Protection Product Selection

Protection elements need to be chosen carefully. Consider the very basic circuit shown in Figure 1 . An input of an integrated circuit is connected to a signal line that enters the system from an unprotected electrical environment. The signal line may be exposed to a variety of external stresses with voltage and current levels well beyond those that the input can withstand. The protection element is needed to insure that the voltage on the input remains within safe limits and shunt current away from the integrated circuit. The protection element choice depends on two things, the nature of the circuit being protected and the nature of the external stress.

Figure 1 Example of protection element use

We will consider the properties of the input being protected first. Figure 2 illustrates the normal voltage range of the input, as well as the voltage beyond which damage will result. Figure 2 also shows the I-V curve of a protection element well suited for this application. For voltages within the normal operating range the protection element's resistance is high, insuring input signal integrity. Beyond the normal operation range, the protection element switches from high impedance to low impedance. How low the on resistance needs to be to prevent the voltage reaching the danger zone depends on how much current the external stress can provide. We therefore need to understand the nature of the external stress.

Figure 2 Voltage limits on circuit

The best way to define the external stress is to use the stress definitions in industry standard tests for system robustness to electrical stress. Test standards specify the voltage and current waveforms that a product needs to survive in a given environment. Consider the example in Figure 3 . The system is required to survive a stress with a peak current of 20A. The circuit being protected has an input with an operating voltage of 0 to 3.6V and damage to the input is expected if the voltage exceeds 8V. The sample protection element turns on at 5V, safely above the normal operating voltage and can carry in excess of 20A without the voltage exceeding the unsafe operating voltage.

There are of course other considerations in the choice of protection elements. Most interfaces are more complex than a single line with respect to ground. Differential signals and systems such as the telephone network which use a combination of Primary and Secondary Protection need special considerations. The capacitance of the protection element is often more important than the low voltage resistance of the protection element, especially for high speed circuits. Physical size of the protection element, placement on the system board and cost are additional issues.

Figure 3 Example of the selection of a protection element for an application requiring a 20A peak current.

Knowing the properties of the system to be protected and the nature of the stress waveform allows an informed choice of protection devices. The protection must not turn on within the normal range of operation of the system node being protected, its capacitance must be low enough not to degrade high frequency signals, the protection element needs to be able to survive the stress itself, and the protection element must have low enough resistance in its on state to keep the voltage of the electrical node being protected below the danger region for the circuit being protected.

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