Method and circuit for generating constant slew rate output signal

[0002] The present invention relates, generally, to memory systems. More particularly, the present invention relates to an output buffer, such as may be utilized for memory applications.

[0003] In the efforts for optimizing and creating new operations in various high-speed microcontroller-based devices, such as portable personal computers (PCs), personal digital assistants (PDAS) and the like, significant attention has been given to the further improvement of memory devices.

[0004] Most new microprocessor-based applications are configured for high processing speed through implementation of dynamic random access memory (DRAM) devices, including synchronous dynamic random access memory (SDRAM) devices that can operate at significantly higher clock speeds than conventional memory devices. In particular, SDRAM devices are synchronized with the clock speed in which the microprocessor is optimized, thus enabling the number of instructions that the microprocessor can perform at a given time to be increased.

[0005] With reference to FIGS. 1A and 1B, an output buffer 100 as may be implemented within an SDRAM device comprises a pair of predriver circuits 102 and 104 and output driver devices 106. A control logic and pull-up predriver circuit 102 is provided for controlling and driving a pull-up transistor M_P0, while a control logic and pull-down predriver circuit 104 is provided for controlling and driving a pull-down transistor M_N0. Pull-up/down transistors M_P0and M_N0are further connected to a bondpad 108. Control logic and predriver circuits 102 and 104 can be configured with an internally supplied voltage V_CCR(or I/O power supply V_CCQ) to drive the gates of pull-up/down transistors M_P0and M_N0to provide an output signal.

[0006] An important characteristic in the design and operation of DRAM devices is the slew rate performance of the output buffers within the DRAM devices. The slew rate is the rate from which the output from an electronic circuit or device can be driven from one limit to another over the dynamic range of the electronic circuit or device. For DRAM devices, an ideal slew rate is between approximately 2 to 4 volts/nanosecond. The slew rate of the output signal of the output buffers in DRAM applications can significantly affect various timing specifications, including TAC, TDQSQ, and the like. As a result, it is desirable for the slew rate to be relatively constant for such output buffers.

[0007] Unfortunately, the slew rate of the output signal of such output buffers is often varied by the power supply, as well as process and temperature variations within the DRAM device. Of these reasons, changes in the power supply is the biggest impediment to constant slew rate operation. For example, for a change in power supply voltage from 2.3 volts to 2.7 volts, the slew rate of the output signal can vary by approximately 40-50% or more.

[0008] In accordance with various aspects of the present invention, a memory system includes an output buffer with an output signal having a substantially constant slew rate. In accordance with an exemplary embodiment, the output buffer comprises a slew rate control circuit and an output driver circuit. The slew rate control circuit is configured at the input terminals of the output driver circuit, for example between a predriver circuit and the output driver circuit. The slew rate control circuit is configured to suitably control the slew rate of an input signal for the output driver circuit, for example an input signal provided by a predriver circuit, based on the level of voltage of a power supply for the output driver circuit. For increases in the voltage of the power supply, the slew rate of the input signal of the output driver circuit is decreased by the slew rate control circuit, while for decreases in the voltage of the power supply, the slew rate of the input signal of the output driver circuit is increased by the slew rate control circuit. As a result of controlling the slew rate of the input signal of the output driver circuit, the variation of the slew rate of the output signal of the output buffer is significantly reduced.

[0009] In accordance with an exemplary embodiment, the slew rate control circuit comprises a first amplifier circuit for controlling the slew rate of an input signal provided to a pull-up element of the output driver circuit, and a second amplifier circuit for controlling the slew rate of an input signal provided to a pull-down element of the output driver circuit. The amplifier circuits can also be configured with current sources configured to facilitate control of the slew rate of the input signals provided to the pull-up and pull-down elements of the output driver circuit based on the level in voltage in the power supply. In an exemplary embodiment, the amplifier circuits comprise operational transconductance amplifiers (OTA's) having voltage-controlled current sources configured for controlling the biasing current for the OTA's based on the level in voltage in the power supply.

[0010] A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, where like reference numbers refer to similar elements throughout the Figures, and:

[0011] FIG. 1 illustrates a block diagram of prior art output buffers with control and predriver circuits;

[0012] FIGS. 2A and 2B illustrate block diagrams of exemplary embodiments of an electronic system with a memory system in accordance with the present invention;

[0013] FIG. 3 illustrates a block diagram of an exemplary output buffer having a slew rate control circuit in accordance with the present invention;

[0014] FIG. 4 illustrates a block diagram of an exemplary output buffer having a slew rate control circuit in accordance with an exemplary embodiment of the present invention;

[0015] FIG. 5 illustrates a schematic diagram of an exemplary voltage-controlled current source in accordance with an exemplary embodiment of the present invention; and

[0016] FIG. 6 illustrates a schematic diagram of an output buffer having a slew rate control circuit in accordance with an exemplary embodiment of the present invention.

