AND823D

they are often inadequate for protecting against surge events that occur in normal product usage. The surge ability of a silicon TVS diode is directly related to its size and external devices are typically a factor of at least ten times larger than the internal IC TVS devices. External TVS diodes provide a higher level of surge protection because it is typically not practical for an IC to incorporate large protection devices. In addition, the internal protection circuit of most ICs is designed to handle only a few ESD events while an external TVS device provides immunity for an indefinite amount of surges.

Knowledge of the IC's internal surge protection circuit can be helpful in selecting an external TVS device with an appropriate power rating and turn−on voltage. Unfortunately, the data sheets of most ICs provide only an ESD rating and do not disclose the internal protection circuit. Internal IC protection circuits can be created using high voltage transistors, Zener diodes, diode arrays, thyristors and overvoltage detection switches. Figure 1 provides two popular IC circuits used to provide internal surge protection. Guidelines to ensure that a surge event will not exceed the power rating of the internal protection circuit will be shown in the following sections.

Transient Voltage Suppression (TVS) diodes  AND823D provide a simple solution to increase the EMI and ESD immunity level of a circuit and only a few guidelines must be followed to provide effective surge protection. This document will analyze several important application features of avalanche TVS and diode arrays. In addition, examples will be provided to demonstrate the advantages and disadvantages of uni and bidirectional TVS devices. The following AND823D circuit design considerations will be analyzed:
• Internal IC versus external TVS protection circuits
• TVS diode turn−on voltage
• Diode array application hints
• Common mode offset voltages
• Back drive protection
• Differential input and output application guidelines