# Patent

## Title
Programmable low power high-speed current steering logic (LPHCSL) driver and method of use

## Abstract

An integrated circuit comprising, a voltage regulator circuit and a programmable low power high-speed current steering logic (LPHCSL) driver circuit coupled to a common supply voltage. The voltage regulator circuit includes a native source follower transistor having a negative threshold voltage to provide more headroom for the voltage regulator to operate. The LPHCSL driver circuit includes a plurality of selectable output driver legs and a plurality of programmable resistors. The ability to use a common supply voltage and the ability to select multiple output impedance drivers reduces the die area without increasing the complexity of the integrated circuit.

## Background
BACKGROUND OF THE INVENTION
Differential signaling is known in the art as a method of transmitting information electronically by means of two complementary signals sent on two separate traces in which a receiving device reads the difference between the two received signals. A driver circuit is generally used to provide the differential signal to be transmitted to the receiver circuit.
Low power high-speed current steering logic (LPHCSL) drivers are known in the art as a means of providing differential signaling. In contrast with traditional HCSL, which steers a constant current between true and complement outputs of a differential pair, low power HCSL (LPHCSL) uses a push-pull voltage drive instead of a current drive. Current consumption is reduced in a programmable LPHCSL driver because supply current flows in the driver only during the round trip flight time, which is equivalent to the time required for a rising edge of the signal from the LPHCSL driver to be received at the receiver and the time required for the signal to be returned back at the driver.
Current architectures of LPHCSL drivers have limitations on the range of the supply voltage of operation. Additionally, LPHCSL driver designs currently known in the art provide multiple driver impedances by adding and/or subtracting driver legs, which has a negative impact on the die area and increases the complexity of the device for slew rate control and impedance load matching.
Accordingly, what is needed in the art is a programmable LPHCSL driver circuit having an improved supply voltage range and a higher degree of programmability for impedance matching that does not negatively impact the die area required for the circuit.
SUMMARY
The present invention describes a programmable LPHCSL driver circuit having an improved supply voltage range and a higher degree of programmability for impedance matching that does not negatively impact the die area required for the circuit and an associated method of use.
In one embodiment, an integrated circuit is provided comprising, a voltage regulator circuit and a programmable low power high-speed current steering logic (LPHCSL) driver circuit coupled to a common supply voltage. The voltage regulator circuit includes a native source follower transistor having a negative threshold voltage to provide more headroom for the voltage regulator to operate. The programmable LPHCSL driver circuit includes a plurality of selectable output driver legs and a plurality of programmable resistors. The ability to use a common supply voltage and the ability to select multiple output impedance drivers reduces the die area without increasing the complexity of the integrated circuit.
In accordance with one embodiment, an integrated circuit is provided including a voltage regulator circuit having a voltage comparator and a native source follower transistor, wherein the common supply voltage is coupled to the voltage comparator and the native source follower transistor. The integrated circuit further includes, a programmable low power high-speed current steering logic (LPHCSL) driver circuit comprising a plurality of selectable output driver legs and a plurality of programmable resistors and a common supply voltage coupled to the voltage regulator circuit and to the programmable LPHCSL driver circuit. The integrated circuit may further include, a control logic circuit coupled to the programmable LPHCSL driver circuit, the control logic circuit for selecting one or more of the selectable output driver legs and for programming one or more of the plurality of programmable resistors.
In a particular embodiment, the native source follower transistor is a native n-channel metal-oxide-semiconductor (NMOS) transistor having a negative threshold voltage of approximately 0.2V.
In a specific embodiment, the common supply voltage is selected to be 3.3V, 2.5V or 1.8V, the reference voltage is between about 0.6V and 1.2V, the output voltage at the second terminal of the voltage regulator and the drain of the native source follower transistor is between about 0.6V and 1.2V and each of the plurality of programmable resistors are programmable to provide a resistance of 100Ω, or 85Ω.
In accordance with the present invention, a method for providing a low power high-speed current steering logic (LPHCSL) driver output signal includes, providing a selected supply voltage to a voltage regulator circuit and a programmable LPHCSL driver circuit, wherein the voltage regulator circuit comprises a native source follower transistor, providing a desired reference voltage to the voltage regulator circuit to set an output level for an output signal from the LPHCSL driver circuit and selecting one or more of a plurality of selectable output driver legs of the LPHCSL driver circuit and programming one or more of a plurality of programmable resistors of the LPHCSL driver circuit to provide an output signal from the LPHCSL driver having a desired impedance across the selected supply voltage.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention, and together with the description, serve to explain the principles of the invention.
FIG. 1 is a block diagram of a programmable low power high-speed current steering logic (LPHCSL) driver, in accordance with an embodiment of the present invention.
FIG. 2 is a schematic diagram illustrating a programmable low power high-speed current steering logic (LPHCSL) driver, in accordance with an embodiment of the present invention.
FIG. 3 is a detailed schematic illustrating an exemplary pre-driver circuit and an exemplary programmable resistor of a programmable low power high-speed current steering logic (LPHCSL) driver, in accordance with an embodiment of the present invention.
FIG. 4 is a flow diagram illustrating a method for providing an output voltage from a programmable LPHCSL driver, in accordance with an embodiment of the present invention.



