Nikkei Electronics Asia -- March 2009
Solutions
Current Limits of Solid-State Overcurrent Protection ICs

Mar 11, 2009 15:30 Nikkei Electronics Asia

Solid-state overcurrent protection ICs, such as USB and card-slot power switches, offer a simple and robust means of protecting pins at risk of overloads or shorts during a product's testing or due to customer abuse. Without accounting for the stored energy in stray inductance, these switches can be subject to overvoltage and even destroyed. The degree of protection is not without limits, and this article explores those limits.

Input inductance can be the limiting factor for peak current, but resistance can also limit the current. One could conclude that the lower input inductance might perform better but only if the current is limited to non destructive values. If the current is not limited, the energy for a low-inductance situation can reach damaging levels very fast! The response time of the switch will dictate the degree of protection. Consequently, care must be exercised to ensure that this does not happen. 

In most PCB applications, with a ground plane under the protection switch and the input and output traces, the inductance should be much lower than 180nH. A 1/16-inch-wide PCB trace over a ground plane will have approximately 10nH per inch. Each application will have its own circumstances which dictate the size of input bypass that must be deployed.

Measurement and analysis of the expected inductance and the switch response time might indicate that a larger bypass is needed to provide reliability. Conversely, it could indicate that a reduced input bypass capacitor is allowed.

by Steve Mannas, Member of the Technical Staff, Applications, Maxim Integrated Products Inc

Full article:
http://techon.nikkeibp.co.jp/NEA/solutions/0903002.pdf

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