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Maxim > Design Support > Technical Documents > Application Notes > Analog Switches and Multiplexers > APP 4226
Maxim > Design Support > Technical Documents > Application Notes > Circuit Protection > APP 4226
Keywords: USB, universal serial bus, power-supply sequencing, mechanical relay, contact bounce, hot-swap
controller, hot swap, power-supply sequencer, hotswap, over current, thermal overload, current sense
amplifier
APPLICATION NOTE 4226
Surge Testing Solid-State USB Switches and Other
Overcurrent Protectors
Apr 08, 2009
Abstract:
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 from customer
abuse. The degree of protection is not without limits, and this article explores those limits.
A similar version of this article appeared in the October 2008 issue of Power Electronics Technology
magazine.
Introduction
With a 1.2A current limit one would think that a circuit-protection IC could maintain complete control in the
event of a fault or short. The reality is that the current limit usually demonstrates a delay time before the
actual shutoff occurs. During a hard short, the current can rise very rapidly, first hitting the DC limit and
starting to turn the switch off. (The DC limit is typically an accurate, but slow threshold. A slow threshold
avoids nuisance trips from inrush and other spurious events.) A short time later the switch opens, but not
before reaching a peak current that can be much higher than the DC limit. Low-inductance leads can cause
the current to rise even faster. See Figure 1.
Limiting Current by Resistance
Using a MAX1558 USB switch with low inductance leads and a hard short, the current is resistively limited by
the internal protection switch. When the protection circuit finally opens, the peak current (I) can be measured.
This process is shown in Figure 2. With peak current flowing through the stray input inductance (L
STRAY
),
energy (E) is stored:
E = ½ × L
STRAY
× I²
Where does this energy go once the circuit breaker or protection switch finally opens the circuit?
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