Abstract:
A low voltage differential swing (LVDS) signal driver having a constant output differential voltage (Vod) over variations in circuit fabrication processes, power supply voltages and operating temperatures (PVT). The minimum and maximum values of the LVDS output signal are monitored and, based upon the difference between them, a signal is provided to the circuit to control the LVDS output signal such that its peak-to-peak value is maintained at a determined value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to high speed digital integrated circuits, and in particular, to low voltage differential swing (LVDS) signal drivers for uses in and with high speed digital integrated circuits. 
     2. Description of the Related Art 
     With the tremendous growth of the Internet, data transfers, in terms of both volume and speed, are increasing dramatically in all areas of communications. For example, data streams for digitized video signals, high definition television (HDTV) and color graphics data require increasing amounts of bandwidth. As a result, increasingly higher speed interconnects between integrated circuits (chips), functional boards and systems become increasingly critical. While virtually all such data is digital in form, it is a high speed analog circuit technique that has become increasingly prevalent in meeting such data transfer needs. This circuitry, i.e., LVDS, provides for multigigabit data transfers on copper interconnects and high speed transmission lines, including fiber optic applications. These LVDS circuits have proven speed, low power, noise control and cost advantages important in point-to-point applications for telecommunications, data communications and video displays. 
     However, while LVDS circuits continue to provide significant advantages in applications requiring high data transfer rates, such circuits are not immune from three major parameters that influence the operation of virtually any circuit or system: circuit fabrication (or manufacture) process variations (“P”); power supply voltage variations (“V”); and operating temperature variations (“T”); often referred to collectively as PVT. 
     With respect to fabrication process variations, it is well known that notwithstanding the stringent quality control measures typically used to fabricate integrated circuits, fabrication processes nonetheless suffer some variations among the various processing parameters. 
     With respect to power supply variations, it is well known that notwithstanding the use of various filters or shielding techniques, noise and especially low frequency noise can be present or induced in the power supply line (e.g, switching noise, electromagnetic interference, etc.). Power supply noise can cause jitter on the rising and falling edges of the signal being processed, as well as frequency skew within the output signal. 
     With respect to operating temperature variations, such variations will virtually never be avoidable, as operating temperatures can vary due to a number of causes, including variations in data transfer rates, ambient temperature, variations in power supply voltage, among others. As operating temperatures vary, so can the amplitude, phase and frequency of some of the signals being processed. 
     SUMMARY OF THE INVENTION 
     A low voltage differential swing (LVDS) signal driver in accordance with the presently claimed invention maintains a constant output differential voltage (Vod) over variations in circuit fabrication processes, power supply voltages and operating temperatures (PVT). The minimum and maximum values of the LVDS output signal are monitored and, based upon the difference between them, a signal is provided to the LVDS circuit to control the LVDS output signal such that its peak-to-peak value is maintained at a predetermined value. 
     In accordance with one embodiment of the presently claimed invention, a low voltage differential swing (LVDS) signal driver includes differential signal driver circuitry and feedback circuitry. The differential signal driver circuitry receives a control signal and in response thereto receives and converts a dual differential input signal to a LVDS signal having minimum and maximum values. The feedback circuitry, coupled to the differential signal driver circuitry, receives the LVDS signal and in response thereto provides the control signal, wherein a difference between the minimum and maximum LVDS signal values is maintained at a predetermined value. 
     In accordance with another embodiment of the presently claimed invention, a low voltage differential swing (LVDS) signal driver includes signal driver means and feedback means. The signal driver means is for receiving a control signal and in response thereto receiving and converting a dual differential input signal to a LVDS signal having minimum and maximum values. The feedback means is for receiving the LVDS signal and in response thereto providing the control signal and maintaining a difference between the minimum and maximum LVDS signal values at a predetermined value. 