[0017] The present invention may be described herein in terms of various functional components. It should be appreciated that such functional components may be realized by any number of hardware or structural devices configured to perform the specified functions. For example, the present invention may employ various integrated components, e.g., buffers, supply references, current sources, signal conditioning devices and the like, comprised of various electrical devices, e.g., resistors, transistors, capacitors, diodes and other components whose values may be suitably configured for various intended purposes. In addition, the present invention may be practiced in any integrated circuit application where an output buffer can be utilized. However for purposes of illustration only, exemplary embodiments of the present invention are described herein in connection with a memory chip application, such as for a DRAM device. Further, it should be noted that while various components may be suitably coupled or connected to other components within exemplary circuits, such connections and couplings can be realized by direct connection between components, or by connection or coupling through other components and devices located thereinbetween.

[0018] An electronic system according to various aspects of the present invention includes a plurality of components operating in conjunction with a supply regulation circuit. The components may comprise any components using a supply regulation circuit, such as multiple integrated circuits and electrical components on a single board, various elements in a single integrated circuit, various components of a computer system, or any other components. For example, with reference to a block diagram illustrated in FIG. 2A, an exemplary electronic system 200 suitably comprises a computer having a processor 210, a supply 212, and a memory system 214. Processor 210 controls the electronic system 200, such as in accordance with a program. Processor 210 may comprise any controlling element, for example a conventional central processing unit, such as an Intel Pentium processor or an Advanced Micro Devices Athlon processor.

[0019] Supply 212 provides power to the various components of electronic system 200, including processor 210 and memory system 214. Supply 212 may comprise any source of power for electronic system 200, such as a conventional electric power supply, a charge pump, and/or other power supplies. In the present embodiment, supply 212 is connected to processor 210 and is configured to supply at least two voltage levels. Although the present embodiment includes the processor 210, supply 212, and memory system 214, electronic system 200 may include any suitable components.

[0020] Memory system 214 stores information for subsequent retrieval. Memory system 214 may comprise any appropriate memory, memory system, or storage device or system. Memory system 214 may comprise or be supplemented by any component or system drawing power from supply 212. Memory system 214 is suitably connected to processor 210 and configured to provide information to processor 210. For example, with reference to FIG. 2B, memory system 214 of the present embodiment suitably comprises a memory 220 and a supply regulation circuit 222. Memory 220 comprises any suitable system for storing data for later retrieval, such as a memory subsystem including a memory controller, multiple memory chips, and associated logic and circuitry. In the present embodiment, memory 220 comprises a DRAM, such as an SDRAM available from Micron Technology, Inc. Memory 220 suitably includes multiple word lines and bit lines used to store information at selected addresses in memory 220.

[0021] Supply regulation circuit 222 controls the supply levels to one or more components of electronic system 200, such as memory 220. In the present embodiment, supply regulation circuit 222 is integrated into memory 220, though supply regulation circuit 222 may be integrated into other components of memory 220 or implemented as a separate circuit. Supply regulation circuit 222 according to various aspects of the present invention provides selected voltage levels to memory 220. In particular, supply regulation circuit 222 is connected to supply 212 to receive power and may be configured to generate, monitor, and regulate one or more particular voltages for memory 220. Supply regulation circuit 222 may comprise any suitable supply regulation circuit, such as a voltage control circuit, current control circuit, or any other supply regulation circuit or suitable combination of circuits. In the present embodiment, supply regulation circuit 222 is configured with an output buffer for regulating voltages within memory 220.

[0022] In accordance with various aspects of the present invention, memory system 214 includes an output buffer with an output signal having a substantially constant slew rate. In accordance with an exemplary embodiment, the output buffer comprises a slew rate control circuit and an output driver circuit. The slew rate control circuit is configured at the input terminals of the output driver circuit, for example between a predriver circuit and the output driver circuit. The slew rate control circuit is configured to suitably control the slew rate of an input signal for the output driver circuit, for example a pull-up signal and/or a pull-down signal provided by a predriver circuit, based on the level of voltage of a power supply for the output driver circuit. As a result of controlling the slew rate of the input signal of the output driver circuit, the variation of the slew rate of the output signal of the output buffer is significantly reduced.

[0023] With reference to FIG. 3, an exemplary output buffer 300 comprises a pair of predriver circuits 302 and 304, an output driver circuit 306, and a slew rate control circuit 308. A control logic and pull-up predriver circuit 302 is configured for providing an input signal, e.g., a pull-up signal P_UP, for controlling and driving a pull-up element of output driver circuit 306, while a control logic and pull-down predriver circuit 304 is configured for providing an input signal, e.g., a pull-down signal P_DOWN, for controlling and driving a pull-down element of output driver circuit 306. Predriver circuits 302 and 304 suitably comprise various logic devices and components for providing pull-up and/or pull-down signals P_UPand P_DOWN. For example, predriver circuits 302 and 304 can comprise a pair of inverter devices in series, and/or various AND gates, NAND gates, OR gates, exclusive-OR gates, in series and/or parallel to provide pull-up and/or pull-down signals P_UPand P_DOWN. In addition, predriver circuits 302 and 304 can be configured within a single predriver circuit, and/or can be configured separate from output buffer 300, i.e., an exemplary predriver circuit can comprise two predriver circuits 302 and 304 or a single predriver circuit for providing pull-up and/or pull-down signals P_UPand P_DOWN, either configured within, or separate from, output buffer 300.

