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Browse Prior Art Database

Base Drive Resistor to Protect against BVCEO

IP.com Disclosure Number: IPCOM000084816D
Original Publication Date: 1976-Jan-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Culican, EF: AUTHOR

Abstract

When using a tri-state driver and a current switch receiver on a bidirectional line, the driver's current sink transistor can exceed BVCEO (break-down voltage collector-to-emitter with base open) under normal operating conditions. Through the proper choice of resistors R1, R2 and R4 the uplevel can be controlled to eliminate this exposure.

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Base Drive Resistor to Protect against BVCEO

When using a tri-state driver and a current switch receiver on a bidirectional line, the driver's current sink transistor can exceed BVCEO (break-down voltage collector-to-emitter with base open) under normal operating conditions. Through the proper choice of resistors R1, R2 and R4 the uplevel can be controlled to eliminate this exposure.

The breakdown exposure occurs when transistor T1 charges the line to an uplevel and then both output devices are turned off. If the driver at the far end of the line is off, the line remains at an uplevel and the load is strictly capacitive. In this case, if the voltage at node A exceeds -V+BVCEO then breakdown can occur.

Normally resistor R1 supplies base drive current for transistor T3 and by design is minimal in order to guarantee maximum uplevel. In this case resistor R4 is smaller and R2 is larger than normal to yield an Eta ratio of 1. This forces Schottky barrier diode (SBD2) to conduct increasing I1 and lowering the node A voltage. The resistor values are adjusted until BVCEO is not exceeded. Typically an uplevel clamp limits the minimum uplevel. This circuit uses the base drive resistor to clamp the maximum uplevel.

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