Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
1998-09-15
2001-09-04
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C323S316000
Reexamination Certificate
active
06285246
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to voltage regulators. More particularly, the present invention relates to low drop-out regulators implemented in a CMOS process.
2. Description of the Related Art
A voltage regulator is a device that produces an approximately constant output voltage. This output voltage will remain constant even if the load current changes. Similarly, the voltage regulator ensures that the output voltage remains constant for a variable input supply voltage. Accordingly, the regulator ensures that the output voltage is constant when at least one of the input voltage and the load current varies.
Referring to
FIG. 1
, a general block diagram illustrating a low drop-out regulator is presented. Typically, a low drop-out regulator
100
is used to maintain a low drop-out voltage. The drop-out voltage is the difference in voltage between an input voltage provided by an input supply
102
and an output voltage drawn by a load
104
. It is desirable to maintain a low drop-out voltage in many instances to maximize the efficiency of a circuit, therefore minimizing voltage and power loss. This is particularly important in applications where the supply voltage is low (e.g., 3-12 volts). By way of example, for an input supply voltage of 5 volts and a drop-out voltage of 3 volts, the maximum output voltage that may be produced is 2 volts. This results in greater than 50% voltage and power loss. Accordingly, such voltage and power loss is typically minimized in low drop-out mode through the use of low drop-out regulators.
Low drop-out regulators are commonly fabricated using bipolar transistors. One beneficial characteristic of bipolar transistors is an approximately constant base-emitter voltage V
BE
. Thus, V
BE
remains constant regardless of the current through the transistor. Bipolar transistors are therefore ideal for use in applications such as the low drop-out regulator.
While low drop-out regulators may be fabricated using bipolar transistors, there are numerous advantages that may be provided by CMOS transistors. Currently, CMOS transistors are commonly used in the semiconductor industry in the fabrication of integrated circuits. Thus, it would be advantageous if a low drop-out regulator could be fabricated and integrated in such integrated circuits using a single process. Moreover, implementing an existing process would allow regulator designers to take advantage of existing research, reducing the design and fabrication costs. It would therefore be desirable if a low drop-out regulator could be fabricated using CMOS transistors rather than bipolar transistors.
Although the development of low drop-out regulators in the CMOS process would be beneficial, it is difficult to achieve a functionality equivalent to low drop-out regulators developed in the bipolar process. As described above, the V
BE
of a bipolar transistor remains constant. The equivalent of the V
BE
in a bipolar transistor is the ground-source voltage V
GS
in a CMOS transistor. However, since the V
GS
is not constant, the functionality of a low drop-out regulator cannot easily be duplicated using a CMOS process.
Various methods for fabricating low drop-out regulators in the CMOS process have been attempted. However, these regulators have been capable of functioning only when the driver transistor is in saturation region which means higher power loss. Accordingly, it would be desirable if a low drop-out regulator could be developed in the CMOS process that would be operable in both the linear and the saturation regions as well as provide reduced power consumption and operating costs.
SUMMARY OF THE INVENTION
The present invention provides a low drop-out regulator and methods for providing a low drop-out regulator implemented in a CMOS process that is capable of functioning in both linear and saturation regions of the driver transistor. This is accomplished through providing accurate mirroring of the load current in both the linear and the saturation regions. In this manner, the voltage differential between the source and drain of a driver transistor is mirrored in a mirroring transistor.
According to one aspect of the present invention, the voltage differential between the drain and the source of the driver transistor is mirrored in the mirroring transistor. A driver transistor and a mirroring transistor are provided. The driver transistor is adapted for connecting to an input supply voltage and producing an output voltage. The mirroring transistor is coupled to the driver transistor and a voltage differential between the drain and the source of the driver transistor is mirrored in the mirroring transistor. This may be accomplished through coupling the source of the driver and mirroring transistors to the input supply voltage. In addition, a mirroring circuit may be provided to sense the output voltage at the drain of the driver transistor and force the voltage at the drain of the mirroring transistor to the sensed voltage. Each transistor may be implemented in a P-channel MOS transistor as well as an N-channel MOS transistor.
The advantages of implementing a low drop-out regulator in a CMOS process are numerous. By way of example, the low drop-out regulator may be fabricated and integrated in integrated circuits using a single process. Since an existing process may be used, the benefits of existing research may be obtained. Moreover, the present invention operates with a power consumption lower than provided by other CMOS low drop-out regulator designs. Accordingly, the present invention provides reduced power consumption and operating costs, as well as reduced manufacturing costs.
REFERENCES:
patent: 5867015 (1999-02-01), Corsi et al.
patent: 5929696 (1999-07-01), Lim et al.
patent: 5982226 (1999-11-01), Rincon-Mora
Gabriel A. Rincon-Mora and Phillip E. Allen A Low-Voltage, Low Quiescent Current, Low Drop-Out Regulator, Jan. 1998, IEEE Journal of Solid State Circuits, vol. 33.
Beyer Weaver & Thomas LLP
California Micro Devices, Inc.
Cunningham Terry D.
Tra Quan
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