[0024] Output driver circuit 306 suitably comprises a pull-up element and a pull-down element for driving an output signal OUTPUT. In the exemplary embodiment, the pull-up and pull-down elements comprise metal-oxide-silicon field-effect transistors (MOSFET's), with the pull-up element suitably comprising a pull-up element transistor M_P0and the pull-down element suitably comprising a pull-down transistor M_N0. Pull-up transistor M_P0includes a source terminal coupled to a power supply V_CCQ, while pull-down transistor M_N0includes a source terminal coupled to ground, with the respective drain terminals connected together for providing output signal OUTPUT. Although output driver circuit 306 illustrates a p-channel pull-up transistor and an n-channel pull-down transistor, output driver circuit 306 can also suitably comprise a pair of n-channel devices for the pull-up element and the pull-down element. Moreover, output driver circuit 306 can also suitably comprise other types of transistor devices, such as bipolar-junction transistors (BJT's), and can comprise any output driver configuration, now known or hereinafter developed, for driving an output signal OUTPUT.

[0025] Slew rate control circuit 308 is coupled to the input terminals of output driver circuit 306, for example, configured between predriver circuits 302 and 304 and output driver circuit 306. Slew rate control circuit 308 receives the pull-up and/or pull-down signals P_UPand P_DOWNfrom predriver circuits 302 and 304 and provides a pair of controlled input signals 310 and 312 for driving pull-up transistor M_P0and pull-down transistor M_N0. Slew rate control circuit 308 is configured to suitably control the slew rate of input signals 310 and 312 for output driver circuit 306 based on the level of voltage of power supply V_CCQfor output driver circuit 306.

[0026] For example, for increases in the voltage of power supply V_CCQ, the slew rates of input signals 310 and 312 of output driver circuit 306 are decreased by slew rate control circuit 308, while for decreases in the voltage of power supply V_CCQ, the slew rates of input signals 310 and 312 of output driver circuit 306 are increased by slew rate control circuit 308. The slew rate of output signal OUTPUT of output driver circuit 306 suitably corresponds to the slew rate of the input signals 310 and 312. In other words, decreasing the slew rate of input signals 310 and/or 312 decreases the slew rate of output signal OUTPUT, and increasing the slew rate of input signals 310 and/or 312 decreases the slew rate of output signal OUTPUT. As a result, the variation of the slew rate of output signal OUTPUT of output buffer 306 is significantly reduced, and a substantially constant slew rate of output signal OUTPUT can be realized.

[0027] Slew rate control circuit 308 can be configured in various manners for controlling the slew rate of output signal OUTPUT of output buffer 306. In accordance with an exemplary embodiment, a slew rate control circuit comprises a first amplifier circuit for controlling the slew rate of the input signal provided to the pull-up element of the output driver circuit, and a second amplifier circuit for controlling the slew rate of the input signal provided to the pull-down element of the output driver circuit. However, an exemplary slew rate control circuit can comprise any configuration for controlling the slew rate of the input signals for an output driver circuit based on the level of voltage of a power supply for the output driver circuit, including a single amplifier circuit, or multiple amplifier circuits, for controlling one or both of the input signals provided to the pull-up and/or pull-down elements of the output driver circuit.

[0028] With reference to FIG. 4, in accordance with an exemplary embodiment, an output buffer 400 comprises an output driver circuit 406 and a slew rate control circuit 408. Slew rate control circuit 408 comprises a first amplifier circuit 414 and a second amplifier circuit 416. In accordance with the exemplary embodiment, first amplifier circuit 414 and a second amplifier circuit 416 comprise operational transconductance amplifiers OTA₁and OTA₂, respectively. In this exemplary embodiment, the pull-up and pull-down signals, as may be provided from a predriver circuit, such as predriver circuits 302 and 304, have been converted to differential input signals IN⁻ and IN⁺; however, the pull-up and pull-down signals can also be configured as single-ended input signals. Amplifiers OTA₁and OTA₂are configured for receiving differential input signals IN⁻ and IN⁺, with a positive input terminal coupled to input signal IN⁻, and a negative input terminal coupled to IN⁺. The output terminals of operational transconductance amplifiers OTA₁and OTA₂are coupled to pull-up transistor M_P0and pull-down transistor M_N0to provide pull-up input signal 410 and pull-down input signal 412, respectively. Operational transconductance amplifiers OTA₁and OTA₂are configured to control pull-up input signal 410 and pull-down input signal 412 independently, i.e., without affecting one another. In addition, the output terminals of operational transconductance amplifiers OTA₁and OTA₂are coupled to parasitic capacitances of pull-up transistor M_P0and pull-down transistor M_N0, represented as C_P1and C_P2, respectively.