DETAILED DESCRIPTION
Representative embodiments of the present invention are described below with reference to various examples wherein like reference numerals are used throughout the description and several view of the drawings to indicate like or corresponding parts and further wherein the various elements are not necessarily drawn to scale.
With reference to FIG. 1, an integrated circuit according to an embodiment of the present invention is illustrated and generally designated 100. In at least some embodiments, the integrated circuit 100 includes a voltage regulator circuit 115, a programmable low power high-speed current steering logic driver circuit 110 and a control logic circuit 160. A common supply voltage 170 is coupled to the voltage regulator circuit 115 and to the programmable LPHSCL driver circuit 110 and a load 120 is coupled across the true output 140 and the complimentary output 142 of the programmable LPHCSL driver circuit 110. In operation of the integrated circuit 100, a common supply voltage 170 is provided to the voltage regulator circuit 115 and the programmable LPHCSL driver circuit and an output level 130 of the voltage regulator circuit 115 is provided to the programmable LPHCSL driver circuit 110 to set the output level of the true output 140 and the complimentary output 142 of the LPHCSL driver circuit 110 in response to a true input 150 and a complimentary input 152 to the programmable LPHCSL driver circuit 110. The impedance of the LPHSCL driver circuit changes based upon the common supply voltage 170 and it is desirable to maintain a substantially constant impedance over the supply voltage range in order to minimize complexity and reduce the cost of system implementations employing the LPHSCL driver circuit 110. In addition, the impedance requirements for the LPHSCL driver circuit 100, for the same common supply voltage 170, can vary based upon the target application. In order to maintain a substantially constant impedance at the output of the LPHCSL driver circuit 110, a control logic circuit 160 is coupled to the programmable LPHSCL driver circuit 110 to provide control signals to the programmable LPHSCL driver circuit 110 to select one or more driver legs and one or more programmable resistors to control the output impedance of the LPHSCL driver circuit 110.
With reference to FIG. 2, an integrated circuit 200 in accordance with one embodiment of the present invention includes a voltage regulator circuit 115 for providing an output level 130 to a programmable LPHCSL driver circuit 110. In at least one embodiment, the voltage regulator circuit 115 includes a voltage comparator 210 having a first input for receiving a reference voltage 205 and a bias input coupled to the common supply voltage 170. In a particular embodiment, the reference voltage 205 is between about 0.6V and 1.2V and the common supply voltage 170 is between about 3.3V and 1.8V. The reference voltage 205 may be generated using techniques known in the art, such as bandgap and switches. The voltage regulator circuit 115 further includes a native source follower transistor 215 coupled to the voltage comparator 210. In particular, the native source follower transistor 215 has a gate coupled to an output of the voltage comparator 205, a drain coupled to the common supply voltage 170 and a source coupled to the LPHCSL driver circuit 110 and to a second input of the voltage comparator 210. The voltage regulator circuit 115 further includes a first capacitor 220 having a first terminal coupled between the output of the voltage comparator 210 and the gate of the native source follower transistor 215 and a second terminal coupled to ground 290. The first capacitor 220 provides required stability margins and bandwidth. The voltage regulator circuit 115 further includes a second capacitor 225 having a first terminal coupled between the source of the native source follower transistor 215 and a second input of the voltage comparator 210 and a second terminal coupled to ground 290 and a first resistor 230 having a first terminal coupled between the source of the native source follower transistor 215 and a second input of the voltage comparator 210 and a second terminal coupled to ground 290. The second capacitor 225 in the feedback path acts as a bypass capacitor and the resistor 230 in the feedback path sets up the output voltage level 130 at the output of the native source follow transistor 215 to match the reference voltage 205 input to the voltage comparator 210. In a particular embodiment, the output voltage level 130 may be between about 0.6V and 1.2V. In the present embodiment the native source follower transistor 215 is a native n-channel metal-oxide-semiconductor (NMOS) transistor. In one embodiment, the native source follower transistor 215 is a native n-channel metal-oxide-semiconductor (NMOS) transistor having a negative threshold voltage. In a specific embodiment, the native source follower transistor 215 may be a native n-channel metal-oxide-semiconductor (NMOS) transistor having a negative threshold voltage of approximately 0.2V.
