Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Having specific delay in producing output waveform
Reexamination Certificate
2002-06-11
2003-11-25
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Having specific delay in producing output waveform
C327S276000, C327S277000, C327S170000
Reexamination Certificate
active
06653882
ABSTRACT:
FIELD OF INVENTION
The invention relates to an output driver for integrated circuits and a method for controlling the output impedance of an integrated circuit.
BACKGROUND OF THE INVENTION
Output drivers are important building blocks in the input/output (I/O) path of integrated circuits like microprocessors and memory systems. They act as the primary interface through which data transmission takes place between the integrated circuit and external systems via transmission lines. For this end, the output driver has to convert the chip-internal logic levels and noise margins to those required for driving the inputs of chip-external circuits in digital systems.
The operation of the output driver of an integrated circuit in certain temperature and supply voltage ranges have to satisfy requirements in both DC and AC conditions, which are specified at the outset of design. The AC operating conditions for the outputs determine the properties of the signal-transients which are to be performed by the output drivers at given DC signal levels, and include the required rise time t
r
and fall time t
f
of the output signal when the data output is connected to a specific load impedance. Thus, in order to avoid undesired degradations of the output data, the output impedance of the output driver has to be properly matched to the characteristic impedance of the transmission line connecting the data output to the chip-external system.
For lossless transmission lines which are terminated by purely ohmic impedances (Z
L
=R), the voltage reflection coefficient &rgr;
v
is given by &rgr;
v
=(R−Z
0
)/(R+Z
0
) wherein Z
0
is the characteristic wave impedance of the transmission line. Therefore, if the characteristic impedance of the transmission line Z
0
matches the output impedance of the output driver, no signal reflection occurs and &rgr;
v
=0.
The characteristic impedance of the transmission may have a slight temperature dependency. Furthermore, the channel resistances of the n- and p-MOS transistors of the output driver will vary with the (manufacturing) process (P), the operating voltage (V) and the operating temperature (T). In order to minimize or avoid impedance mismatches caused by such PVT-variations, so called PVT-sensors have been applied in conventional output drivers. These sensors detected PVT changes and, using predetermined impedance correction data, correspondingly adjust the output impedance of the output driver. However, applying PVT-sensors in integrated circuits has severe drawbacks in that they are complicated circuits with limited impedance correction accuracy.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an output driver for integrated circuits (such as memory or microprocessor circuits), which allows for a simple and efficient control of the output impedance thereof. It is further an object of the invention to provide a method for controlling the output impedance of an integrated circuit.
These problems are solved by an output driver according to claim
1
and a method according to claim
10
, respectively. Preferred embodiments are subject of the dependent claims.
According to the invention an output driver for integrated circuit includes:
at least one driver circuit for driving an external circuit, the driver circuit having
at least one data input connectable to the integrated circuit;
at least one data output connectable to a transmission line leading to the external circuit; and
impedance adjusting means for adjusting the output impedance of the driver circuit according to determinable impedance adjusting data;
at least one dummy circuit comprising a dummy driver circuit and a dummy transmission line, the dummy driver circuit and the dummy transmission line being electrical replica of the driver circuit and of the transmission line, respectively; and
at least one impedance control circuit for controlling the output impedance of the driver circuit, the impedance control circuit being connected to the dummy circuit and the impedance adjusting means;
wherein the impedance control circuit is adapted to control the impedance of the driver circuit by determining the impedance adjusting data necessary for matching the output impedance of the dummy driver circuit to the characteristic impedance of the dummy transmission line and outputting the determined impedance adjusting data to the impedance adjusting means of the driver circuit.
Hence, according to one aspect of the invention, the output driver includes an additional “dummy” circuit, which is essentially used to determine the impedance adjusting data necessary for matching the output impedance of the driver circuit to the characteristic impedance of the transmission line. This dummy circuit includes a replica of the (actual) driver circuit as well as a replica of the (actual) transmission line. Thus, the actual driver circuit and the actual transmission line are electrically or physically modeled as dummy replica circuits in the output driver. It is to be understood that the dummy circuit does not have a data output connectable to the external circuit, i.e. the dummy transmission line is not connected to the external circuit.
Similarly, the dummy driver circuit also includes electrical replica of the impedance adjusting means. The impedance control circuit uses this dummy circuit to determine which impedance adjusting data, under the present operating voltage and temperature, are necessary for matching the impedances. Since the dummy driver circuit and the dummy transmission line are designed to have the same electrical characteristics as the actual driver circuit and the actual transmission line, the impedance adjusting data thus determined are suitable for an efficient matching of the driver circuit to the transmission line connected thereto. Consequently, a complicated PVT-sensor is not necessary. Instead, changes of the channel resistances of the driver transistors of the driver circuit resulting in changes of the output impedance are detected by the impedance control circuit using the dummy circuit.
Preferably, the dummy driver circuit is a scaled down replica of the driver circuit. The scaling factor might, for example, lie within the range of two to ten. Scaling down the transistor dimensions of the dummy driver circuit relative to those of the driver circuit is favorable, since smaller transistors in the dummy circuit will dissipate less power. Therefore, less heat is generated by the dummy circuit so that such an output driver is also applicable in low-energy applications.
The dummy driver circuit does not need to be an exact replica of the driver circuit in a geometrical sense. Although the dummy driver circuit might be designed to represent an exact geometrical miniature replica of the driver circuit (for example scaled down by the geometrical factor of eight), it is equally possible to use a different (favorably simpler) design for the dummy driver circuit as long as its electrical characteristics closely resemble those of the (actual) driver circuit. Increases of the channel resistances of scaled down transistors in the dummy driver circuit have to be accounted for when determining the impedance adjusting data used for matching the output impedance of the driver circuit. Beneficially, if the transistors in the dummy driver circuit are scaled down by a geometrical scaling factor of k (resulting in an increase of the channel resistances by this factor of k), the dummy transmission line is designed to have a characteristic impedance of k*Z
0
, wherein Z
0
is the characteristic impedance of the actual transmission line. Thus, the impedance of the dummy transmission line is scaled up by k, if the transistors of the dummy driver circuit are scaled down by k.
Favorably, the dummy transmission line includes at least one polysilicon resistor and/or metal resistor. As described above, impedance mismatches between the driver circuit and the transmission line connected thereto can arise due to variations of the electrical characteristics of the transistors o
Fish & Richardson P.C.
Infineon - Technologies AG
Tran Toan
LandOfFree
Output drivers for IC does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Output drivers for IC, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Output drivers for IC will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3184519