[0029] Although in the exemplary embodiment the pair of amplifier circuits 414 and 416 comprise a pair of operational transconductance amplifiers OTA₁and OTA₂, amplifier circuits 414 and 416 can comprise any other types of amplifiers for suitably controlling the slew rate of input signals 410 and 412. In addition, while amplifier circuits 414 and 416 include two amplifiers OTA₁and OTA₂, an exemplary slew rate control circuit 408 can comprise additional or fewer amplifiers. For example, with momentary reference to FIG. 6, an exemplary slew rate control circuit 608 can comprise a first amplifier circuit including a p-channel differential pair circuit 614 and an n-channel differential pair circuit 618, and a second amplifier circuit including a p-channel differential pair circuit 622 and an n-channel differential pair circuit 626.

[0030] With reference again to FIG. 4, first amplifier circuit 410 and second amplifier circuit 412 can also be configured with current sources configured to facilitate control of the slew rate of input signals 410 and 412 provided to the pull-up and pull-down elements of output driver circuit 406 based on the level in voltage in the power supply. For example, in accordance with the exemplary embodiment, operational transconductance amplifiers OTA₁and OTA₂can be configured with a pair of voltage-controlled current sources I_S1and I_S2, respectively. Voltage-controlled current sources I_S1and I_S2are configured for controlling biasing current for amplifiers OTA₁and OTA₂based on the level of the voltage of power supply V_CCQ. Voltage-controlled current sources I_S1and I_S2can be suitably varied to control the biasing current for amplifiers OTA₁and OTA₂, and thus control the slew rate of input signals 410 and 412 for output driver circuit 406 based on the level of the voltage of power supply V_CCQ. For example, the slew rate of pull-up signal 410 can be determined by the current of current source I_S1divided by the capacitance of parasitic capacitor C_P1, i.e., PU_SLEW=I_S1/C_P1; likewise, the slew rate of pull-down signal 412, i.e., PD_SLEW, can be determined by IS₂/C_P2.

[0031] Accordingly, if the current in current source I_S1is suitably decreased when the level of voltage of power supply V_CCQis increased, the slew rate of input signal 410 will decrease. This decrease in the slew rate of input signal 410 can be configured to suitably offset or otherwise cancel the increase in the slew rate of output signal OUTPUT caused by increases in the level of voltage of power supply V_CCQ. On the other hand, if the current in current source I_S1is suitably increased when the voltage of power supply V_CCQis decreased, the slew rate of input signal 410 will increase, resulting in the offset or otherwise cancellation of any decrease in the slew rate of output signal OUTPUT caused by decreases in the voltage of power supply V_CCQ. As a result, the slew rate variation of output signal OUTPUT can be minimized despite variations or changes in the voltage level in power supply V_CCQ, for example, to a variance of approximately 10% to 15% or less, as compared to 40% or 50% or more for prior art output buffers. Further, the slew rate variation of output signal OUTPUT of output buffer 400 can be controlled over a greater range of voltage for power supply V_CCQ, for example from approximately 2.2 volts to 2.8 volts, as opposed to a 2.3 volt to 2.7 volt specification of prior art output buffers.

[0032] Although operational transconductance amplifiers OTA₁and OTA₂are configured in the exemplary embodiment with a pair of voltage-controlled current sources I_S1and I_S2, additional current sources can be provided for controlling biasing current for amplifiers OTA₁and OTA₂based on the level of the voltage of power supply V_CCQ, e.g., operational transconductance amplifier OTA₁can be configured with a pair of voltage-controlled current sources I_S1Aand I_S1B, or additional voltage-controlled current sources I_S1X. In addition, with momentary reference again to FIG. 6, an exemplary slew rate control circuit 608 can comprise a first amplifier circuit including a current source 616 for p-channel differential pair circuit 614 and a current source 620 for n-channel differential pair circuit 618, and a second amplifier circuit including a current source 624 for p-channel differential pair circuit 622 and a current source 628 for n-channel differential pair circuit 626. Further, voltage-controlled current sources I_S1and I_S2can be configured in various manners and arrangements for controlling biasing current for the first and second amplifier circuits based on the level of the voltage of power supply V_CCQ.

[0033] With reference to an exemplary embodiment of FIG. 5, an exemplary voltage-controlled current source 500 suitably comprises a differential pair of transistors M_N1and M_N2, a third transistor M_N3, and a pair of current sources I_Aand I_B. Voltage-controlled current source 500 is configured to provide a controlled current I_Sfor biasing amplifiers OTA₁and OTA₂. Although MOSFET devices are implemented in the exemplary embodiment, other transistor-based configurations can be realized, such as other FET or BJT configurations.