The prior art limitations on low supply voltages are primarily a result of headroom issues related to the voltage regulator driving the output source follow transistor are commonly resolved by utilizing separate supply voltages for the voltage regulator and the LPHSCL driver circuit. However, utilizing separate supply voltages limits the programmable lower supply voltage range. To overcome this limitation, the present invention utilizes a high voltage tolerant native source follower transistor 215 having a negative threshold voltage, thereby providing more headroom, which allows the voltage regulator circuit 115 to operate at a much lower voltage. The implementation of the present invention allows for low voltage (i.e. 1.8V) operation of the voltage regulator and the LPHCSL driver circuit and for sharing of a common supply voltage, which allows a more dynamic range of voltages (i.e. 3.3V-1.8V), resulting in reduced complexity in circuitry and ESD protection schemes as result of a simplified supply voltage distribution requirement. Accordingly, the use of a native source follower transistor 215 in the voltage regulator circuit 115 provides the implementation of a highly versatile LPHCSL driver circuit 110 that is capable of operating across multiple supply voltages (3.3V-1.8V), with a reduced die area and an acceptable signal profile.
With reference again to FIG. 2, the integrated circuit 200 further includes a programmable LPHCSL driver circuit 110 coupled to the voltage regulator circuit 115 and the programmable LPHCSL driver circuit 110 further includes a plurality of programmable resistors 270, 272, 274, 276 and a plurality of selectable output driver legs. In the embodiment illustrated in FIG. 2, the plurality of selectable output driver legs include at least one first transistor 240 coupled to an output of first pre-driver circuit 280 that is driven by a true input signal 150 and at least one second transistor 242 coupled to an output of a second pre-driver circuit 282 that is driven by the complement 152 of the true input signal 150. A first variable resistor 270 and a second variable resistor 272 are coupled in series with the first transistor 240 and the second transistor 242, thereby providing a true output signal 140 at a node between the first variable resistor 270 and the second variable resistor 272. In a specific embodiment, one of the plurality of selectable output driver legs may be considered as including a selectable pull-up transistor 240 having a drain coupled to an output 130 of the voltage regulator circuit 110 and a selectable pull-down transistor 242 having a source coupled to ground 290 and at least two of the plurality of programmable resistors 270, 272 coupled in series between a source of the pull-up 240 transistor and a drain of the pull-down transistor 242 to provide an output signal of the programmable LPHCSL driver coupled between the series programmable resistors 270, 272. In this circuit configuration, the pull-up transistor 240 is coupled to a first pre-driver circuit 280 that is driven by a true input signal 150 and the pull-down second transistor 242 is coupled to a second pre-driver circuit that is driven by a complementary input signal 152 provides a true output signal 140 at an intersection of a pair of series programmable resistors 270, 272 based upon a selected common supply voltage 170 and a desired impedance output.
In the exemplary embodiment, illustrated in FIG. 2, three selectable driver legs and four programmable resistors are used to provide selected combinations of three voltage levels (3.3V, 2.5V and 1.8V) and at two resistances (100Ω, and 85Ω) to provide a selected combination of series resistance and driver strengths of: 3.3V/100Ω, 3.3V/85Ω, 2.5V/100Ω, 2.5/85Ω, 1.8V/100Ω and 1.8V/85Ω. The control logic circuit 160 is coupled to the programmable LPHCSL driver to provide input signals for selecting one or more of the selectable output driver legs and for programming one or more of the plurality of programmable resistors, as will be explained in more detail with reference to FIG. 3.
