Notable customer concerns regarding the performance of the fast switching characteristics of the SP720, SP721, and SP723 family of protection ICs, in the event of an electrostatic discharge(ESD), was addressed by LittleFuse in an application note issued on March 1997.
LittleFuse states that the SCR cell structure was first introduced to this family for the purpose of protecting the more sensitive ICs, which were subject to harsher conditions than normal human body model stress, against ESDs. The SCR accomplishes this by first absorbing energy and then diverting it away from the signal interface of the sensitive circuits.
Image via littelfuse.com.
The combination of having a wide dynamic operating range of 35v, with load input capacitance and low leakage, and minimal loading times make the SP720, SP721, and SP723 ICs capable of meeting the needs of both high performance application as well as more regular signal interfaces.
The SCR cell structure was selected because of its low series resistance path to the high current of an ESD and for its fast turn-on response. Additionally, the SCR cell structure is capable of decreasing resistance with increasing current. Active switches are paired with positive and negative SCR cells to absorb and shunt the energy of an ESD to the supply line or ground. After the energy is dispersed there is no current to support the latch holding conditions of the SCR and it returns to its off state.
By Maximilian Teodorescu
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