Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
1998-05-22
2001-12-04
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S278000, C327S288000, C327S404000, C326S085000
Reexamination Certificate
active
06326821
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the output buffer portion of integrated circuit devices. More particularly, the invention relates to the control of the impedance of an integrated circuit device output buffer.
2. Description of the Related Art
The increase in the operating speeds of integrated circuits (ICs) has resulted in faster rise and fall times of output voltages in response to inputs generating more system switching noise. Controlling output edge rates and output impedances is required to minimize this noise when transmitting data, e.g., at high rates. The characteristics of output voltages typically are a function of, e.g., temperature variations, process variations and voltage variations (i.e., PVT variations). Another important factor includes the actual load presented to an output terminal. For applications such as adding devices to transmission lines, it is desirable for the impedance of the output driver as seen by the PAD to be constant despite the output driver output waveform, i.e., despite varying output driver output voltage values. Furthermore, it is desirable for the impedance of the output driver to match the load impedance of the transmission line. See, e.g., Gabara et al., “Digitally Adjustable Resistors in CMOS for High-Performance Applications”, IEEE Journal of Solid-State Circuits, Vol. 27, No. 8, August 1992, pp. 1176-1185.
Conventional systems use, e.g., arrangements that include constant current sources and some type of analog circuitry to address the problems of variations of the output characteristics as a function of load. However, such conventional systems typically require a higher power drain due to the use of the constant current sources and analog circuitry fabricated on the integrated circuit. With most complementary metal-oxide semiconductor (CMOS) systems, such arrangement is relatively difficult to implement and often requires the use of bipolar-CMOS (BiCMOS) technology in order to realize these functions.
Alternatively, conventional integrated circuit (IC) output buffers include, e.g., a pre-driver stage and an output driver stage. The output driver stage includes a plurality of control bit terminals and a PAD operably connected thereto. Conventional output drivers use single transistors as the pull-up and pull-down resistors within the output driver stage. However, the performance of these pull-up and pull-down resistor arrangements typically is non-linear across the operating voltage swing at the PAD. Also, the non-linear performance is dependent on external factors such as the aforementioned PVT variations. See a discussion of such problems, e.g., in U.S. Pat. Nos. 5,194,765 and 5,243,229.
Therefore, a need exists for a method and apparatus for controlling the output impedance such that it can be maintained within acceptable limits independent of the voltage swing at the PAD and independent of PVT variations.
SUMMARY OF THE INVENTION
The invention is embodied in an integrated circuit output buffer having linear performance independent of the voltage swing. Embodiments of the invention provide an integrated circuit output buffer having a pre-drive stage and an output driver stage. The output driver stage includes a pull-up resistor arrangement having a plurality of branches connected in parallel and, alternatively at least one pull-down resistor arrangement having a plurality of branches connected in parallel. According to an embodiment of the invention, the branches of the pull-up resistor arrangement include at least one resistor and at least one transistor serially connected between a supply voltage and the output buffer output terminal connectable to the PAD external to the output buffer. The transistor in the pull-up arrangement is connected to both the data terminals from the pre-drive stage and the control bit terminals. According to another embodiment of the invention, the branches of the pull-down resistor arrangement include a resistor and at least one transistor serially connected between the ground voltage potential of the integrated circuit and the output buffer output terminal that is to be connected to the PAD. The transistor in the pull-down arrangement is connected to both data terminals from the pre-drive stage and control bit terminals. Alternatively, the pull-up transistor is replaced by a pair of transistors, e.g., a pull-up control transistor and a pull-up data transistor. Similarly, according to another alternative embodiment of the invention, the pull-down transistor is replaced by a pull-down control transistor and a pull-down data transistor. Control information received from the control bit terminals cause the resistance of the output buffer to be adjusted in a manner that compensates for variations in ambient temperature, operating voltage and processing variations (i.e., PVT variations). In this manner, the novel resistor and transistor arrangements provide output buffer performance that is linear across the operating voltage range of the output driver (e.g., 0.75-1.50 volts) and that is independent of external factors such as PVT variations.
REFERENCES:
patent: 5001369 (1991-03-01), Lee
patent: 5194765 (1993-03-01), Dunlop et al.
patent: 5220216 (1993-06-01), Woo
patent: 5243229 (1993-09-01), Gabara et al.
patent: 5258663 (1993-11-01), Tamaki
patent: 5589789 (1996-12-01), Kamiya
patent: 5621335 (1997-04-01), Andresen
patent: 5917358 (1999-06-01), Sanchez et al.
patent: 253914 (1986-07-01), None
patent: 752677 (1997-01-01), None
T.J. Gabara and S.C. Knauer, “Digitally Adjustable Resistors in CMOS for High-Performance Applications,”IEEE Journal of Solid-State Circuits, vol. 27, No. 8, Aug. 1992, pp. 1176-1185.
Agere Systems Guardian Corp.
Callahan Timothy P.
Gamburd Nancy R.
Harman John M.
Nyuyen Minh
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