Referring again to FIG. 2, the programmable LPHCSL driver 110 further includes a plurality of pre-driver circuit 280, 282, 284, 286, wherein each of the plurality of pre-driver circuits 280, 282, 284, 286 are coupled to one of a plurality of selectable output driver legs and the common supply voltage 170 is coupled to an enable input of each of the plurality of pre-driver circuits 280, 282, 284, 286. In general, each of the pre-driver circuits has an input coupled to a true input or a complimentary input signal and one or more outputs coupled to a gate of one of each of the plurality of selectable output driver legs. Additionally, each of the plurality of selectable output driver legs includes a first n-channel metal-oxide-semiconductor (NMOS) transistor having a gate coupled to a pre-driver circuit and a drain coupled to an output from the voltage regulator circuit 115, a first programmable resistor having a first terminal coupled to a source of the first NMOS transistor, a second programmable resistor having first terminal coupled to a second terminal of the first programmable resistor and a second n-channel metal-oxide-semiconductor (NMOS) transistor having a gate coupled to the pre-driver circuit, a drain coupled to a second terminal of the second programmable resistor and a source coupled to ground.
In the exemplary embodiment shown in FIG. 2, employing NMOS transistors in the programmable LPHCSL driver circuit, a first pre-driver circuit 280 and a second pre-driver circuit 284. The first pre-driver circuit 280 having an input coupled to a true input 150 and the second pre-driver circuit 284 having an input coupled to a complimentary input 152. The output of the first pre-driver circuit 280 is coupled to the gate of each of three parallel-coupled NMOS transistors 240, 250, 260 and the output of the second pre-driver circuit 284 is coupled to the gates of three parallel-coupled NMOS transistors 244, 254, 264. The drain of each of the NMOS transistors 240, 250, 260, 244, 254, 264 is coupled to the output 130 of the voltage regulator circuit 115 and the source of each of the NMOS transistors 240, 250, 260 is coupled to a first terminal of a first variable resistor 270 and the source of each of the NMOS transistors 244, 254, 264 is coupled to a first terminal of a second variable resistor 274. The programmable LPHCSL driver circuit further includes, a third pre-driver circuit 282 and a fourth pre-driver circuit 286. The third pre-driver circuit 282 having an input coupled to a complimentary input 152 and the fourth pre-driver circuit 286 having an input coupled to a true input 150. The output of the third pre-driver circuit 282 is coupled to the gate of each of three parallel-coupled NMOS transistors 242, 252, 262 and the output of the fourth pre-driver circuit 286 is coupled to the gates of three parallel-coupled NMOS transistors 246, 256, 266. The drain of each of the NMOS transistors 242, 252, 262 is coupled to a second terminal of a third variable resistor 272 and the first terminal of the third variable resistor 272 is coupled to a second terminal of the first variable resistor 270. The drain of each of the NMOS transistors 246, 256, 266 is coupled to a second terminal of a fourth variable resistor 276 and the first terminal of the fourth variable resistor 276 is coupled to a second terminal of the second variable resistor 274. The source of each of the NMOS transistors 242, 252, 262, 246, 256, 266 is coupled ground 290. In this configuration, three selectable driver legs are provided for driving three voltage levels of 3.3V, 2.5V and 1.8V and the four programmable resistors are provided output resistances of 100Ω or 85Ω to provide impedance matching. As such, a combination of selected driver legs and resistance values provides 100Ω or 85Ω impedances across 3.3V, 2.5V and 1.8V.
With reference to FIG. 3, an integrated circuit 300 is illustrated showing the voltage regulator circuit 115 and a partial view of the programmable LPHCSL driver circuit 110. The partial view of the programmable LHPCSL driver circuit illustrates one true and complementary circuit configuration in additional detail. As shown in FIG. 3, and as previously described, the voltage regulator circuit 115 includes a voltage comparator 210 having a first input for receiving a reference voltage 205 and a bias input coupled to the common supply voltage 170 and in a particular embodiment, the reference voltage 205 is between about 0.6V and 1.2V and the common supply voltage 170 is between about 3.3V and 1.8V. The voltage regulator circuit 115 further includes a native source follower transistor 215 coupled to the voltage comparator 210. In particular, the native source follower transistor 215 has a gate coupled to an output of the voltage comparator 205, a drain coupled to the common supply voltage 170 and a source coupled to the LPHCSL driver circuit 110 and to a second input of the voltage comparator 210. The voltage regulator circuit 115 further includes a first capacitor 220 having a first terminal coupled between the output of the voltage comparator 210 and the gate of the native source follower transistor 215 and a second terminal coupled to ground 290. The voltage regulator circuit 115 further includes a second capacitor 225 having a