM diodes at a cost of up to USD 2.0.

An effective ESD test should be at the highest test voltage within the entire voltage range. Because some may pass the 10kV test, but fail in 4kV test, so that the IC virtually has no anti-static capabilities. Human Body Model and IEC1000-4-2 standard specifies a test voltage range must be based on 200V as an interval of testing, but also to simultaneously detect positive and negative voltages. In other words, start testing from ± 200V, ± 400V, ± 600V, until the maximum test voltage. Of all possible IC work pattern should have a complete ESD testing respectively, including power-on work status, power shutdown status, if the serial interface device with automatic turn-off sleep mode, should also conduct another ESD test. All the relevant testing standards and procedures provide that, in each test voltage point, the measured pin should be continuously discharge 10 times, taking into account the positive and negative voltage to be measured, the actual discharge times is 20. After the completion of each run of discharge, the device should be measured the corresponding parameters under test to determine whether any damage to the device. For the serial interface device (RS-232, RS-485) should be guided by the following criteria:

● whether the supply current is normal (supply current increase in general means that deadlock took place);

● signaling output of the output level is still within the parameters range;

● signal receiving terminal is normal (usually between 3kΩ and 7kΩ).

Only these indicators are qualified before they can go to the next voltage test point. Points at all voltages have been tested then the IC should also have a full functional test to see if IC is still within the parameters of standard definition. It is only pass all of these ESD tests that the IC is a real anti-static IC. Note that, it does not judge good or bad IC according to general criteria for the completion of ESD testing, as some ESD tester comes with a number of parameter measurement capabilities, but not a specific device parameter measurement, therefore the general test methods can only serve as a reference. Rigorous testing as described above is recommended to be test procedures and test criteria.
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