This document describes the features and characteristics of Fairchild Switch (FST) Bus Switch products, including the advantages of FST Bus Switch products, the possible faults of FST Bus Switch products in the presence of large amounts of system signal under-shoot voltage on I/O ports, and the anti-undershoot methods of FST Bus Switch products
The document discusses the problems of signal undershoot in bus switch applications, particularly how it can cause data corruption in standard NMOS devices. It details the physics behind these failures, including the activation of parasitic NPN transistors.
The note evaluates four potential solutions for undershoot hardening: Schottky clamp diodes, charge pumps, series NMOS configurations, and Fairchild's proprietary Undershoot Hardening Circuit (UHC). It concludes that the UHC solution found in the FSTU family provides fail-safe protection without the high power consumption associated with charge pump solutions.
An undershoot on the source node greater than or equal to 650 mV will create a positive Vgs and enable the bus switch to activate.
The device will still cause large leakage currents ranging from 10 mA to 15 mA on the output.
The quiescent current increases 100-fold over standard non-charge pump solutions, from 3uA to 3mA.