first terminal coupled between the source of the native source follower transistor 215 and a second input of the voltage comparator 210 and a second terminal coupled to ground 290 and a first resistor 230 having a first terminal coupled between the source of the native source follower transistor 215 and a second input of the voltage comparator 210 and a second terminal coupled to ground 290. In the embodiment illustrated in FIG. 3, the output voltage level 130 of the voltage regulator 115 is coupled to a drain of three parallel-coupled pull-up NMOS transistors 240, 250, 260 and the source of the three parallel-coupled NMOS transistors 240, 250, 260 is coupled to a first terminal of a first variable resistor 270. A gate terminal of each of the three parallel-coupled pull-up NMOS transistors 240, 250, 260 is coupled to a first selectable pre-driver circuit 370 driven by a true input 150. A second terminal of the first variable resistor 270 is coupled to a first terminal of a second variable resistor 272 and the second terminal of the second variable resistor 272 is coupled to a drain of three parallel-coupled pull-down NMOS transistors 242, 252, 262. A source terminal of each of the three parallel-coupled pull-down NMOS transistors 242, 252, 262 is coupled to ground 290 and a gate terminal of each of the three parallel-coupled pull-down NMOS transistors 242, 252, 262 is coupled to an output of a second selectable pre-driver circuit 375 driven by a complimentary input 152.
In the prior art, the transistors of the LPHCSL driver circuit are utilized to provide secondary protection, thereby sharing the load for ESD events. Accordingly, in the device layout of the LPHCSL driver in the prior art, the drain contact to gate spacing (DCGS) is large so that the LPHCSL driver is capable of handling large currents under snapback (ESD) conditions. In contrast, in the present invention a non-snapback technique is implemented in the LPHCSL driver circuit to reduce the die area and to enhance the driver performance. The non-snapback technique in accordance with the present invention is a non-self protecting implementation of the device layout for the parallel-coupled pull-up NMOS transistors 240, 250, 260 and the parallel-coupled pull-down NMOS transistors 242, 252, 262. The device layout implemented in the present invention reduces the drain contact to gate spacing (DCGS) of the transistors to minimum layout dimension/rules, thereby resulting in less area and offering lower capacitance. Accordingly, an ESD device relying on a dedicated RC trigger may be added in parallel with the transistors of the LPHCSL driver circuit.
In the embodiment illustrated in FIG. 3, the first selectable pre-driver circuit 370 and the second selectable pre-driver circuit 275 are shown in additional detail to include a primary pre-driver circuits and three secondary pre-driver circuits. The first selectable pre-driver circuit 270 includes the primary pre-driver circuit 280 have an input coupled to a true input signal 150, an enable input 305 and an output coupled to each of three secondary pre-driver circuits 380, 382, 384, each of the three secondary pre-driver circuits 380, 382, 384 coupled to the control logic circuit 160 and each of the secondary pre-driver circuits 380, 382, 384 comprising an enable input 310, 315, 320. The output of each of the second pre-driver circuits is coupled to a gate of one of the three parallel-coupled pull-up transistors 240, 250, 260. The second selectable pre-driver circuit 375 includes the primary pre-driver circuit 282 having an input coupled to a complimentary input signal 152, an enable input 325 and an output coupled to each of three secondary pre-driver circuits 386, 388, 390, each of the three secondary pre-driver circuits 386, 388, 390 coupled to the control logic circuit 160 and each of the secondary pre-driver circuits 386, 388, 390 comprising an enable input 330, 335, 340. The output of each of the secondary pre-driver circuits 386, 388, 390 is coupled to a gate of one of the three parallel-coupled pull-up transistors 242, 252, 262. In this embodiment, the secondary pre-driver circuits 380, 382, 384, 386, 388, 390 are used to select one or more driver legs and variable resistor values to provide the desired driver impedance at an output of the programmable LPHCSL driver circuit. The secondary pre-driver circuit 380, 382, 384, 386, 388, 390 for each selected driver leg is controlled separately to allow for slew rate programming. The selection of driver impedance, including the driver stages and variable resistor value, slew rate and output levels are determined by the control logic circuit 160 which may be based on values written in a register, such as an I2C register.