[0034] The gate terminals of differential pair of transistors M_N1and M_N2are configured to receive a scaled-down supply voltage V_CCQ/K and a reference voltage V_REF(which is relatively constant over the entire power supply range). For example, in the exemplary embodiment, the gate terminal of transistor M_N1is coupled to a scaled-down supply voltage, e.g., V_CCQ/2, while the gate terminal of transistor M_N2is coupled to a reference voltage V_REF. The scaled-down supply voltage, e.g., V_CCQ/2, is assessed to determine the magnitude of the voltage of power supply V_CCQ, and can be compared to reference voltage V_REFto confirm any variations or changes in magnitude, whether due to process or temperature variations, or due to design criteria.

[0035] Scaled-down supply voltage V_CCQ/K can comprise any other suitably scaling of the voltage of power supply V_CCQ, for example, between approximately 99.9% and 1% or less of the voltage of power supply V_CCQ. In addition, reference voltage V_REFcan comprise various reference voltage levels, for example, between approximately 1.0 volt and 1.5 volts, or any other voltage levels less than or greater than the exemplary voltage. Reference voltage V_REFcomprises a value corresponding to the scaling of the voltage of power supply V_CCQ. For example, for an output buffer having a voltage of power supply V_CCQthat may vary between approximately 2.2 and 2.8 volts, and with the scaling K of the voltage of power supply V_CCQconfigured at approximately 2, i.e., power supply V_CCQis scaled down by approximately 50%, a reference voltage V_REFmay be configured with approximately 1.2 volts. Scaled-down supply voltage V_CCQ/K is assessed for any changes in the voltage of power supply V_CCQ, and can be compared to reference voltage V_REFto confirm any such variations.

[0036] The respective source terminals of differential pair of transistors M_N1and M_N2are coupled to current source I_A. The drain terminal of transistor M_N1is coupled to power supply V_CCQ. Third transistor M_N3also includes a gate terminal coupled to reference voltage V_REF, and includes a drain terminal coupled to current source I_B. The source terminals of transistor M_N2and transistor M_N3are configured for coupling to amplifiers OTA₁and OTA₂. Current sources I_Aand I_Bcan comprise any current source configuration. In the exemplary embodiment, current sources I_Aand I_Bcomprise fixed current sources, at least during operation of slew rate control circuit 408.

[0037] Voltage-controlled current source 500 provides a controlled current I_Sequal to the current flowing through transistor M_N2and transistor M_N3. The total current flowing through transistors M_N1and M_N2is equal to the current of current source I_A. In the instance where the gate-source voltages V_GSof transistors M_N1and M_N2are equal, the current flowing through each of transistors M_N1and M_N2is equal to ½ the current of current source I_A. The current flowing through transistor M_N3is equal to the current of current source I_B. Accordingly, under these conditions when the gate-source voltages V_GSof transistors M_N1and M_N2are equal, the total current from voltage-controlled current source 500 is equal to ½ the current of current source I_Aplus the current of current source I_B, i.e., I_S=½I_A+I_B.

[0038] With reference to FIGS. 4 and 5, for reference voltage V_REFequal to 1.2 volts, power supply V_CCQhaving a voltage of 2.4 volts, and the scaled-down supply voltage, e.g., V_CCQ/2, equal to 1.2 volts, the voltage at the gates of transistors M_N1and M_N2are equal. In that the respective source terminals of differential pair of transistors M_N1and M_N2are coupled to current source I_A, and the respective gate terminals have a voltage of approximately 1.2 volts applied, the gate-source voltages V_GSof transistors M_N1and M_N2are equal. As a result, the current flowing through transistors M_N1and M_N2is approximately equal. Accordingly, the total current I_Sfrom voltage-controlled current source 500 is equal to ½ the current of current source I_Aplus the current of current source I_B, i.e., I_S=½I_A+I_B.

[0039] In the event that the voltage in power supply V_CCQis decreased, for example to a voltage of 2.2 volts, the scaled-down supply voltage, e.g., V_CCQ/2, is equal to 1.1 volts, and thus is less than the voltage at reference voltage V_REFprovided to the gate of transistor M_N2. As a result, the gate-source voltage V_GSof transistor M_N1is less than the gate-source voltage V_GSof M_N2, and the current flowing through transistor M_N1is less than the current flowing through transistor M_N2.

[0040] Since the total current flowing through transistors M_N1and M_N2is equal to the current of current source I_A, the current flowing through transistor M_N2will be greater than ½ the current flowing through current source I_A. As a result, the total current from voltage-controlled current source 500 is greater than ½ the current of current source I_Aplus the current of current source I_B. i.e., I_S>½I_A+I_B. This increase in current I_Sresults in increased biasing of amplifiers OTA₁and/or OTA₂, thus resulting in an increase in the slew rate of input signals 410 and 412. Accordingly, a corresponding increase in the slew rate of output signal OUTPUT for output driver circuit 406 can be realized to offset decreases in the slew rate caused by decreases in the voltage of power supply V_CCQ.

