Fairchild AN-5008 Data Manual

-5008 F - U ersh t P ro tected airch ild itch am ily

AN-5008 Fairchild Semiconductor Application Note March 1999 Revised October 2006

FSTU - Undershoot Protected Fairchild Switch Family

Undershoot conditions can occur on busses that are poorly

Introduction

terminated, on signals with very fast edge rates, or on bus-

Fairchild Switch (FST) Bus Switch products have become

ses that allow hot-swapping. The first solution to under-

powerful tools in the modern personal computer as well as

shoot events is to ensure proper termination of

increasing in popularity in telecommunications and data

transmission lines. Some systems are designed to be

communications applications. The Fairchild Switch Family

reflective-wave and therefore rely on the impedance mis-

has a number of positive attributes over the standard bus

match to increase signal levels at the receiver. The PCI

transceivers. FST parts introduce virtually no propagation

Bus is an example of an intentionally reflective-wave bus. It

delay into the line, consume a negligible amount of power,

is reflected-wave architecture that minimizes the amount of

and introduce none of the noise issues associated with the

drive required for PCI devices. Yet these reflections can

typical high-drive bus transceiver. In most applications, use

also cause undershoot conditions.

of FST Bus Switches will alleviate the problems of the bus

Another potential source of undershoot are the early volt-

transceiver without creating new ones.

ages present during the hot-swapping of boards in applica-

In the presence of large amounts of system signal under-

tions such as ser vers and telecommunications systems.

shoot voltage on I/O po rts, the typical NMOS bus switch

Changes are continuing to be made to hot-swap sequenc-

can malfunction. Usually concern with undershoot voltages

ing and electrical requirements within standards specifica-

pertains to latch-up. This is not a con cern with FST prod-

tions such as Compact PCI. These changes make it even

ucts. FST Bus Switches have been characterized for latch-

more critical to ensure that bus switches do not malfunction

up and exceed the 500 m A upper limit of the latch-up

under such conditions.

tester. The real issue is the potential coupling of data from

For the example in Figure 1, a ssume that OE is a HIGH

input to output when the switch device is intended to be

and Bus A is switching as data is written from one device to

disabled (OPEN).

another. If Bus A transitions from a 0V to 5V, and is poorly

This application note will describe how undershoot can

terminated, the end of the bus will try to double to 2 * 5V.

cause data corruption, various methods for undershoot

This positive excursion, while poor for the system, will not

hardening (protection), and proprietary Undershoot

affect the NMOS bus switch. A LOW-going transition, how-

Hardening Circuit (UHC®) design that is now available in

ever, is a different story. In the LOW-going transition case,

Fairchild’s FSTU Undershoot Hardened Bus Switches.

5V to 0V, the voltage swing will attempt to double to –5V. This undershoot may cause problems with standard NMOS

The Undershoot Condition bus switch de vices. Although on standard bus switch

devices, there exists a P -N diode on the input of the bus

In the application of Figure 1, the bus switch is used to iso-

switch that will clamp the undershoot voltage to approxi-

late data on Bus A from the data on Bus B. When the pin

mately –650 m V, by then data corruption on the bus may

from OE is HIGH, Bus B is expected to be isolated with any

have already occurred.

activity that occurs on the Bus A.

FIGURE 1. Typical Bus Switch Application

UHC® is a registered trademark of Fairchild Semiconductor Corporation. Compact PCI is a registered trademark of Industrial Computer Manufacturer Group

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