As shown in FIG. 3, the programmable LPHCSL driver circuit further includes a plurality of NMOS transistors 350, 352, 354, 360, 362, 364 coupled in parallel across each of the plurality of programmable resistors 356, 366 and each of the plurality of NMOS transistors 350, 352, 354, 360, 362, 364 having a gate coupled to the control logic circuit 160. The plurality of NMOS transistors 350, 352, 354, 360, 362, 364 are used to program the programmable resistor 270, 272 by programming the gate of each of the NMOS transistors 350, 352, 354, 360, 362, 364 added in parallel. This is an exemplary embodiment, and various other circuit configurations for programming the programmable resistors are known in the art and within the scope of the present invention.
With reference to FIG. 4, in accordance with the present invention, a method 400 for providing a output voltage from a programmable LPHCSL driver includes, providing a selected supply voltage to a voltage regulator circuit and a programmable LPHCSL driver circuit of a programmable LPHCSL driver circuit, wherein the voltage regulator circuit comprises a native source follower transistor 405. With reference to FIG. 2, in one embodiment, a selected supply voltage may be provided by a common supply voltage 170 to a voltage regulator circuit 115 and a programmable LPHCSL driver circuit 110 and the voltage regulator circuit 115 may include a native source follower transistor 215. In an exemplary embodiment, the common voltage supply 170 may be 3.3V, 2.5V or 1.8V.
After the voltage supply has been provided the method of the present invention continues by providing a desired reference voltage to the voltage regulator circuit to set an output level for an output signal from the LPHCSL driver circuit 410. With reference to FIG. 2, a desired reference voltage 205 may be provided to a voltage comparator 210 of the voltage regulator circuit 115 to set an output level for an output signal 130 from the LPHCSL driver circuit 110. In an exemplary embodiment, the desired reference voltage may be between 0.6V and 1.2V.
Following the setting of the output level, the method continues by selecting one or more of a plurality of selectable output driver legs of the LPHCSL driver circuit and programming one or more of a plurality of programmable resistors of the LPHCSL driver circuit to provide an output signal from the LPHCSL driver having a desired impedance across the selected supply voltage 415. With reference to FIG. 3, selecting one or more of the plurality of selectable output driver legs of the LPHCSL driver circuit 110 and programming one or more of a plurality of programmable resistors of the LPHCSL driver circuit may be implemented by a plurality of pre-driver circuits 280, 282, 284, 286 in combination with a control logic circuit 160.
The present invention describes a programmable LPHCSL driver circuit having an improved supply voltage range and a higher degree of programmability for impedance matching that does not negatively impact the die area required for the circuit, and an associated method of use.
Exemplary embodiments of the invention have been described using CMOS technology. As would be appreciated by a person of ordinary skill in the art, a particular transistor can be replaced by various kinds of transistors with appropriate inversions of signals, orientations and/or voltages, as is necessary for the particular technology, without departing from the scope of the present invention.
In one embodiment, the voltage regulator and the LPHCSL driver circuit may be implemented in an integrated circuit as a single semiconductor die. Alternatively, the integrated circuit may include multiple semiconductor dies that are electrically coupled together such as, for example, a multi-chip module that is packaged in a single integrated circuit package.
In various embodiments, the system of the present invention may be implemented in a Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC). As would be appreciated by one skilled in the art, various functions of circuit elements may also be implemented as processing steps in a software program. Such software may be employed in, for example, a digital signal processor, microcontroller or general-purpose computer.
For purposes of this description, it is understood that all circuit elements are powered from a voltage power domain and ground unless illustrated otherwise. Accordingly, all digital signals generally have voltages that range from approximately ground potential to that of the power domain.
Although the invention has been described with reference to particular embodiments thereof, it will be apparent to one of ordinary skill in the art that modifications to the described embodiment may be made without departing from the spirit of the invention. Accordingly, the scope of the invention will be defined by the attached claims not by the above detailed description.

