Accurate active termination device

Details Accurate active termination device Accurate active termination device

1999-10-28

2001-10-02

Tokar, Michael (Department: 2819)

Electronic digital logic circuitry

Signal sensitivity or transmission integrity

Bus or line termination

C326S082000, C326S083000, C710S120000

Reexamination Certificate

active

06297664

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method for creating an active precision termination onto a silicon semiconductor and to a so obtained active precision termination and, in particular, the invention relates to a termination of the above mentioned type comprising a matching impedance connected in series to a switch formed by a MOS transistor.
BACKGROUND OF THE INVENTION
The present invention is the fruit of the development of a Bus Supplayer device, that is to say a voltage regulator for feeding a Bus which is portable with some standards: SCSI (Small Computer System Interface), SCSI-2, SCSI-3, as well as provided with at least nine active termination with high precision (110&OHgr; at 2%), although the description hereinbelow is made with reference to this specific field of application with the only purpose of simplifying the illustration.
A device of the above-mentioned type essentially operates as a voltage regulator and feeds every line of the Bus via a so called active termination comprising a matching impedance with a switch associate thereto. A voltage regulator and the active termination (switch+resistance) are formed inside the integrated circuit.
A regulator of known type is schematically illustrate in FIG.
1
. In conditions of operation, the regulator feeds every line of the bus with a matching impedance of high precision; in alternative, if required by the CPU that manages the Bus, the regulator can set the terminations into a state of high impedance disconnecting itself from the Bus. Hence the output of the regulator is interfaced with the Bus by means of a switch that is usually formed with a MOS transistor connected in series to a respective resistor for every line of the Bus.
A conclusive condition for the efficiency of the regulator is the precision of the matching impedance, that corresponds to the resistance “seen” by the output pin of the regulator connected to the line of the Bus in direction of the inside of the integrated circuit.
R
TERM
=R
DS
—
MOS
+R
  (1)
The termination connects to the output of the regulator, which represents a node at low impedance.
The precision of the resistance R
TERM
must be further maintained in presence of potential changes in the power supply, in temperature, and also with respect to the fluctuations of the voltage of the bus line between V
IN
and GND.
In the past, for SCSI applications the regulators of this kind were always integrated and formed interfacing them with each one of the Bus lines via resistors, and possibly switches, external to the integrated circuit. Obviously this created serious problems of overall circuit dimensions, as well as higher costs and management burden of the regulator Bus system.
In
FIG. 2
an example of a regulator formed according to this approach proposed by the prior art is shown.
A further method for obtaining a resistive termination with high precision, internal to the integrated circuit incorporating the bus, is that of using a technique known as “Laser Trimming”. Essentially, during the EWS (Electrical Wafer Sort) test phase of the integrated circuit, the resister connected in series to the MOS switch undergoes a trimming action by a laser beam in such a way to calibrate the resistive layer up to a desired value.
It is clear though that such a known solution needs an engineered system comprising the laser trimming and the technology for creating a device able to stand the trimming action.
In
FIG. 3
a schematic view of a resistive termination cell is shown, which undergoes a trimming action by means of the laser trimming technique.
SUMMARY OF THE INVENTION
Embodiments of the present invention devise a new method for forming a resistive precision termination on a silicon semiconductor substrate, thus obtaining a new type of resistive precision termination having structural and functional characteristics such to overcome the drawbacks highlighted with reference to the prior art and, in particular not to require the use of the laser trimming technique.
A cell is formed by a plurality of circuit branches provided in parallel and leading to a unique output terminal, each branch comprising an input coupled to the series of an impedance and a switch and receiving a control voltage signal. The control of the various branches by means of the control voltages is such to allow the value of the termination resistance to pass from an initial value to a final desired value with a range of variation corresponding to steps of a trimming action. In this way it is possible to form a precision resistor without using the laser trimming technique.
The features and the advantages of the method and of the circuit according to the invention will become clear from the following specification of an embodiment thereof, which is herein set as example for descriptive and non-limiting purposes, with reference to the attached drawings.


REFERENCES:
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patent: 5239559 (1993-08-01), Brach et al.
patent: 5362991 (1994-11-01), Samela
patent: 5382841 (1995-01-01), Feldbaumer
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patent: 5553250 (1996-09-01), Miyagawa et al.
patent: 5635873 (1997-06-01), Thrower et al.
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patent: 6084432 (2000-07-01), Dreps et al.
patent: 6157215 (2000-12-01), Gabara et al.
patent: 0 531 630 A1 (1993-03-01), None

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