[0041] On the other hand, in the event that the voltage in power supply V_CCQis increased from a voltage of 2.4 volts, for example to a voltage of 2.6 volts, the scaled-down supply voltage, e.g., V_CCQ/2, is equal to 1.3 volts, and thus is greater than the voltage at reference voltage V_REFprovided to the gate of transistor M_N2. As a result, the gate-source voltage V_GSof transistor M_N1is greater than the gate-source voltage V_GSof M_N2, and the current flowing through transistor M_N1is greater than the current flowing through transistor M_N2.

[0042] Again, since the total current flowing through transistors M_N1and M_N2is equal to the current of current source I_A, the current flowing through transistor M_N2will be less than ½ the current flowing through current source I_A. As a result, the total current from voltage-controlled current source 500 is less than ½ the current of current source I_Aplus the current of current source I_B, i.e., I_S<½I_A+I_B. This decrease of current I_Sresults in decreased biasing of amplifiers OTA₁and/or OTA₂, thus resulting in a decrease in the slew rate of input signals 410 and 412. Accordingly, a corresponding decrease in the slew rate of output signal OUTPUT for output driver circuit 406 can be realized to offset increases in the slew rate caused by increases in the voltage of power supply V_CCQ.

[0043] Voltage-controlled current sources I_S1, and I_S2can be configured in various manners for controlling the biasing current for amplifiers OTA₁and OTA₂. For example, the scaling of the device areas and/or sizes of the various transistors, the sizes of the current sources, or the sizes and characteristics of other components can be suitably modified to provide various levels of biasing for given changes in the voltage of power supply V_CCQ.

[0044] With reference to voltage-controlled current source 500, for example, scaled-down supply voltage V_CCQ/K provided to the gate of transistor M_N1can be configured in other scaling arrangements besides V_CCQ/2, e.g., V_CCQ/3, V_CCQ/4, V_CCQ/5 or less. In other words, scaled-down supply voltage V_CCQ/K can be configured in any other scaling arrangement. However, while K can be less than 1, values greater than 1 provide a scaled-down voltage that may be more desirable to operation of voltage-controlled current source 500. Further, the value K can be suitably selected based on desired voltage levels to be compared to the voltage at reference voltage V_REF. The scaling of supply voltage V_CCQto provide scaled-down supply voltage V_CCQ/K can be suitably generated by a scaling circuit, such as a scaling circuit 604 illustrated in FIG. 6, or any other scaling circuit configuration.

[0045] In addition, reference voltage V_REFcan be suitably adjusted to different levels to cause the increasing or decreasing of current I_Sprovided for the biasing of amplifiers OTA₁and/or OTA₂. For example, higher/lower voltage levels of reference voltage V_REFcan require higher/lower voltage levels of scaled-down supply voltage V_CCQ/K to be obtained before the current flowing within transistor M_N2is decreased/increased, i.e., before current I_Sis decreased/increased, and thus before the slew rate of input signals 410 and 412 is decreased/increased. Further, the device areas, e.g., the width/length (W/L) ratios, of transistors M_N1and M_N2can be suitably scaled to change the relative proportions of current from current source I_Athat flow through transistors M_N1and M_N2for a given scaled-down supply voltage V_CCQ/K and reference voltage V_REF. Still further, current sources I_Aand I_Bcan be suitably scaled in various fixed amounts. In accordance with an exemplary embodiment, current source I_Acan comprise a current source having an average, fixed current of between approximately 100 μA and 1 μA, while current source I_Bcan comprise a current source having an average, fixed current of between approximately 300 μA and 700 μA, e.g., approximately 500 μA. Moreover, while current sources I_Aand I_Bcan be suitably maintained at a constant value during operation, current sources I_Aand I_Bcan also be suitably tuned or adjusted to different levels prior to, or during operation.

[0046] Accordingly, voltage-controlled current sources I_S1and I_S2, for example voltage-controlled current sources 500, can be configured in various manners for controlling the biasing current for amplifiers OTA₁and OTA₂, and thus the adjustment of the slew rate of input signals 410 and 412. These adjustments of the scaling of the device areas and/or sizes of the various transistors, the sizes of the current sources, or the sizes and characteristics of other components can be suitably configured based on the amount of slew rate modification of input signals 410 and 412 desired for given changes in the voltage of power supply V_CCQ.

[0047] An exemplary slew rate control circuit, such as slew rate control circuits 308 and 408, can comprise various other configurations for controlling the slew rate of the input signals for an output driver circuit based on the voltage of a power supply for the output driver circuit, and can include various other circuits and components to facilitate predriving, scaling, or signal or reference voltage generating mechanisms.