## Claims

1. An integrated circuit comprising:
a voltage regulator circuit including a voltage comparator and a native source follower transistor;
a programmable low power high-speed current steering logic (LPHCSL) driver circuit comprising a plurality of selectable output driver legs and a plurality of programmable resistors; and
a common supply voltage coupled to the voltage regulator circuit, the voltage comparator, the native source follower transistor and to the programmable LPHCSL driver circuit.

2. The integrated circuit of claim 1, further comprising a control logic circuit coupled to the programmable LPHCSL driver circuit, the control logic circuit for selecting one or more of the selectable output driver legs and for programming one or more of the plurality of programmable resistors.

3. The integrated circuit of claim 1, wherein the native source follower transistor is a native n-channel metal-oxide-semiconductor (NMOS) transistor.

4. The integrated circuit of claim 1, wherein the native source follower transistor is a native n-channel metal-oxide-semiconductor (NMOS) transistor having a negative threshold voltage.

5. The integrated circuit of claim 1, wherein the native source follower transistor is a native n-channel metal-oxide-semiconductor (NMOS) transistor having a negative threshold voltage of approximately 0.2V.

6. The integrated circuit of claim 1, wherein the common supply voltage is coupled to a bias input of the voltage comparator and a drain of the native source follower transistor, and the voltage regulator circuit further comprises:
a reference voltage coupled to a first input of the voltage comparator;
a source of the native source follower transistor coupled to a second input of the voltage comparator;
a first capacitor having a first terminal coupled between an output of the voltage comparator and a gate of the native source follower transistor and a second terminal coupled to ground;
a second capacitor having a first terminal coupled between the source of the native source follower transistor and a second input of the voltage comparator and a second terminal coupled to ground; and
a first resistor having a first terminal coupled between the source of the native source follower transistor and a second input of the voltage comparator and a second terminal coupled to ground.

7. The integrated circuit of claim 1, wherein the supply voltage is between about 3.3V and about 1.8V, the reference voltage is between about 0.6V and 1.2V and an output voltage at the second terminal of the voltage regulator and the source of the native source follower transistor is between about 0.6V and 1.2V.

8. The integrated circuit of claim 1, wherein each of the plurality of programmable resistors are programmable to provide a resistance of 100Ω or 85Ω.

