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Test Methodology for Detecting Missing Emitter Follower Resistors for Differential Current Switch Circuits

IP.com Disclosure Number: IPCOM000039881D
Original Publication Date: 1987-Aug-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 37K

Publishing Venue

IBM

Related People

Rizzolo, RF: AUTHOR

Abstract

A method is described for detecting missing emitter follower pull-down resistors in a differential cascode current switch circuit (DCS circuit) such as is shown in the drawing. The emitter follower pull-down resistors are REL, RER, RSL and RSR. If any of these resistors is missing, or the metal lines broken at the arrows 10, this circuit fails. This failure is a delay failure, and under normal operating conditions, it is not DC detectable, meaning the steady-state voltage levels on the chip outputs do not indicate a failure. The method described herein enables DC detection of such faults. In practice, standard stuck-fault or random patterns may be used to test for these faults, along with other stuck faults on a standard LT tester.

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Test Methodology for Detecting Missing Emitter Follower Resistors for Differential Current Switch Circuits

A method is described for detecting missing emitter follower pull-down resistors in a differential cascode current switch circuit (DCS circuit) such as is shown in the drawing. The emitter follower pull-down resistors are REL, RER, RSL and RSR. If any of these resistors is missing, or the metal lines broken at the arrows 10, this circuit fails. This failure is a delay failure, and under normal operating conditions, it is not DC detectable, meaning the steady-state voltage levels on the chip outputs do not indicate a failure. The method described herein enables DC detection of such faults. In practice, standard stuck-fault or random patterns may be used to test for these faults, along with other stuck faults on a standard LT tester. When a pull-down resistor is missing from an emitter follower, the current through the emitter greatly decreases because the only current flowing is base current due to the loading of the following stage. This decrease in current causes a corresponding decrease in VBE . This decrease is given qualitatively by: Typically, iL' = iL = ir/100. Inserting these values yields a VBE of -120mV. This means that with a missing emitter follower resistor the emitter is 120 mV higher than it should be. The fault is DC detected in accordance with this method by decreasing VX until the signal swing (difference between out and -out) is less than 1...