Protecting against ESD

AN3960

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Doc ID 022082 Rev 1

3.5

Adding diodes

Input/output lines that are susceptible to ESD stress are sometimes protected by adding

‘external’ diodes which shunt the high energy of the ESD stress before it can reach the

device input pin. These diodes may either pass the current to the power supply rails or they

may internally dissipate the unwanted power. External diodes are similar to the diodes built

into a device (internal diodes) for protection but, they are designed differently. External

diodes have two significant advantages:

●

They can switch faster and at a lower excursion voltage than the internal diodes of the

device.

●

They can have much better connections to the supply rails and can carry more power.

The effects of external diodes on circuit operation are different from internal diodes,

because the connections used internally cannot be achieved with external devices.

Two types of protection diode are typically used against ESD stress. Zener diodes or

transient voltage suppression (TVS) avalanche diodes can be placed between an input

signal and ground. In this configuration, the diode protects the CMOS input by reverse

conduction whenever its voltage rises above the specified diode breakdown voltage.

Negative ESD excursions are shunted to ground through normal diode action. In another

configuration, diode pairs (typically Schottky diodes due to their lower forward voltage drop)

are placed between the input line and the power and ground rails. These devices protect the

CMOS input by normal diode conduction whenever the input line voltage moves outside the

range of the power supply rails.

Diodes placed on capacitive sensed lines present the same problems to capacitive sensing

circuits as they do with any analog circuit input: they can be highly capacitive (over 100 pF)

and leaky. Some Schottky pairs leak over 20 µA; some avalanche diodes leak over 1 mA

when operated near their reverse-standoff voltage (generating significant noise voltage as

well). Although these given numbers are for the least suitable devices, the most commonly-

used Schottky and TVS diodes have parasitic parameters that make them unacceptable for

use in capacitive sensing applications. If the diode circuit can be designed to add only a very

small amount of additional capacitance, capacitance sensing solutions can be adjusted to

match. This is because compensation mechanisms are usually built into the touch sensing

device for adaptation to the naturally-occurring changes in capacitance that result from

environmental changes. However, leakage and bulk capacitance can create problems for

any sort of capacitive sensing method, some more than others.

External diodes with high reverse leakage make the test capacitance look larger because

their leakage drains test current from the circuit. This disappearing test current (which

should fill the capacitance under test) has no dV/dt effect on the test load. As diode leakage

currents approach the level of the test current, the apparent load capacitance approaches

infinity. Also, the amount of current required to detect a 0.1 pF change in capacitance is less

than 20 pA, many orders of magnitude less than the leakage current for some protection

diodes. For this reason, where external diodes must be used, it is essential to specify

devices with extremely low reverse leakage.

The ESDAULC6 diode from STMicroelectronics was designed to resist multiple ESD

stresses. It has low capacitance (1 pF) and low leakage (less than 100 nA), both of which

reduce the problems encountered when using Schottky protection diodes. The bidirectional

protection ESDAXLC6 diode, with even lower capacitance (0.5 pF), can be used instead of

the ESDAULC6 diode to prevent the occurrence of negative and positive pulses.

Note:

Although small and inexpensive, an external diode circuit can be two to four times larger and

four times more expensive than adding a serial resistor.