9. The integrated circuit of claim 1, wherein the programmable LPHCSL driver circuit further comprises a plurality of pre-driver circuits coupled to each of the plurality of selectable output driver legs, wherein the common supply voltage is coupled to each of the plurality of pre-driver circuits.

10. The integrated circuit of claim 9, wherein the common supply voltage is coupled to an enable input of each of the plurality of pre-driver circuits.

11. An integrated circuit comprising:
a voltage regulator circuit;
a programmable low power high-speed current steering logic (LPHCSL) driver circuit comprising a plurality of selectable output driver legs and a plurality of programmable resistors, the programmable LPHCSL driver circuit including:
a plurality of pre-driver circuits, each of the pre-driver circuits having an input coupled to a true input or a complimentary input signal and one or more outputs coupled to a gate of one of each of the plurality of selectable output driver legs, wherein each of the plurality of selectable output driver legs further comprises a plurality of selectable pull-up transistors having a drain coupled to an output of the voltage regulator circuit and a plurality of pull-down transistors have a source coupled to ground;
at least two of the plurality of programmable resistors coupled in series between a source of each of the plurality of pull-up transistors and a drain of each of the plurality of pull-down transistors; and
an output signal of the integrated circuit coupled between the at least two of the plurality of programmable resistors coupled in series; and

a common supply voltage coupled to the voltage regulator circuit and to the programmable LPHCSL driver circuit.

12. The integrated circuit of claim 11, wherein each of the plurality of selectable output driver legs further comprises:
a first n-channel metal-oxide-semiconductor (NMOS) transistor having a gate coupled to the pre-driver circuit and a drain coupled to an output from the voltage regulator circuit;
a first programmable resistor having a first terminal coupled to a source of the first NMOS transistor;
a second programmable resistor having first terminal coupled to a second terminal of the first programmable resistor; and
a second NMOS transistor having a gate coupled to the pre-driver circuit, a drain coupled to a second terminal of the second programmable resistor and a source coupled to ground.

13. The integrated circuit of claim 11, wherein the pre-driver circuit further comprises a plurality of selectable pre-driver circuits coupled to the control logic circuit and to one of the plurality of selectable output driver legs.

14. The integrated circuit of claim 11, further comprising a plurality of n-channel metal-oxide-semiconductor (NMOS) transistors coupled in parallel across each of the plurality of programmable resistors and each of the plurality of NMOS transistors having a gate coupled to the control logic circuit, wherein the plurality of NMOS transistors are used to program the programmable resistor.

15. An integrated circuit comprising:
a voltage regulator circuit comprising a voltage comparator and a native source follower transistor, wherein the common supply voltage is coupled to the voltage comparator and the native source follower transistor;
a programmable low power high-speed current steering logic (LPHCSL) driver circuit comprising a plurality of selectable output driver legs and a plurality of programmable resistors;
a common supply voltage coupled to the voltage regulator circuit and to the programmable LPHCSL driver circuit; and
a control logic circuit coupled to the programmable LPHCSL driver circuit, the control logic circuit for selecting one or more of the selectable output driver legs and for programming one or more of the plurality of programmable resistors.

16. A method comprising:
providing a selected supply voltage to a voltage regulator circuit and a programmable low power high-speed current steering logic (LPHCSL) driver circuit, wherein the voltage regulator circuit comprises a native source follower transistor;
providing a desired reference voltage to the voltage regulator circuit to set an output level for an output signal from the programmable LPHCSL driver circuit; and
selecting one or more of a plurality of selectable output driver legs of the programmable LPHCSL driver circuit and programming one or more of a plurality of programmable resistors of the programmable LPHCSL driver circuit to provide an output signal from the programmable LPHCSL driver having a desired impedance across the selected supply voltage.

17. The method of claim 16, wherein the native source follower transistor is a native NMOS transistor having a negative threshold voltage.

18. The method of claim 16, wherein the supply voltage is selected from 3.3V, 2.5V and 1.8V.

19. The method of claim 16, wherein the desired reference voltage is between about 0.6V and 1.2V.