[0048] For example, with reference to FIG. 6, an exemplary embodiment of an output buffer circuit 600 is illustrated. An output buffer circuit 600 suitably comprises predriver circuit 602, an output driver circuit 606, and a slew rate control circuit 608. Predriver circuit 602 is configured for providing a pull-up signal 634 for controlling and driving a pull-up transistor M_P0of output driver circuit 606, and a pull-down signal 636 for controlling and driving a pull-down transistor M_N0of output driver circuit 606. Predriver circuit 602 can comprise various logic devices and components for providing pull-up and/or pull-down signals 634 and 636. For example, predriver circuit 602 comprises a pair of inverter devices in series such as predriver circuits 302 and 304, and/or various other AND gates, NAND gates, OR gates, exclusive-OR gates, in series and/or parallel to provide pull-up and/or pull-down signals 634 and 636.

[0049] In the exemplary embodiment illustrated in FIG. 6, predriver circuit 602 comprises an input terminal 640 receiving an input signal V_INthrough an inverter I_X, with input terminal 640 coupled through a series of inverters INV₁through INV_Mto an input of a NAND₁gate, and coupled directly to NAND₁gate. An output of NAND₁gate is coupled to an inverter INV_Zto provide pull-down signal 636. To provide a pull-up signal 634, a NAND₂gate has an input terminal coupled through an inverter INV_Yto input terminal 640, and another input terminal coupled to the output of inverter INV_Mthrough a series of inverters INV_M+1through INV_N. Again, however, various other logic devices can be included, modified, rearranged, or removed to provide pull-up and/or pull-down signals 634 and 636.

[0050] Output driver circuit 606 suitably comprises a pull-up transistor M_P0and a pull-down transistor M_N0for driving an output signal OUTPUT, for example to a load R. The device areas of pull-up transistor M_P0and pull-down transistor M_N0can be scaled in various arrangements. Moreover, while output driver circuit 606 illustrates a p-channel pull-up transistor and an n-channel pull-down transistor, output driver circuit 606 can also suitably comprise a pair of n-channel devices, or multiple n-channel and p-channel devices. Further, output driver circuit 606 can also suitably comprise other types of transistor devices, such as bipolar-junction transistors (BJT's). Accordingly, output driver circuit 606 can comprise any output driver configuration for driving an output signal OUTPUT.

[0051] In the exemplary embodiment, slew rate control circuit 608 comprises a first amplifier circuit and a second amplifier circuit, each including two differential input pairs, e.g., the first amplifier circuit comprises p-channel differential pair circuit 614 and n-channel differential pair circuit 618 configured with voltage-controlled current sources 616 and 620, respectively, and the second amplifier circuit comprises p-channel differential pair circuit 622 and n-channel differential pair circuit 626 configured with voltage-controlled current sources 624 and 628, respectively. To generate a differential input signal from each of pull-up and/or pull-down signals 634 and 636 for the first and second amplifier circuits, output buffer circuit 600 can also comprise differential signal generator circuits 610 and 612.

[0052] Differential signal generator circuits 610 and 612 can be configured in various arrangements. In the exemplary embodiment, differential signal generator 610 comprises a transmission gate g_mand an inverter INV_G1configured to receive pull-up signal 634 and generate differential signals 642 and 644, while differential signal generator 612 comprises a transmission gate gm and an inverter INV_G2configured to receive pull-down signal 636 and generate differential signals 646 and 648. However, any other circuit configuration for generating a differential signal from a single-ended output signal can be provided for differential signal generator circuits 610 and 612.

[0053] With respect to the first amplifier circuit, p-channel differential pair circuit 614 comprises an input differential pair of p-channel devices M_P1and M_P2having gate terminals coupled to differential signals 642 and 644, source terminals coupled to voltage-controlled current source 616, and drain terminals coupled to ground through a pair of n-channel devices M_N4and M_N5, while n-channel differential pair circuit 618 comprises an input differential pair of n-channel devices M_N6and M_N7having gate terminals coupled to differential signals 642 and 644, source terminals coupled to voltage-controlled current source 620, and drain terminals coupled to power supply V_CCQthrough a pair of p-channel devices M_P3and M_P4. With respect to the second amplifier circuit, p-channel differential pair circuit 622 comprises a differential pair of p-channel devices MP₆and MP₇having gate terminals coupled to differential signals 642 and 644, source terminals coupled to voltage-controlled current source 624, and drain terminals coupled to ground through a pair of n-channel devices M_N9and M_N10, while n-channel differential pair circuit 626 comprises an input differential pair of n-channel devices M_{N11 and MN12}having gate terminals coupled to differential signals 642 and 644, source terminals coupled to voltage-controlled current source 628, and drain terminals coupled to power supply V_CCQthrough a pair of p-channel devices M_P8and M_P9. Although dual p-channel/n-channel pair configurations are illustrated in the exemplary embodiments for p-channel differential pair circuits 614 and 622 and n-channel differential pair circuits 618 and 626, other differential pair configurations can be suitably implemented.