     In accordance with still another embodiment of the presently claimed invention, a method for generating a low voltage differential swing (LVDS) signal driver includes: 
     receiving a control signal; 
     receiving a dual differential input signal; 
     converting the dual differential input signal to a LVDS signal having minimum and maximum values in response to the control signal; and 
     generating the control signal in response to the LVDS signal and thereby maintaining a difference between the minimum and maximum LVDS signal values at a predetermined value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a schematic diagram of a LVDS Signal Driver in accordance with one embodiment with a presently claimed invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
     Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. 
     Referring to the FIGURE, an LVDS Signal Driver Circuit  10  (preferably in integrated circuit form) in accordance with one embodiment of the presently claimed invention includes a differential signal driver circuit  12  and feedback circuitry  14  which, as discussed in more detail below, controls the signal driver circuit  12  in such a manner as to provide an output signal voltage Vout, in which the peak-to-peak differential signal amplitude Vod is maintained at a substantially constant value (e.g., approximately 350 millivolts) notwithstanding PVT variations. 
     The LVDS driver circuitry  12  includes P-type metal oxide semiconductor field effect transistors (P-MOSFETs) M 0 , M 1 , M 2 , M 3  and M 4 , all interconnected substantially as shown. In accordance with well known LVDS circuit principles, transistors M 1 , M 2 , M 3  and M 4  form the output signal “switchbox” with differential pair transistors M 1  and M 2  receiving the primary differential phase Vina and differential pair transistors M 3  and M 4  receiving the inverse differential phase Vinb of the input signal Vin. (It should be understood that while this driver circuitry  12  has been implemented using P-MOSFETs exclusively, similar circuitry can be implemented using N-MOSFETs exclusively or a complementary arrangement of P- and N-MOSFETs.) Transistor M 0  serves as a tail current source for the driver current Id flowing between the positive power supply terminal VDD and the negative power supply terminal VSS/GND. This transistor M 0  is biased, or controlled, by a control voltage Vcon which, as discussed in more detail below, establishes and maintains the driver current Id such that the output signal Vout is maintained at the desired amplitude Vod notwithstanding PVT variations. 
     The feedback circuitry  14  includes signal peak detection circuits  16 ,  18 , a signal combiner (e.g., summer)  20  and a voltage comparison circuit  22 , all interconnected substantially as shown. 
     In accordance with well known LVDS circuit principals, the interconnected drain and source terminals of transistors M 1  and M 3  and transistors M 2  and M 4  provide the differential output signal Vout. The gate terminals of transistors M 1  and M 2  receive the primary (“positive”) differential signal phase Vina and the gate terminals of transistors of M 3  and M 4  receive the inverse (“negative”) differential signal phase Vinb of the input signal Vin. When transistors M 1  and M 4  are turned on, transistors M 2  and M 3  are turned off, while conversely when transistors M 2  and M 3  are turned on, transistors M 1  and M 4  are turned off. Accordingly, the driver current Id is steered through an external load resister (not shown) to produce the output voltage Vout. 
     The differential signal components Vouta and Voutb of the output signal Vout are processed by the signal peak detection circuits  16 ,  18 . The positive peak detection circuit  16  detects the maximum (e.g., most positive or least negative) signal value of the output signal Vout and provides an output signal Voh indicative of that value. Similarly, the negative peak detection circuit  18  detects the minimum (e.g., most negative or least positive) signal value of the output signal Vout and provides an output signal Vol indicative of that value. 
     The signal combiner circuitry  20  differentially sums these detection signals Voh, Vol by subtracting the minimum value detection signal Vol from the maximum value detection signal Voh and provides a resultant, or difference, signal Vres indicative of the difference between these two signals Voh, Vol. 
     This resultant signal Vres is compared against a reference signal Vref in the signal comparison circuitry  22 . As a result of this comparison, the control signal Vcon is generated to indicate the difference between the resultant signal Vres and the reference Vref. 
     In accordance with the foregoing discussion, the feedback circuitry  14  monitors the output LVDS signal Vout and, through the feedback control signal Vcon, establishes and maintains the driver current Id such that the amplitude Vod of the output signal Vout is maintained notwithstanding PVT variations, as well as variations in the external load (not shown) through which the driver current Id flows. 
     Various other modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.