[0054] Voltage-controlled current sources 616 and 620 and voltage-controlled current sources 624 and 628 are configured for controlling biasing current for the first and second amplifier circuits, respectively, based on changes in the voltage of power supply V_CCQ. In this exemplary embodiment, voltage-controlled current sources 616, 620, 624 and 628 comprise current sources similar in configuration to exemplary voltage-controlled current source 500; voltage-controlled current sources 620 and 628 have current sources I_Aand I_Bcoupled to ground, while voltage-controlled current sources 616 and 624 have current sources I_Aand ¹B coupled to power supply V_CCQ. However, any other current source configuration for controlling biasing current for the first and second amplifier circuits, based on the level of the voltage of power supply V_CCQcan be suitably implemented.

[0055] To generate a reference voltage, e.g., reference voltage V_REF, for voltage-controlled current sources 616, 620, 624 and 628, e.g., for coupling to the gates of transistors M_N1of current sources 616 and 624, or to the gates of transistors M_N3of current sources 620 and 628, output buffer circuit 600 can be coupled to a reference generating circuit. The reference generating circuit can comprise, for example, a bandgap voltage reference circuit configured to provide a selected reference voltage V_REF, or any other reference generating circuit, arrangement or device configured to provide a selected reference voltage V_REF.

[0056] For the scaling of supply voltage V_CCQto provide scaled-down supply voltage V_CCQ/K to voltage-controlled current sources 616, 620, 624 and 628, output buffer circuit 600 can include a scaling circuit, such as a scaling circuit 604. In the exemplary embodiment, scaling circuit comprises a series of transistors configured in resistor divider network and coupled to supply voltage V_CCQto provide scaled-down supply voltage V_CCQ/K to voltage-controlled current sources 616, 620, 624 and 628. The appropriate scaling K can be suitably configured by the number and device areas of the various transistors configured in the resistor divider network. Although transistor devices are illustrated in the resistor divider network, any other component or device, for example, resistors, capacitors and the like, can be suitably implemented in a resistor divider network. Further, scaling circuit 604 is not limited to resistor divider network configurations, and can comprise any circuit, device or configuration for generating scaled-down supply voltage V_CCQ/K to voltage-controlled current sources 616, 620, 624 and 628.

[0057] To provide controlled slew rate drive signals to output driver circuit 606, the first and second amplifier circuits comprise output stage circuits 630 and 632, respectively. Output stage circuit 630 comprises a pull-up transistor M_P5and a pull-down transistor M_N8, while output stage circuit 632 comprises a pull-up transistor M₁₀and a pull-down transistor M_N13. Output stage circuit 630 is configured to receive controlled signals from p-channel differential pair circuit 614 and n-channel differential pair circuit 618, and is configured to provide a controlled drive signal 650 to output drive circuit 606, i.e., to pull-up transistor M_P0. Output stage circuit 632 is configured to receive controlled signals from p-channel differential pair circuit 622 and n-channel differential pair circuit 626, and is configured to provide a controlled drive signal 652 to output drive circuit 606, i.e., to pull-up transistor M_N0. While output stage circuits 630 and 632 are configured for receiving a pair of drive signals from a pair of p-channel/n-channel circuits in the exemplary embodiment, output stage circuits 630 and 632 can be configured to receive only a single drive signal. Moreover, output buffer circuit 600 can be suitably configured to provide drive signals 650 and 652 directly from one or more p-channel differential pair circuits and/or n-channel differential pair circuits, or other amplifier circuits.

[0058] During operation, predriver circuit 602 can provide pull-up signal 634 and pull-down signal 636 to the first and second amplifier circuits through differential signal generator circuits 610 and 612. Slew rate control circuit 608 is configured to provide controlled input signals 650 and 652 to output driver circuit 606, with the slew rate of input signals 650 and 652 being controlled by the first and second amplifier circuits based on the level of voltage of power supply V_CCQ. Current sources 616, 620, 624 and 628 are configured to compare scaled-down supply voltage V_CCQ/K to reference voltage V_REF, and then suitably increasing or decreasing the biasing current to the first amplifier circuit comprising p-channel differential pair circuit 614 and n-channel differential pair circuit 618 and the biasing current to the second amplifier circuit comprising p-channel differential pair circuit 622 and n-channel differential pair circuit 626. As a result, the slew rate of input signals 650 and 652 can be suitably increased/decreased, resulting in the offset or otherwise cancellation of any increase or decrease in the slew rate of output signal OUTPUT caused by decreases in the voltage of power supply V_CCQ. Accordingly, the slew rate of output signal OUTPUT can be maintained substantially constant despite variations or changes in the voltage level in power supply V_CCQ.

[0059] The present invention has been described above with reference to various exemplary embodiments. However, various other changes and modifications may be made to the exemplary embodiments without departing from the scope of the present invention. In addition, any type of transistor devices configured for performing the intended functions can be utilized. These and other changes or modifications are intended to be included within the scope of the present invention, as set forth in the following claims.